WATER
Q UALITY
MANAGEMENT
AERATION
SHRIMP
FARMING
Fisheries and Allied Aquacultures Departmental Series No. 2
Alabama Agricultural Experiment Station
Auburn University
Auburn University, Alabama
Lowell T. Frobish, Director
February 1989
AND
IN
Conversion Factors for English and Metric Units
To convert columnTo convert column
1 to column 2, 
2 to column 1,
multiply by Column 1 Column 2 multiply by
Length
2.540 ................ Inches Centimeters......................... 0.3937
0.3048 ................ Feet Meters........... a................... 3.281
1.609 ................ Miles (statute) Kilometers.......................... 0.6214
30.48 . ............... Feet Centimeters......................... 0.0328
0.9144 ............... Yards Meters.............................. 1.094
Area
0.4047 ................ Acres Hectares............................ 2.471
6.452 ............... Square Inches Square centimeters.................. 0.1550
Volume
0.9463 ............... Quart, liquid, U.S. (32 ounce) Liter............ .................... 1.057
1.136 ..... .......... Quart, imperial (40 ounce) Liters.............................. 0.8799
3.785 ............... Gallon, U.S. (4 quarts) Liters.............................. 0.2642
4.546 ............... Gallon, imperial Liters............................... 0.2200
29.57 . ................ Ounce (U.S. fluid) Milliliters......................... 0.0338
Weight
28.35 .... ........... Ounces (avoirdupois) Grams............................... 0.0353
0.4536 ............... Pounds (avoirdupois) Kilograms........................... 2.205
1.016 ......... ..... . Tons (gross or long) Metric ton.......................... 0.9842
0.9072 ............... Tons (short or net) Metric ton.......................... 1.102
Pressure
70.31 ............... Pounds per square inch Grams per square centimeter......... 0.0142
0.0703 ............... Pounds per square inch Kilograms per square centimeter..... 14.22
Other conversions
1.12 ................ Pounds per acre Kilograms per hectare............... 0.892
10.76 ............... Foot candels Lux................................. 0.0929
CONTENTS
Page
Effect of Salinity on hip.....................6
Factors Influencing Salinity ....................... 7
Fa ctors Af fe c ting pH .. ...................................
9
Effects on Ship..........................10
LIMING .....................................
10
Acid-Sulfate Sis..........................10
SUSPENDED SOLIDS AND ORGANIC MTE..................12
D ecompo s ition...................................................... 
1290 0 4 0a 0 6 60 1
Bacterial Amendments ................................ 15
NUTRIENTS, PLANKTON, FERTILIZER, AND FED..............16
Turb id ity. ............................................................
17
DISSOLVED OYE.........................~ 
23
S olub ili ty............................................................230* 0 0 * 0* 00 a 0 2
Effects of DO on Srm.......................25
Fluctuations in DO Concentrations.................................. 26
Feeding and DO .................................. 27
Oxygen Budgets ...............................28
Aerat ors. ................................
29
Paddle Wheel Aerator Dsg.....................30
Aeration of Pns..........................33
Aerator Placement ............................... 35
Operating Cost and Service Lf...................36
WATER CIRCULATION ................................. 37
TOXIC MTBLTS...........................38
Carbon Doie............. . . . . . . . 39
Hydrogen Slie...........................40
WATER SPL..............................41
WATER ECAG.............................43
POND BTOS..............................45
HEAVY MTL..............................46
MEASUREMENT OF WATER QUALITY..........................................48
ACKNOWLEDGMENTS..................o.....................................49
FIGUES.................................................51
BIBIORAPH................................79

Water Quality Management and Aeration in Shrimp Farming
Claude E. Boyd*
Shrimp farming in brackishwater ponds is an important and rapidly
growing industry in many tropical nations. Satisfactory techniques for
producing shrimp larvae to stock in ponds are available, and procedures
for rearing shrimp in ponds are highly developed. However, production
of shrimp in ponds often is limited by water quality degradation. A
large amount of information on water quality management in freshwater
ponds is available (13, 59). This knowledge has not been used much in
shrimp farming, but many of the methods used in freshwater aquaculture
are directly applicable to brackishwater aquaculture. The purpose of
this manual is to provide a simple account of water quality management
for application to brackishwater shrimp ponds. The information also
will be useful to those culturing other species of aquatic animals in
brackishwater.
SALINITY
Salinity is defined as the total concentration of dissolved ions in
water. Salinity often is expressed in milligrams per liter (mg/1), but
in aquaculture, it is more convenient to express salinity in parts per
thousand (ppt or 0/00). One can divide salinity values expressed in
milligrams per liter by 1,000 to get parts per thousand; e.g., 5,500
mg/l = 5.5 ppt.
Salinity of freshwater often is considered to be 0 ppt, but most
inland waters contain from 0.05 to 1 ppt salinity. In arid regions,
inland waters may be highly saline. Water containing more than 0.5 ppt
salinity usually is not suitable for domestic purposes, and 1 ppt
salinity will impart a salty taste. Seawater has a salinity of 30 to 35
ppt. Estuarine waters may range from near 0 ppt to more than 30 ppt.
If sea water or estuarine water is held in a pond during dry weather,
evaporation may cause salinity to increase. Fast (37) classified waters
based on salinity as follows: fresh water, <0.5 ppt; oligohaline, 0.5 -
3.0 ppt; mesohaline, 3.0 - 16.5 ppt; polyhaline, 16.5 - 30.0 ppt;
marine, 30.0 - 40.0 ppt; brine or hypersaline, >40.0 ppt.
Seven ions (sodium, potassium, calcium, magnesium, chloride, sul-
fate, and bicarbonate) contribute most of the salinity to water, table
1. The other dissolved substances in water usually contribute little to
salinity or total dissolved solids, but they may be biologically
important. Water usually contains only trace amounts of phosphorus,
inorganic nitrogen, iron, manganese, zinc, copper, boron, and certain
other elements, but small amounts of these elements are essential for
phytoplankton growth.
*Professor of Fisheries and Allied Aquacultures.
Table 1. Typical Concentrations of Major Ions (mg/l) in Seawater,
Brackishwater, and Freshwater
Ion Seawater Brackishwater Freshwater
Chloride 19,000 12,090 6
Sodium 10,500 7,745 8
Sulfate 2,700 995 16
Magnesium 1,350 125 11
Calcium 400 308 42
Potassium 380 75 2
Bicarbonate 142 156 174
Other 86 35 4
Total 34,558 21,529 263
Effect of Salinity on Shrimp
Shrimp larvae are produced or captured in waters with salinities of
28 to 35 ppt, but post larval stages often are stocked in ponds where
salinity is much lower. When shrimp post larvae are stocked into ponds,
they should be acclimated gradually to lower salinity to reduce stress
and mortality. The acclimation rate should not exceed 1 or 2 ppt change
in salinity per hour. Acclimation can be accomplished by gradually
adding low salinity water to the higher salinity water in which shrimp
are held. The acclimation procedure can be done at the hatchery if a
source of low salinity water is available and the salinity of pond water
is known. If the acclimation procedure will take more than 6 or 8
hours, post larvae should be fed. Aeration of the acclimation tank is
desirable. Acclimation also may be accomplished in the pond where post
larvae will be stocked. During nighttime, bags of water containing the
post larvae can be floated in a pond and pond water can be poured
gradually into the bags until the salinity in the bags equals the
salinity of the pond water. The rate of salinity change should not
exceed 1 to 2 ppt per hour.
Two species of shrimp, Penaeus merguiensis (banana prawn) and P.
monodon (tiger prawn), are commonly cultured in Asia. Best survival and
growth of P. merguiensis occur at salinities above 15 ppt, but P.
monodon will survive and grow well at lower salinity (17, 47). P.
monodon can tolerate freshwater for about 1 month. In South and Central
America, P. vannamei is commonly cultured. This species is native to
the Pacific coast. Salinities of 15 to 25 ppt are considered ideal, but
P. vannamei can be cultured successfully at lower and higher salinities.
In Ecuador, many farmers claim that this species can be grown in
freshwater. Although P. vannamei can tolerate freshwater for several
weeks, practical experience indicates that a salinity of at least 0.5 to
1.0 ppt is necessary for survival and growth. Hand-held refractometers
(salinometers) used by shrimp farmers do not measure salinity accurately
when salinity is low. Therefore, the salinity is not necessarily 0 ppt
when a salinometer indicates 0 ppt. Low salinities can be estimated
from chloride concentrations; chloride analysis is not difficult and may
be made with water analysis kits.
Factors Influencing Salinity
Shrimp farming is done in low-lying coastal areas where seawater
and freshwater mix. Salinity concentrations in rivers and canals that
supply water for shrimp ponds are regulated by the proportions in which
seawater and freshwater mix. Where a small river runs into the sea,
estuarine water has high salinity. Salinity is lower where a large
river flows into the sea. Most shrimp farming is conducted in the
tropics where wet and dry seasons are well defined. During the wet
season, river discharge increases and salinity declines in estuaries.
Conversely, river discharge decreases during the dry season and salinity
increases in estuaries. Data from Thailand (17) will be used to
illustrate this concept. Monthly rainfall at Bangkok, Thailand, and
salinity of water at mouths of four major rivers which discharge into
the Gulf of Thailand in the vicinity of Bangkok are presented in table
2. A short distance upstream from the river mouths, salinity may be
extremely low during the wet season. Shrimp ponds are filled with water
from canals which receive their flow from the rivers. Hence, during the
wet season, water for shrimp ponds in some areas of Thailand will have
low salinity.
Table 2. Normal Monthly Rainfall at Bangkok, Thailand,
of Water at Mouths of Four Major Rivers
and Salinity
Salinity (ppt)
Month Rainfall, Bang Tha Mae Chao
mm Pakong Chin Klong Phraya
January 10.3 31.2 24.5 27.6 24.0
February 30.7 
31.2 28.8 
26.9 -
March 23.7 
32.1 32.8 28.7 
-
April 63.5 32.0 23.8 23.1 23.2
May 185.3 29.3 19.5 24.2 15.9
June 159.8 10.3 18.6 23.7 22.2
July 170.8 7.3 8.0 6.5 23.1
August 198.2 5.6 13.5 3.3 15.4
September 341.8 7.2 8.6 16.1 14.3
October 221.3 18.5 22.2 13.5 18.3
November 44.0 28.2 30.2 26.6 19.3
December 8.0 27.9 22.6 28.8 31.4
The situation described for Thailand
areas of other nations. Rainfall and sali
is common to
inity of pond
low-lying coastal
water on a shrimp
farm near Guayaquil, Ecuador, are provided in table 3. Ponds were
filled with water from the Guayas River, and 10% water exchange was made
daily. The salinity of the river dropped drastically during the wet
season and pond water contained less than 5 ppt salinity for 3 months.
Further upstream, river water may become fresh during the wet season and
the water for shrimp ponds may be between 0 and 0.5 
ppt-for 3 or 4
months. In some areas where ponds have low salinity for long periods,
it may be possible to develop salt-water wells for discharging saline
ground water into ponds to increase salinity.
Evaporation
dissolved ions in
some areas during
ponds is the only
rates are high in the tropics and this concentrates
pond water. Salinities above 35 ppt are common in
the dry season. Continual flushing of water through
procedure for reducing salinity.
Table 3. Monthly Rainfall Totals and Average Salinity of Pond
Water at a Shrimp Farm Near Guayaquil, Ecuador
Month Rainfall, Salinity in shrimp ponds,
mm ppt
January 521 23.8
February 206 8.4
March 51 4.2
April 203 4.1
May 10 3.8
June 0 5.0
July 0 9.2
August 0 10.2
September 0 14.6
October 0 20.7
November 0 22.3
December 76 23.1
pH
The term "pH" refers to the hydrogen ion concentration in water;
more generally, pH refers to how acidic or basic a water is. For
practical purposes, water with a pH of 7 is considered neither acidic
nor basic; it is said to be neutral. When the pH is below 7, a water is
said to be acidic. Water with a pH above 7 is considered basic. The pH
scale extends from 0 to 14; the greater the departure of the pH from 7,
the more acidic or basic a water. The above explanation of pH will
suffice for some, but more details are provided below:
The pH concept is based on the ionization of water. A fixed number
of water molecules dissociate to give hydrogen ion (H+) and hydroxyl ion
(OH-). The product of the molar concentrations of H+ and OH- will
always equal 10
-
14. We may write:
H20 = H
+ 
+ OH
(H
+ )
(OH) 
=10
14
Concentrations of H and OH are equal in pure water, so we may
substitute H+ for OH and get:
(H
)
(H
+
) =-10
14
(H) 
1C7
(H = 10-n.
By definition, pH is the negative logarithm of the hydrogen ion
concentration:
pH 
= 
-log(H+).
Foprewaer(+
)  
0-7
For pure water, (H+)=10 molar, and it follows that pH is 7:
pH = -log(10
-7 
) = -(-7) = 7
, H+
Pure water is neither acidic nor basic, because H+ (acidity) and OH
(basicity) are equal in concentration.
Factors Affecting pH
The pH of normal brackishwater usually is between 7 and 9. Phyto-
plankton blooms often develop in shrimp ponds. Phytoplankton obtain CO
2
(carbon dioxide) for use in photosynthesis from the HCO
3
7
(bicarbonate)-CO
2 
equilibrium system as follows:
2HC C2-
2HCO3= C02 
+ 
CO
3  
+H
2
0.
As CO
2 
is removed, the reaction progresses to the right of the equation
and C
3
2
- (carbonate) accumulates. Hydrolysis 
of CO
3
2
-occurs according
to the following reaction:
2- + -
CO2 + H = HCO
3
.
33
- 2-
Notice that two HCO
3  
ions give one CO
2 
molecule and one CO
3  
ion, but
hydrolysis of one C03
2
- ion only replaces one HCOf ion. Furthermore,
only a portion of the C0
3
2  
hydrolyzes. Hydrogen ion is derived from
the dissociation of water:
H
2
0 = H
+ 
+ OH-.
The equilibrium constant for the ionization of water must be maintained,
that is:
(H
+
) (Oi-) = 10
-
14.
Hence, when H
+  
is used in the hydrolysis of CO
3 2 -
, more water must
dissociate to maintain the equilibrium constant of water. As a 
result,
there is more OH- and less H+ than when photosynthesis started. 
Thus,
pH rises as phytoplankton removes 002 from water.
Because phytoplankton uses 002 during daylight, the pH of water in-
creases during the day. At night, no 002 is removed by 
phytoplankton,
but all pond organisms release 002 from respiration. This 
002 reacts
2- -
with CO03 and H
2
0 to form HCO
3 
; the HCO
3  
dissociates to release H
and the pH declines. The daily cycle in pH is illustrated in figure 1.
However, the daily fluctuation in pH is not always as great as shown in
this figure.
For those not adept in chemistry, it is sufficient to remember that
CO02 is an acid. Removal of CO2 will make the water less acidic (pH will
rise), while addition of CO2 will make the 
water more acidic (pH will
fall).
Effects on Shrimp
There are few data on the effect of pH on shrimp, but it is safe
to assume that shrimp respond to pH in much the same way as fish. The
effect of pH on aquaculture species is generalized below (13):
pH Effect
4 Acid death point
4-6 Slow growth
6-9 Best for growth
9-11 Slow growth
11 Alkaline death point
Brackishwaters are well buffered against pH change, and pH will
seldom fall below 6.5 or rise above 9. Therefore, adverse effects of pH
on shrimp are uncommon. There are instances where acidic soils are a
problem in shrimp farming; this will be discussed later.
LIMING
Total alkalinity is defined as the total concentration of titrat-
able bases in water. Primary bases in water are HCO
3 - 
and C032- ions.
Total alkalinity traditionally has been expressed as milligrams per
liter of equivalent calcium carbonate (CaCO3). Total hardness 
is
defined as the total concentration of divalent cations in water, also
expressed as milligrams per liter of CaCO
3
. Calcium and magnesium ions
are the major divalent cations in water.
In freshwater aquaculture, alkalinity and hardness often are impor-
tant considerations. The concentration of each variable should exceed
20 mg/l as CaCO
3 
. When alkalinity and hardness are too low, lime is
applied to increase their concentrations. In brackishwater, alkalinity
and hardness are usually high, so these variables are seldom important
in management of shrimp ponds. However, there are areas where soils are
acidic and lime may be required.
Acid-Sulfate Soils
Shrimp farms sometimes are constructed in areas once covered by
saline and brackishwater tidal swamps and marshes (34, 53). When rivers
with a heavy sediment load emptied into the sea, sediment was deposited
near the shore. After the deposits rose above mean low water level,
vegetation became established. As deposition continued, the coast
slowly accreted, and a swamp forest developed. In the swamp forest,
10
tree roots trapped organic and inorganic debris, and decomposition of
dense masses of organic debris resulted in anaerobic conditions. As a
result, sulfur reducing bacteria became abundant, and 
sulfide produced
by the bacteria accumulated in pore spaces in sediment as 
hydrogen
sulfide (H
2
S) or combined with iron to form precipitates of iron
sulfides. Iron sulfides underwent further chemical reaction 
to form
iron disulfide that crystallized to form iron pyrite.
As long as sediments containing 
pyrites are submerged and anaer-
obic, they remain reduced and change little. However, if they are
drained and exposed to the air, oxidation results, and sulfuric 
acid is
formed. The summary reaction for sulfuric acid formation from iron
pyrite (34) is:
FeS +3.7502 
+ 3.5 H20 
-- Fe(OH)+2SO2- 
+ 4H+
FeS
2 
2
The Fe(OH)3 crystallizes as a reddish brown 
material in the sediment.
After draining, a sediment containing pyrite is called an acid-sulfate
soil or a "cat's clay."
When aerobic, acid-sulfate soils will have a pH below 4.0. The pH
of acid-sulfate soils often will decrease as much as 3 units upon 
drying
(39). Field identification of acid-sulfate soils can sometimes be made
by the smell of hydrogen sulfide from disturbed soil, but 
the positive
test is to measure pH before and after drying.
In ponds, the problem with acid-sulfate soils usually 
originates on
the levees. Pond bottoms are usually flooded and anaerobic, so 
sulfuric
acid does not form. However, levees dry and sulfuric acid formed 
during
dry periods enters ponds in runoff after rains. Acidity on 
levees can
be controlled by liming and establishing good cover with 
an
acid-resistant species of grass. A method for determining 
the lime
requirement of acid-sulfate soils is provided by Boyd 
(13).
Brinkman and Singh (27) developed a method for rapid 
reclamation of
ponds with acid-sulfate soils. The procedure is as 
follows:
1. In the early part of the dry season, dry the 
pond and harrow
thoroughly.
2. Fill with brackishwater. Measure the pH of the 
water frequently.
The pH will drop from that of sea water (7 to 9) 
to below 4. Once the
pH has stabilized, drain the pond. Repeat this procedure 
until the pH
stabilizes at a pH above 5. Often, three or more 
drying and filling
cycles may be required.
3. At the same time the pond is being reclaimed, 
acid must be
removed from the surrounding levees. To achieve this, level the levee
tops and build small bunds along each side of the levee tops to produce
shallow basins. Fill basins with brackishwater. When the pond is
drained for drying, also drain the small basins on the levee tops for
drying. Repeat if necessary. Finally, remove the bunds and broadcast
agricultural limestone over the tops and sides of levees at 0.5 to 1.0
kilograms per square meter (kg/in
2
).
4. Once the last drying and refilling cycle is complete, broadcast
agricultural limestone over the pond bottom at 500 kilograms per hectare
11
(kg/ha). Fertilize the pond with manures or chemical fertilizers as
necessary to promote phytoplankton blooms.
5. To prevent fish kills by seepage of acid from levees, check pH
frequently, and apply agricultural limestone if necessary.
Soils which do not contain iron pyrite may be acidic because of ex-
change acidity (13). However, brackishwater has a high alkalinity, and
lime applications would probably be of little value to shrimp ponds
without highly acid-sulfate soils on levees. A method for determining
the lime requirement of pond soils with only exchange acidity is
provided by Pillai and Boyd (50). It should be noted that acid-sulfate
soils also contain exchange acidity.
SUSPENDED SOLIDS AND ORGANIC MATTER
Where shrimp are produced in brackishwater ponds, the estuarine
water used to fill the ponds often has a heavy load of suspended solids.
These solids consist of soil particles and particles of organic matter
held in suspension by turbulence. The origin of suspended soil
particles is erosion in the drainage basins of rivers. Deforestation
and poor soil management practices in agriculture are major causes of
erosion. After waters from canals or rivers are pumped into ponds,
turbulence decreases and suspended solids settle. Over time, this
process will cause a pond to fill with sediment and become more shallow.
Sediment often has a large organic matter content, and degradation of
this organic matter by bacteria may be a major demand for oxygen.
Water also contains dissolved and particulate matter which will not
settle out. This material often is primarily organic, and it is derived
from contact of runoff with vegetative remains, from phytoplankton
production within the water, and from pollution. Dissolved and
particulate matter also exert an oxygen demand.
Decomposition
As mentioned above, settleable solids are deposited on the pond
bottom, and organic matter has an oxygen demand. Although sediment
accumulation may be troublesome, the oxygen demand of sediment and of
particulate and dissolved organic matter has more serious consequences.
Degradation of organic matter is accomplished by microorganisms that use
it for food. Factors affecting the rate of organic matter decomposition
are temperature, pH, and the nature of the organic matter.
Decomposition increases with increasing temperatures between 0*C and
3500 and with increasing pH up to about pH 8.5. Simple organic
molecules (sugars, proteins, and fats) tend to degrade faster than more
complex ones (cellulose, lignin, and tannins). Microorganisms capable
of degrading organic matter are present in all ponds, and their
abundance will increase when organic matter is increased. Temperature
and pH in shrimp ponds usually are ideal for growth of microorganisms.
Organic matter that has a high concentration of nitrogen relative to its
concentration of carbon (low C/N ratio) will decompose faster than a
material with a high C/N ratio. Additionally, more nitrogen will be
released to the environment as ammonia when a substance with a low C/N
ratio decomposes (2,4).
12
Bacteria contain about 10% N and 50% C on a dry weight basis. They
have a carbon assimilation efficiency of about 5%. This means that for
every 100 grams (g) of substrate carbon, bacteria will convert 5 g to
bacterial tissue. Bacteria are roughly 50% carbon, so 100 g of
substrate carbon would yield 10 g of bacteria. A 10-g mass of bacteria
would contain 1 g of nitrogen.
Suppose that 100 kg of dry organic matter, containing 2% N and 40%
C (C/N ratio = 40/2 = 20), is decomposed by bacteria in a pond. The
organic material contains 40% C, or 40 kg C (100 x 0.4 = 40 kg).
Bacteria will convert about 5%, or 2 kg (40 x 0.05 = 2 kg) of the carbon
in the organic matter to bacterial carbon. This amounts to 4 kg (2 +
0.5 = 4 kg) of bacteria. The bacteria are 10% N, and 4 kg of bacteria
will contain 4x 0.1 = 0.4 kg of nitrogen. The organic matter contained
2 kg of nitrogen (100 x 0.02 = 2 kg). Hence, 1.6 kg of nitrogen would
be released (or mineralized) to the environment as ammonia.
If the 100 kg of dry organic matter contained 40% C but only 0.3% N
(C/N ratio = 40/0.3 = 133), decomposition would be slow and no nitrogen
would be released to the environment:
(i) 100 kg organic matter x 40% C = 40 kg C in the organic matter.
(ii) 100 kg organic matter x 0.3% N = 0.3 kg N in organic matter.
(iii) 40 kg C in organic matter x 5% carbon assimilation efficiency
= 2 kg C in bacteria.
(iv) 2 kg C in bacteria 50% carbon in bacteria = 4 kg bacteria.
(v) 4 kg bacteria x 0.1 kg N/kg bacteria = 0.4 kg N in bacteria.
(vi) 0.4 kg N needed by bacteria - 0.3 kg N in organic matter = 0.1
kg N shortage.
If an organic material is very low in nitrogen, there will not be
enough nitrogen in it to effect 
complete decomposition. In 
this event,
bacteria will remove nitrate and ammonia from pond water (immobilization
of nitrogen) for use in decomposition of matter. Also, bacteria may die
and the nitrogen in dead bacterial cells may be recycled. Nevertheless,
the rate of decomposition generally increases as the proportion of
nitrogen to carbon in an organic residue increases (C/N ratio becomes
smaller). This principle is illustrated below by the rates of oxygen
consumption by bacteria decomposing aquatic plant residues of different
nitrogen concentrations (4):
% N in Oxygen Consumption
residue .(mg/liter per day)
1.48 0.22
3.29 0.64
3.83 0.93
5.92 1.86
13
The amount of oxygen needed to completely decompose organic matter
to CO
2 
and water can be calculated approximately. 
Suppose that 100 kg
of dry organic matter containing 40% C is decomposed. There is 40 kg of
C in the organic matter. The approximate equation for decomposition is:
Organic C + 02 CO +H0
We may treat the equation stoichiometrically as follows:
(i) 40 kg organic C produces 40 kg 
C in CO2
(ii) 40 kg C in 002 multiplied by the ratio of C02 to C (44/12)
will give the weight of C002 produced, that is:
40 kg C x (44/12) = 146.7 kg 
CO2
(iii) Amount of oxygen used may be obtained by the following
proportionality because, in final analysis, each molecule of
carbon will react with one molecule of oxygen to produce one
molecule of carbon dioxide:
X 146.7 kg
C + 02 = 0C02
32 44
X 106.7 kg oxygen
Thus, 106.7 kg oxygen would be consumed by bacteria while completely
degrading 100 kg of organic matter containing 40% C. Expressed in more
general terms, each kg of organic carbon would require 2.67 kg of oxygen
for conversion to carbon dioxide.
In reality, organic matter usually does not decompose completely.
The actual oxygen demand of organic matter in shrimp ponds over a 1-year
period likely would probably be about 70% of the demand calculated by
the procedure illustrated above.
Suppose that the sedimentation rate in a shrimp pond is 100,000
kg/ha per crop [a layer of sediment of approximately 1.5 centimeters
(cm) in depth over the entire pond bottom] and the sediment contains 4%
organic matter. Organic matter is usually about 40% C. The organic
matter input rate in sediment would be 4,000 kg/ha (I00,000 x 0.04).
This amount of carbon would be 1,600 kg/ha (4,000 x 0.4). The oxygen
demand of the sediment would be:
1,600 kg organic C/ha x 2.67 kg 0
2
/kg organic C x 0.70 = 2,990 kg 0
2
/ha.
Over a 135-day production cycle, this oxygen demand is equal to 22.1 kg
O2/ha per day. For a pond 1 m deep, the oxygen demand calculated above
would equal the loss of 2.2 mg/l of dissolved oxygen from the water each
14
day. Such a decomposition is not unreasonable; rates similar to this
have been reported for ponds (9, 24).
Organic matter suspended or dissolved in water also exerts an oxy-
gen demand. This subject will be considered in the section on plankton.
Control of Sediment
The present methods of sediment control involve removing sediment
from ponds periodically at the end of each growing season when ponds are
drained, dredging sediment from undrained ponds, and using sediment
ponds or canals to remove the bulk of sediment before water is
discharged into production ponds. The use of sediment ponds is the best
approach to sediment control.
Sediment ponds should be as deep as possible to maximize storage.
If water levels in sediment ponds are 40 to 50 cm higher than those in
production ponds, this will facilitate transfer of water to production
ponds. Sediment ponds should be subdivided so that sedimentation will
occur in stages. A sediment pond is illustrated in figure 2. In
extensive culture of shrimp, sediment ponds could be constructed inside
existing production ponds. Separate sediment ponds would be needed on
intensive farms. It also is possible to construct a large water supply
canal and use the canal as a settling basin, figure 3. This design is
particularly suitable for new construction.
In Ecuador, sedimentation is especially heavy, for water from the
Guayas River contains a tremendous sediment load. Many shrimp farms use
the water supply canal for sedimentation and employ a floating dredge
for removing sediment. On some farms, the initial part of the water
supply canal is dual. In this way, the dredging operation can be
alternated between canals.
The size of sediment ponds must be determined on a farm to farm
basis. The size will depend upon sediment pond depth, size of
production ponds, rate that solids settle from the water, and desired
water exchange rate. Sedimentation of some soils is accelerated by
dissolved salts in brackishwater, figure 4. However, for other soils,
sedimentation is not rapid even at high salinity.
Aluminum sulfate (alum) often is used to flocculate suspended sol-
ids so that they will settle quickly (10). Alum is too expensive to be
used in shrimp farming.
Bacterial Amendments
Several companies are selling suspensions of bacteria which they
claim will destroy organic matter in ponds and improve conditions for
growth of fish and shrimp. These suspensions contain ordinary bacteria
which are already in ponds. Research has demonstrated that treatment of
aquaculture ponds with bacterial amendments is not worthwhile (26).
15
NUTRIENTS, PLANKTON, FERTILIZER, AND FEED
Phytoplankton is the base of the food web in shrimp ponds. In
semi-intensive and intensive culture, shrimp are provided commercial
feeds. Phytoplankton use inorganic nutrients and sunlight to produce
organic matter by photosynthesis. The photosynthetic process may be
illustrated by the following equation:
6C0
2 
+ 6H20 light, pigments, and ) CH 0 + 60
2 inorganic nutrients 6 12 6 2
Photosynthesis is a phototropic reaction in which plant pigments such as
chlorophyll utilize light energy to reduce inorganic carbon (CO
2
) to
carbohydrate. Oxygen is released in the process; photosynthesis is the
primary source of dissolved oxygen (DO) in aquaculture ponds.
Plant Nutrients
A large number of inorganic elements are required for phytoplankton
growth. Most species require at least the following: carbon, hydrogen,
oxygen, nitrogen, sulfur, phosphorus, chloride, boron, molybdenum,
calcium, magnesium, sodium, potassium, zinc, copper, iron, and
manganese. Diatoms also require silicon. Phytoplankton make oxygen by
photosynthesis, and they obtain hydrogen from water. Carbon dioxide
enters water from the atmosphere, so it usually is present at sufficient
concentration.
Nitrogen and phosphorus are most likely to limit phytoplankton
growth. Typical concentrations of plant nutrients in pond water and in
phytoplankton are shown in table 4. Concentration factors indicate how
much of each element is accumulated by phytoplankton above concentration
of the element in water. There is less nitrogen and phosphorus in pond
water relative to phytoplankton needs than for other elements.
In brackishwater ponds with moderate or high salinity, diatoms are
the dominant phytoplankton. Diatoms require fairly large amounts of
nitrogen, and nitrogen often is as important or even more important than
phosphorus as a limiting factor. When salinity is low, blue-green algae
may become abundant in shrimp ponds. Many species of blue-green algae
can fix elemental nitrogen (N2), so they can grow well without a
combined source of nitrogen (nitrate or ammonia) provided phosphorus is
plentiful.
In many nations, shrimp ponds are supplied waters that are polluted
with domestic sewage. This water contains moderate to high
concentrations of nitrate, ammonia, and phosphate. When held in ponds,
this water usually will produce phytoplankton blooms. In ponds where
feeds are applied, excretory products from shrimp add ammonia and
phosphate to water. If ponds are constructed in areas where waters are
not polluted with sewage, it will be necessary to apply inorganic
fertilizers or manures to foster phytoplankton growth.
16
Table 4. Concentrations of Elements in Seawater and Phytoplankton
1 Concentration
Element Seawater, ppm Phytoplankton, ppm factor
Phosphorus 0.02 187 9,350
Nitrogen .25 2,000 8,000
Iron .01 10 1,000
Manganese .002 2 1,000
Copper .003 1 333
Silicon 3.0 500 167
Zinc .01 1 100
Carbon 28 12,000 43
Potassium 380 250 .7
Calcium 400 250 .6
Sulfur 900 175 .2
Boron .8 .1 .1
Magnesium 1,340 125 .1
Sodium 10,500 25 .002
1
Wet weight basis; assumed 2.5% dry weight.
Turbidity
There are two basic types of turbidity in shrimp ponds: (1) that
resulting from phytoplankton blooms, and (2) that caused by suspended
soil particles. Both kinds of turbidity restrict light penetration into
pond waters, and less light at pond bottoms discourages the growth of
troublesome filamentous algae and aquatic weeds. However, turbidity
from phytoplankton is much more desirable than turbidity from suspended
soil particles. Phytoplankton represents the base of the food web that
culminates in shrimp. Clear ponds have little phytoplankton, and there
may be little natural food for shrimp. In some clear ponds, algae grow
on the bottom (benthic algae) and provide natural food for shrimp, but
most pond managers prefer a pond with a phytoplankton bloom. Clear
ponds usually have low concentrations of plant nutrients, so chemical
fertilizers or manures should be applied to promote phytoplankton blooms
and provide more natural food for shrimp and greater turbidity.
Excessive phytoplankton blooms in shrimp ponds can cause oxygen
depletion (see section on Dissolved Oxygen).
Turbidity in pond waters can be measured most easily with a Secchi
disk, figure 5. The Secchi disk is a 20-cm-diameter disk which is
weighted on the underside. Its upper surface usually is painted with
alternate black and white quadrants. The disk is attached to a
graduated rope. To use a Secchi disk, lower it into the water until it
just disappears and record the depth. Raise the disk until it just
reappears and again record the depth. The average of depth readings is
the Secchi disk visibility. Optimum Secchi disk visibility for shrimp
ponds is 40 to 60 cm. It must be noted that the Secchi disk visibility
is affected by both types of turbidity. The individual taking Secchi
17
disk readings must decide if the turbidity is from phytoplankton,
suspended soil particles, or both.
Fertilization
Shrimp production can be greatly increased by applications 
of fer-
tilizer, but much greater production can be achieved through 
the
combination of fertilization and feeding. In ponds, 
shrimp use a lot of
natural food, so it is not clear how much of the 
increased production
realized from feeding occurs from the shrimp eating the feed 
directly
and how much occurs from the degradation products 
of the feed
stimulating the production of natural food organisms 
for shrimp.
Production can be increased more by feeding than by 
fertilization, so
shrimp apparently derive considerable nutriment from 
feed. There has
been little research on fertilization of shrimp ponds, but 
practical
experience suggests the following:
(i) Waters with high concentrations of nitrogen and phosphorus
(polluted waters) do not require as much fertilizer as 
water
with low concentrations of these nutrients.
(ii) Fertilization becomes less important as feeding rates
increase.
(iii) Diatoms are good food organisms for shrimp, and 
diatoms can be
encouraged by adding large amounts of nitrogen fertilizer
relative to phosphate fertilizer.
(iv) High feeding and fertilization rates cause heavy 
phytoplankton
blooms and increase the probability of low DO concentrations.
(v) Low salinity coupled with high phosphorus 
concentrations
favors blue-green algal blooms.
(vi) Fertilization and feeding creates turbidity 
which helps
control the growth of underwater weeds.
(vii) The development of a suitable fertilization schedule 
for a
particular shrimp farm must be done by trial and 
error,
because research findings are inconclusive.
(viii) Water exchange flushes out fertilizer nutrients.
The food web in a shrimp pond is illustrated in 
figure 6. Of
course, feed may be applied to supplement the natural food.
Chemicali Fert ilIizers
Inorganic fertilizers are substances which contain nitrogen, phos-
phorus, and potassium either alone or in combination. Examples 
of
common fertilizer compounds are listed in table 5. When two or more 
of
these materials are blended, the resulting mixture is called 
a mixed
fertilizer. Fertilizers are classified by plant nutrient content;
nitrogen is expressed as N, phosphorus pentoxide P
2
0
5
, and potash K20.
18
Nitrogen is present in fertilizers as nitrate (NO
2
), ammonium (NH
4
+),
or urea [(NH)
2
COI, phosphorus as phosphate (H
2
PO
4 - ) 
and potassium as
potassium ion (K+). Use of N, P
2
0
5
, and K20 is traditional instead of
descriptive, i.e., fertilizers do not contain elemental N, P
2
0
5
, and
K20.
Table 5. Approximate Grades of Common Commercial Fertilizers
Percentage
Substance
N P205 K20
Urea 45 0 0
Calcium nitrate 15 0 0
Sodium nitrate 16 0 0
Ammonium nitrate 33 0 0
Ammonium sulfate 21 0 0
Superphosphate 0 20 0
Triple superphosphate 0 46 0
Monoammonium phosphate 11 48 0
Diammonium phosphate 18 48 0
Ammonium polyphosphate 10-13 34-39 0
Muriate of potash 0 0 60
A fertilizer with a grade of 15-15-5 contains 15% N, 15% P
2
0
5
, and
5% K20. The grade of a commercial fertilizer 
is provided by the vendor.
It will be instructive to consider how to make a mixed fertilizer like
the 15-15-5 mentioned above. This fertilizer is not necessarily the
best for shrimp ponds; it is used here simply for illustration. A
100-kg quantity of mixed 15-15-5 fertilizer will be made of urea, triple
superphosphate (TSP), and potassium chloride (KCI). In 100 kg of
15-15-5, there will be 15 kg N, 15 kg P
2
0
5
, and 5 kg K20. Necessary
amounts of 
fertilizer 
compounds 
are:
(i) 15 kg N 1 0.45 kg N/kg urea = 33.3 kg urea
(ii) 15 kgP
2
0
5
0.46 kg P
2
0
5
/kg TSP = 32.6 kg TSP
(iii) 5 kg K20 L 0.60 kg K
2
0/kg KC1
(iv) Total fertilizer compounds
= 8.3 kg KCl
= 74.2 kg
(v) Filler (agricultural limestone) = 25.8 kg
(vi) Total = 100.0 kg
Fertilizer compounds must be diluted to 100 kg with filler so that the
mixed fertilizer will have the proper N-P
2
0
5
-K
2
0 percentages. This is
done by adding a material like agricultural limestone (pulverized
19
limestone) to the mixture of fertilizer compounds. In the 
examplc
above, 25.8 kg of agricultural limestone were added as filler.
When granular fertilizers are broadcast over pond surfaces, 
they
settle to the bottom before dissolving completely (12). Phosphate 
is
adsorbed strongly by mud, and much of it never reaches the 
water.
Liquid fertilizers are much more efficient than granular fertilizers,
because they will not settle to the bottom (46). Three common 
liquid
fertilizers suitable for aquaculture are liquid urea (45-0-0),
phosphoric acid (0-55-0), and ammonium polyphosphate (10-34-0 
to
13-39-0). If these materials cannot be purchased locally, suitable
liquid fertilizers may be made by dissolving granular fertilizers 
in a
container of water. Liquid fertilizers should be diluted with water 
and
splashed over pond surfaces. In a large pond, liquid fertilizer 
may be
released into the propeller wash of an outboard motor as a boat is
driven over the pond surface. It might also be efficient to 
splash
liquid fertilizer into the water that flows into a shrimp pond. 
Never
pour liquid fertilizers directly into pond water, because they 
are
heavier than water and will settle to the bottom.
An alternative means of applying granular fertilizer is 
to pour it
on an underwater platform so that water currents 
move the nutrients
throughout the pond after they dissolve (43). Some workers 
also put
fertilizer in porous bags and suspend these bags in ponds so 
that
nutrients are released to the water as fertilizers dissolve. 
Either
procedure prevents the fertilizer granules from dissolving 
while in
contact with pond bottom soils. Nevertheless, regardless 
of the
technique used for applying fertilizers, the phosphorus is eventually
assimilated by plants, adsorbed by pond muds, or lost in outflowing
water. Nitrogen is not strongly absorbed by pond muds, but is lost
through denitrification and in outflow.
Most pond fertilization research has been conducted in freshwater
(13). Application rates usually have consisted of 2 to 9 kg/ha of P
2
0
5
alone, or applications of 2 to 9 kg/ha of both N and P
2
0
5
. One of the
best fertilization programs for freshwater ponds consists of periodic
applications of 2 to 4 liters per hectare (1/ha) of a 10-34-0 
liquid
fertilizer (11, 13). This amounts to 3.9 to 7.8 kg/ha of P
2
0
5 
and 1.1
to 2.2 kg/ha of N. Fertilizer applications normally are made 
at 3- to
4-week intervals.
Nitrogen is more important as a limiting factor in brackishwater
ponds than in freshwater ponds. Furthermore, a wide N:P ratio 
is
thought to encourage the growth of diatoms (6). For these reasons, a
fertilization program for brackishwater ponds should employ more
nitrogen and less phosphorus than recommended for freshwater ponds. In
a recent experiment (Daniels and Boyd, unpublished data), fertilization
with 30 kg N/ha and 1 kg P/ha (2.29 kg P
2
0
5
/ha) or 15 kg N/ha and 1 kg
P/ha (2.29 kg P
2
0
5
/ha) resulted in good phytoplankton blooms that
consisted of 50% to 90% diatoms. Both urea and sodium nitrate were
suitable as nitrogen sources. Ammonium chloride did not perform well.
Triple superphosphate was used as a source of phosphorus. The water was
low in silica (<1 ag/l), and additions of silicate at 30 kg Si/ha per
application with N:P ratios of 30:1 or 15:1 further enhanced 
the
20
production of diatoms. The general benefits of silica fertilization is
unknown. Experiments on silica fertilization might prove valuable.
However, silica concentrations are fairly high in brackishwater in most
tropical nations.
The ideal frequency of fertilization for shrimp ponds is not known.
However, if water exchange rates of 5 to 10% per day are used,
fertilization two or more times per week is needed. Water exchange
flushes nutrients and plankton from ponds and counteracts fertilization.
When ponds are first stocked with shrimp, applying fertilizers once or
twice weekly and limiting water exchange is suggested. If feed is not
applied to ponds and waters are infertile, heavy fertilization must be
continued until shrimp are harvested. However, if feed is applied,
feeding rates will gradually increase as the standing crop of shrimp
increases, and nitrogen and phosphorus from the feed will supplement the
fertilizer. Therefore, fertilizer applications may be reduced and
possibly discontinued as feeding rates increase. Heavy plankton blooms
result from excessive nitrogen and phosphorus concentrations, and can
cause DO depletion.
Much is unknown about fertilization of shrimp ponds, so the shrimp
farmer may use the suggestions presented as a basis for developing
experimental fertilization programs. Different fertilizers, N:P ratios,
application rates, and frequencies of application may be tested in
production ponds until a suitable fertilization program is found. For
those who do not desire to experiment, a 20:1 ratio of N:P is suggested.
Infertile waters need 20 kg N/ha and 1 kg P/ha two or more times per
week. Where waters are more fertile, the amounts may be reduced by 50%
or more, but the 20:1 ratio of N:P should be maintained in
brackishwater. Fertilization should be suspended when plankton blooms
become dense during periods of heavy feed applications.
Fertilizers should be used with caution to prevent excessive plank-
ton blooms. Once Secchi disk visibility is less than 30 to 40 cm, fer-
tilizers should be applied less frequently and in smaller doses.
However, fertilizer applications should not be reduced to the point that
plankton blooms disappear. Where feeding is employed, fertilizer
applications will probably not be required after feeding rates reach 20
or 30'kg/ha per day.
I have by convention reported the phosphorus content of fertilizers
in terms of P
2
0
5
, but I gave N:P ratios. To convert from P205 to P$
divide by 2.29. For example, 46% P
2
0
5 
is 46-" 2.29 = 20.1% P.
Suppose that one desires to calculate the amounts of urea and
triple superphosphate required to provide 20 kg N/ha and 1 kg P/ha in a
15-ha pond. From table 5, urea is 45% N and 0% P and triple
superphosphate is 0% N and 46% P
2
0
5 
(or 46% " 2.29 = 20.1% P). The
amounts are calculated as follow:
(20 kg N/ha "0.45 kg N/kg urea) x 15 ha = 667 kg urea
(1 kg P/ha "0.201 kg P/kg triple superphosphate) x 15 ha = 75 kg
triple superphosphate
21
Manures
Manures are animal excrements and other agricultural wastes. 
Ma-
nures are largely organic matter, but they contain some nitrogen,
phosphorus, and potassium, table 6. It takes a large quantity 
of manure
to give the same amounts of fertilizer nutrients found in a small
quantity of inorganic fertilizer. For example, on a dry weight 
basis,
100 kg of poultry manure contains about 1.2 kg nitrogen 
(100 kg x
0.012). Urea is 45% nitrogen, and 2.67 kg of urea would 
contain as much
nitrogen as 100 kg of poultry manure 
(1.2 * 0.45 2.67 kg).
Nevertheless, in some places manures are readily 
available and cheaper
than inorganic fertilizers.
Table 6. Fertilizer Constituents 
in Fresh Manure of Selected
Farm Animals
Average composition, pct.
Manure Moisture N P205 K20
Dairy cattle 85 0.5 0.2 0.5
Beef cattle 85 
.7 .5 .5
Poultry 72 1.2 
1.3 .6
Swine 82 .5 .3 .4
Sheep 77 1.4 .5 
1.2
When manures are applied to ponds, they decay 
and release inorganic
nutrients which can be used by phytoplankton 
(13). Because manures have
an oxygen demand, the shrimp farmer must be careful 
not to apply enough
to cause an oxygen depletion. Manure 
also serves as food for
zooplankton, shrimp, and fish, but it is low 
in protein and cannot be
considered a high quality food. The combination of manure 
and inorganic
fertilizer sometimes is used to stimulate zooplankton 
blooms in ponds.
Larger quantities of manures must be applied 
to ponds because they
are not concentrated sources of plant nutrients. Many 
times, initial
applications of 500 to 1,000 kg/ha of fresh manure 
are made to ponds,
followed by applications of 200 to 300 kg/ha at 1- to 2-week 
intervals.
Manures have high C/N ratios and little phosphorus, 
so they do not
decompose rapidly. Some aquaculturists apply 
inorganic fertilizer to
enhance the nutrient supply and hasten decomposition 
of manure. In
manured ponds, particles of manure on the 
pond bottom or suspended in
water provide a substrate for microbial growth. 
Bacteria enhance the
protein content of manure particles and improve 
the nutritive value of
particles for shrimp food. Zooplankton feed 
directly on particles of
manure, and manuring is an effective procedure 
for rapidly increasing
zooplankton abundance.
22
Some shrimp farmers initially fill their ponds 
with only a small
volume of water (10 to 20% total capacity) and apply 
a large amount of
chicken manure. After a few days a heavy 
plankton bloom develops.
Water then is added slowly over a few days 
to fill the pond. This
procedure assures abundant plankton to serve 
as food for young shrimp.
DISSOLVED OXYGEN
Dissolved oxygen is the most critical water quality 
variable in
aquaculture. Aquaculturists need a thorough 
understanding of factors
affecting the concentration of DO in pond 
water. They also should be
aware of the influence of low DO concentrations 
on aquatic organisms.
Solubility
The atmosphere contains 20.95% oxygen (33). At 
standard barometric
pressure, 760 millimeters (mm) of mercury, the 
pressure of oxygen in air
is 159.2 mm (760 x 0.2095). The pressure 
of oxygen in air varies with
barometric pressure. Elevation has a marked 
effect on barometric
pressure; pressure decreases with increasing 
elevation. At a given
location, barometric pressure varies during 
the day and from day to day.
Local variation in barometric pressure seldom 
is more than 5% above or
below normal barometric pressure. Shrimp 
ponds are in low-lying coastal
areas, so elevation is near sea level. 
Therefore, for practical
purposes, one may assume that barometric pressure 
acting on surfaces of
shrimp ponds is 760 mm and that the pressure 
of oxygen in air above
shrimp ponds is 159.2 mm.
The pressure of oxygen in air drives oxygen 
into water until the
pressure of oxygen in water is equal to the pressure 
of oxygen in air.
When the pressure of oxygen in water and air 
are equal, net movement of
oxygen molecules from air to water ceases. 
When this occurs, DO is said
to be at equilibrium, or at saturation. The 
concentration of DO at
saturation varies with barometric pressure, 
temperature, and salinity.
As already mentioned, the effect of barometric 
pressure will be ignored
because it will be small for water in 
shrimp ponds. As water
temperature or salinity increase, the 
DO concentration at saturation
decreases, table 7. At 250C and 0 ppt salinity, 
the DO concentration at
saturation is 8.24 mg/l. If temperature 
increases to 300C with no
change in salinity, the DO concentration 
at saturation drops to 7.54
mg/l. The DO concentration at saturation 
is 6.39 mg/l for 30 ppt
salinity and 30
0
C.
Plants growing in pond water produce oxygen 
by photosynthesis, and
during daylight, plants may produce oxygen so fast that DO
concentrations in water rise above saturation. Water containing 
more DO
than expected for the existing barometric pressure, water 
temperature,
and salinity is said to be supersaturated with DO.
Water also may contain less DO than expected at saturation 
for pre-
vailing conditions. Respiration by organisms in ponds 
may cause DO
levels to decline; DO typically declines at night.
23
Table 7. The Solubility of Oxygen in mg/L as Functions of Temperature
and Salinity 0-40 ppt (Moist Air, Barometric Pressure = 760 mm Hg)
Temp Salinity, parts per thousand (ppt)
(C) 0 5 10 15 20 25 30 35 40
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
14.60
14.20
13.81
13.44
13.09
12.76
12.44
12.13
11.83
11.55
11.28
11.02
10.77
10.52
10.29
10.07
9.86
9.65
9.45
9.26
9.08
8.90
8.73
8.56
8.40
8.24
8.09
7.95
7.81
7.67
7.54
7.41
7.29
7.17
7.05
6.93
6.82
6.72
6.61
6.51
6.41
14.11
13.72
13.36
13.00
12.67
12.34
12.04
11.74
11.46
11.18
10.92
10.67
10.43
10.20
9.98
9.77
9.56
9.36
9.17
8.99
8.81
8.64
8.48
8.32
8.16
8.01
7.87
7.73
7.59
7.46
7.33
7.21
7.09
6.98
6.86
6.75
6.65
6.54
6.44
6.34
6.25
13.64
13.27
12.91
12.58
12.25
11.94
11.65
11.36
11.09
10.83
10.58
10.34
10.11
9.89
9.68
9.47
9.28
9.09
8.90
8.73
8.56
8.39
8.23
8.08
7.93
7.79
7.65
7.51
7.38
7.26
7.14
7.02
6.90
6.79
6.68
6.58
6.47
6.37
6.28
6.18
6.09
13.18
12.82
12.49
12.16
11.85
11.56
11.27
11.00
10.74
10.49
10.25
10.02
9.80
9.59
9.38
9.19
9.00
8.82
8.64
8.47
8.31
8.15
8.00
7.85
7.71
7.57
7.44
7.31
7.18
7.06
6.94
6.83
6.72
6.61
6.51
6.40
6.31
6.21
6.12
6.02
5.94
12.74
12.40
12.07
11.76
11.47
11.18
10.91
10.65
10.40
10.16
9.93
9.71
9.50
9.29
9.10
8.91
8.73
8.55
8.38
8.22
8.06
7.91
7.77
7.63
7.49
7.36
7.23
7.10
6.98
6.87
6.75
6.64
6.54
6.43
6.33
6.24
6.14
6.05
5.96
5.87
5.79
12.31
11.98
11.67
11.38
11.09
10.82
10.56
10.31
10.07
9.84
9.62
9.41
9.21
9.01
8.82
8.64
8.47
8.30
8.14
7.98
7.83
7.68
7.54
7.41
7.28
7.15
7.03
6.91
6.79
6.68
6.57
6.47
6.36
6.26
6.17
6.07
5.98
5.89
5.81
5.72
5.64
11.90
11.58
11.29
11.00
10.73
10.47
10.22
9.98
9.75
9.53
9.32
9.12
8.92
8.73
8.55
8.38
8.21
8.05
7.90
7.75
7.60
7.46
7.33
7.20
7.07
6.95
6.83
6.72
6.61
6.50
6.39
6.29
6.19
6.10
6.01
5.91
5.83
5.74
5.66
5.58
5.50
11.50
11.20
10.9.1
10.64
10.38
10.13
9.89
9.66
9.44
9.23
9.03
8.83
8.65
8.47
8.29
8.13
7.97
7.81
7.66
7.52
7.38
7.25
7.12
6.99
6.87
6.75
6.64
6.53
6.42
6.32
6.22
6.12
6.03
5.94
5.85
5.76
5.68
5.59
5.51
5.44
5.36
11.11
10.82
10.55
10.29
10.04
9.80
9.57
9.35
9.14
8.94
8.75
8.56
8.38
8.21
8.04
7.88
7.73
7.58
7.44
7.30
7.17
7.04
6.91
6.79
6.68
6.56
6.46
6.35
6.25
6.15
6.05
5.96
5.87
5.78
5.69
5.61
5.53
5.45
5.37
5.30
5.22
When water is below saturation with DO, there is
oxygen molecules from air to water.
a net movement of
At saturation with DO, the number
of oxygen molecules leaving the water equals the number entering; there
24
,, v rr v 7V V LIY V VV ~
is no net movement of oxygen molecules. Net movement 
of oxygen
molecules from water to air occurs when water is supersaturated 
with DO.
The larger the difference between the pressure of oxygen in 
water and
air, the greater the movement of oxygen molecules.
The degree of saturation of water with DO frequently is
as percent saturation. The equation for estimating
saturation is:
expressed
percentage
% Saturation =
DO concentration in water
DO concentration at saturation
For example, suppose that pond water with 20 ppt salinity 
at
contains 4 mg/l of DO. To estimate percent saturation, 
look up
oxygen saturation value for 28
0
C and 20 ppt salinity in table 7.
value is 6.98 mg/l, and the percent saturation is:
28
0
C
the
The
% Saturation = x 100 = 57.3%.
6.98
If water at the same temperature and pressure mentioned 
above contained
8.5 mg/l DO instead of 4 mg/l DO, percent saturation would 
be:
8.5
% Saturation 
=  
.x 100 = 121.8%.
6.98
Effects of DO on Shrimp
The influence of DO concentrations on shrimp and 
fish (13) cultured
in ponds is summarized below:
DO concentration
Less than 1 mg/l
1-5 mg/1l
5 mg/l-saturation
Above saturation
Effect
Lethal if exposure lasts more than a few hours.
Growth will be slow if exposure to low Do is
continuous.
Best condition for good growth.
Can be harmful if supersaturated conditions
exist throughout pond volume. Normally,
there is no problem.
Concentrations of DO can fall so low that shrimp in ponds 
are killed.
However, adverse effects of low DO more often are expressed 
as reduced
growth and greater susceptibility to disease. In ponds with 
chronically
low DO concentrations, fish will eat less and they 
will not convert food
to flesh as efficiently as in ponds with normal DO concentrations 
(5,
13i 42). The same relationship to DO probably 
is exhibited by shrimp.
25
x 100
Fluctuations in DO Concentrations
Light passing through pond water is rapidly quenched, and the rate
of quenching increases as the amount of particulate matter (turbidity)
in the water increases. Therefore, plankton blooms reduce light
penetration, and the heavier the bloom the less light available for
photosynthesis at a given depth. As a result, photosynthesis occurs
most rapidly in the surface layer of water, and DO concentrations
decline with depth (8, 13, 40). In deep ponds, DO concentrations may
fall to 0 mg/l at depths of 1.5 or 2 m, figure 7. The rate at which DO
declines with depth increases with greater turbidity; in ponds,
phytoplankton often is the major source of turbidity (3). For this
reason, it is advantageous to have shallow ponds (75 to-150 cm deep).
It is especially important to have fairly shallow ponds for shrimp,
because they dwell primarily on the bottom, and low DO concentrations at
the pond bottom would be especially harmful.
Concentrations of DO exhibit a daily cycle. The lowest concen-
trations of DO occur at dawn. During daylight, photosynthesis causes DO
concentrations to increase, and maximum DO concentrations are reached in
the afternoon. During the night, photosynthesis ceases and respiration
by organisms in the pond consumes oxygen and causes DO concentrations to
fall. The daily cycle in DO is most pronounced in ponds with heavy
phytoplankton blooms, figure 8. The DO cycle shown in figure 8 is for
surface water. However, studies of shrimp ponds in Ecuador (Daniels and
Boyd, unpublished data) revealed that DO concentrations at the pond
bottom exhibit the same cycle. Of course, light intensity at the pond
bottom is less than at the pond surface, and DO concentrations sometimes
are lower at the bottom than at the surface. The influence of the daily
cycle of DO on growth of aquaculture species is poorly understood, but
most workers feel that good growth can be achieved as long as the DO
concentration does not fall below 25 or 30% of saturation during the
night and does not remain at this low level for more than 1 or 2 hours
(13). The procedure outlined in figure 9 may be used to determine how
low DO will fall in a pond during the night (24).
Cloudy weather profoundly influences DO concentrations as illus-
trated in figure 10. This results because DO concentrations do not
increase as much on a cloudy day as on a clear day, but just as much
oxygen is consumed at night by the pond biota. The influence of cloudy
weather is more pronounced in a pond with a heavy phytoplankton bloom
than in a pond with less phytoplankton.
Phytoplankton in ponds may suddenly die and decompose, causing DO
depletion (7, 23). An example of a phytoplankton die-off is shown in
figure 11, and the influence of the die-off on DO concentrations is
illustrated in figure 12. The DO concentrations did not return to
normal until a new phytoplankton bloom was
? 
established. Most
phytoplankton die-offs involve species of blue-green algae. During calm
weather, blue-green algae often form scums at pond surfaces. Intense
sunlight may result in sudden death of algae in this scum. Blue-green
algae have high concentrations of nitrogen in their tissue, so they
decompose rap idly.
26
Mats of filamentous algae which develop on pond bottoms .may, under
certain conditions, float to the surface of a pond and die (35). This
phenomenon also can 
cause depletion of DO.
Water supply canals for shrimp farms in some.countries are polluted
with organic substances. On occasion, unusually large quantities 
of
organic matter enter canals. If highly polluted water is pumped into
ponds, decomposition of organic matter by bacteria may cause DO
depletion.
Feeding and DO
It already has been shown that phytoplankton abundance is 
con-
trolled by nutrient supply, and that DO concentrations are regulated to
a large extent by phytoplankton abundance. Feed applied for shrimp
results in pollution of pond waters by organic and inorganic metabolic
wastes from shrimp. Uneaten feed also decomposes, releasing nutrients
into the water. Consequently, phytoplankton abundance increases as a
function of increasing feeding rate. As phytoplankton abundance
increases, the daily cycle in DO becomes more extreme. Concentrations
of DO at dawn are lower and concentrations of DO in the afternoon are
higher, figure 8. Additionally, DO concentrations decline more rapidly
with depth as phytoplankton abundance increases in response to higher
feeding rates, figure 7. If phytoplankton blooms are extremely dense,
DO may be low on pond bottoms, even in ponds where the water depth does
not exceed 1 m. The probability of DO depletion during cloudy weather
and the likelihood of phytoplankton die-offs are greater in ponds with
high feeding rates and abundant phytoplankton (54).
The influence of feeding rate on DO concentrations is illustrated
in figures 13 and 14. These data suggest that feeding rates above 40 
or
50 kg/ha per day will result in unacceptably low DO. Higher 
feeding
rates may be used in ponds if aeration is applied. Aeration 
is
discussed in another section.
Feeding is a proven technique for increasing production. However,
feeding results in deterioration of water quality, in greater
phytoplankton abundance, and in lower DO concentrations during the
night. If feeds are applied in excessive quantity, DO depletion can
result in mbrtality of shrimp.
Feed conversion value is determined as the quantity of feed applied
divided by the weight of shrimp harvested. For example, suppose that a
1-ha pond yielded 1,600 kg of shrimp and 3,100 kg of feed had been
applied. The feed conversion value is:
3,100 kg feed
= 1.94.
1,600 kg shrimp
A low feed conversion value indicates greater efficiency than a high
value. With good management practices, feed conversion values of 1.5 to
2.0 may be achieved with many species of fish. With shrimp, feed
27
conversion values less than 2.0 can be achieved at low feeding rates.
However, in highly intensive shrimp production, feed conversion values
usually are between 2.0 and 3.0. One of the effects of overfeeding in
shrimp ponds is to increase the feed conversion value. Notice that a
low feed conversion value indicates better use of feed by shrimp than a
high value. As feeding rate increases, DO concentrations during the
night decline. Chronically low DO concentrations have an adverse effect
on the appetite and metabolism of shrimp, and feed conversion values
tend to increase drastically if feeding rates are increased to a level
where DO concentrations fall below 2 or 3 mg/l each night.
Oxygen Budgets
Inputs and outputs of oxygen over an 8-month period were determined
for three channel catfish ponds in Alabama (16). These ponds were 1 m
average depth and had maximum daily feeding rates of 45 kg/ha per day.
Water exchange was not practiced, but emergency aeration was used on
several occasions in each pond. The budget which is presented below is
instructive even to those interested only in shrimp ponds:
Dissolved oxygen
Item kg/ha
Input
Photosynthesis 4,130 76.9
Inflowing water 94 1.7
Aeration 99 1.8
Diffusion (net) 1050 19.6
5,373 100.0
Output
Overflow and draining of water 32 0.6
Phytoplankton respiration 3,090 57.5
Respiration by organisms in sediment 1,040 19.4
Fish respiration 1)210 22.5
5,372 100.0
Most of the oxygen (76.9%) was derived from photosynthesis by
phytoplankton, and most of the oxygen produced by phytoplankton was
consumed in respiration by these same organisms. Respiration by fish
accounted for only 22.5% of the total output of oxygen.
Oxygen budgets are not available for shrimp ponds, but shrimp ponds
receive more oxygen in inflowing water than catfish ponds because of
water exchange. For the same reason, more oxygen would be lost from
shrimp ponds in outflowing water. Respiration in sediment also would be
somewhat greater in many shrimp ponds than in catfish ponds because many
shrimp ponds receive large amounts of organically enriched sediment. In
shrimp ponds, both phytoplankton and water exchange will be dominant
factors in the oxygen budget, and in some ponds benthic respiration also
will be quite important.
28
AERATION
Aerators
Aerators are mechanical devices that increase the rate at which ox-
ygen enters water. There are two basic techniques for aerating pond
water: water is splashed into the air or bubbles of air are released
into the water (18). Hence, we have "splasher" and "bubbler" aerators.
Splasher aerators include vertical pump, pump-sprayer, and 
paddle
wheel aerators. A vertical pump aerator consists of a motor with an
impeller (propeller) attached to its shaft. The motor is suspended
below a float with a center opening and the impeller jets water into the
air at low velocity, figure 15. A pump-sprayer aerator employs a
centrifugal pump to spray water at high velocity through holes in a
manifold and into the air, figure 16. A paddle wheel aerator splashes
water into the air as the paddle wheel rotates, figure 17.
Bubbler aerators include diffused-air systems and propeller-aspir-
ator-pumps. In a diffused-air system, an air blower or air compressor
is employed to deliver air through an air line, and the air is released
through air diffusers located on the pond bottom or suspended in the
water, figure 18. The propeller-aspirator-pump aerator has a high
velocity, uncased impeller at the end of a hollow shaft and housing. In
operation, air flows down the shaft by the venturi principle and is
released into the water in fine bubbles, figure 19.
Aerators usually are powered by electric motors. Where electricity
is unavailable, aerators may be driven by the power-take-off (PTO) of
farm tractors or by diesel or gasoline engines.
Boyd and 'Ahmad (18) evaluated more than 30 aerators for aquacul-
ture. The oxygen transfer efficiencies of the aerators in kilograms of
oxygen transferred per kilowatt*hour of power applied to aerator shafts
is presented below for the basic types of aerators:
Average oxygen transfer efficiency
Type of aerator kg O/kwohr
Paddle wheels 2.13
Propeller-aspirator-pumps 1.58
Vertical pumps 1.28
Pump sprayers 1.28
Diffused air systems 0.97
The purchase prices of different types of aerators do not differ greatly
per kilowatt of motor size. Therefore, paddle wheel aerators will
transfer oxygen to pond water at a lower cost than other types of
aerators. However, all types of aerators have been used successfully in
aquaculture.
The standard aeration efficiency (SAE) of an aerator is a measure
of the amount of oxygen that an aerator will transfer to water under
standard conditions of 20
0
C, 0 mg/l DO, and clean, freshwater. These
conditions seldom exist in shrimp ponds, and the aerator will transfer
29
less oxygen to pond water than it will transfer under standard
conditions (18, 56). Nevertheless, SAE values permit one to compare 
the
efficiencies of different aerators.
Salinity has little effect on oxygen-transfer 
efficiency of aera-
tors (19). However, water with high salinity is corrosive. Therefore,
aerators for salt water must be protected 
from corrosion. It is
possible, but expensive, to use stainless steel or 
plastic construction.
An alternative is to use mild steel construction and 
employ a hot-dip
galvanization procedure to coat the steel 
with a layer of
corrosion-resistant material. After the galvanized 
surfaces have aged
for about 6 months, a coat of epoxy paint over the 
galvanized surfaces
will afford further protection against corrosion. 
Some aquaculturists
have used coal-tar epoxy paint over mild steel to 
retard corrosion.
Methods of corrosion prevention have not been tested, 
and work in this
area is needed.
Paddle Wheel Aerator Design
In an extensive study of design and performance 
of floating, elec-
tric, paddle wheel aerators (1), it was found that 
the best design for a
paddle wheel consisted of a 90-cm-diameter 
paddle wheel with paddles
triangular (1350 interior angle) in cross section. 
The Width of paddles
is unimportant from the standpoint of oxygen 
transfer. However,
fabrication of a paddle wheel with wide paddles 
(10 to 15 cm) is easier
than for one with narrow paddles, because fewer 
paddles have to be made
and welded to the hub. There were four paddles per 
row around the
circumference of the hub, and the paddles were 
attached in a pattern
which provided a spiral arrangement, figure 20. With 
the spiral of
paddles, a constant paddle area moves through the 
water and the torque
necessary to turn the paddle wheel is relatively 
constant. Torque
constantly changes with other paddle arrangements. Paddle 
depth should
be 10 to 15 cm, and paddle wheel speed should be 
about 80 to 90 rpm.
The paddle wheel requires approximately 1 kw of power for each 50 cm of
paddle wheel length. Of course, power 
requirements vary with paddle
wheel speed and paddle depth.
The paddle wheel design described above is ideal for 
paddle wheel
aerators of 2 to 10 kw. For smaller paddle wheel 
aerators, it is more
convenient to reduce the diameter of the paddle 
wheel from 90 cm to
about 60 cm and to use paddles of 5-cm width.
Power requirement for a paddle wheel aerator of a 
given diameter
increases linearly, with increasing speed and with 
increasing paddle
depth (1, 22). For a particular speed and paddle 
depth, the power
requirement increases linearly as paddle wheel diameter increases. The
oxygen transfer efficiency of paddle wheel aerators is related to paddle
tip speed. Paddle wheels of 50, 70, and 90 cm diameter and 12.5 cm
paddle depth were run at different speeds. The greatest aeration
efficiencies were achieved at 80 rpm for the 90-cma-diameter paddle
wheel, at 105 rpm for the 70-cm-diameter paddle wheel, and at 130 rpm
for the 50-cma-diameter paddle wheel. Paddle tip speeds (angular
velocities of tips) were similar at optimum speeds (50 cm, 3.5 m/sec; 70
cm, 3.9 m/sec; 90 cm, 3.8 m/sec). The 90-cma-diameter paddle wheel had a
30
greater oxygen-transfer efficiency than the two smaller diameter paddle
wheels. The power requirements for a paddle wheel with a specific
combination of diameter, depth, and speed will increase in direct
proportion to paddle wheel length. However, for small paddle wheel
aerators (2 kw and less), it is not practical to use a 90-cm-diameter
paddle wheel because it would be too short. It is probably best to make
the paddle wheel 1 m or 1.5 m long, but only 60 cm in diameter, for
small aerators. The load can be adjusted to the motor size by
regulating the depth of paddles.
Several aerator manufacturers have taken the paddle wheel aerator
design described above and used it, or a slight modification of it, to
fabricate floating, electric, paddle wheel aerators powered by 2.25- to
7.5-kw electric motors. A 7.5-kw electric paddle wheel aerator is shown
in figure 21. The aerator had a 3.66-m-long by 20-cm-diameter hub made
of steel tubing. A short, 5.08-cm-diameter shaft was welded to each of
two circular plates (20-cm-diameter), and the plate and shaft were
welded to each end of the hub. Paddles were made of 0.32-cm-thick,
35.6-cm-long, by 15-cm-wide steel plates. The paddles were placed in a
brake press and bent along the centers of their long axes to provide a
rectangular cross section (135' interior angle). Paddles were welded
directly to the hub. There were four paddles welded 90' apart in each
row around the circumference of the hub. Paddles were positioned in
spiral fashion on the hub; the first paddle in each row was offset 15*
from the first paddle in the preceding row of paddles to produce the
spiral. A total of 24 rows of paddles (96 paddles) was attached to the
hub. The paddle wheel was mounted in pillar block bearings, and the
bearings were attached to the steel frame. Four 200-1 barrels were
filled with styrofoam and used for flotation. Two barrels were attached
with straps and turnbuckles to each end of the frame. By adjusting the
turnbuckles, the frame and paddle wheel could be leveled and the paddle
depth adjusted. A 7.5-kw, 230/460-volt, 3-phase electric gear motor was
bolted to one end of the frame. The output shaft of the gear motor was
connected to one end of the paddle wheel shaft with a flexible coupling.
The paddle wheel aerator was rotated at 88 rpm and the paddle depth was
approximately 9 cm. The aerator was maintained in position in a pond by
use of two 6-m-long steel pipes (3.8-cm-diameter) attached between the
aerator frame and steel posts driven into the pond bank.
A 2.25-kw electric paddle wheel aerator is shown in figure 22. The
aerator is constructed in the same manner as the 7.5-kw aerator
described above, but it is smaller. The paddle wheel is 90 cm in
diameter and approximately 1 m long.
Variation in steel paddle wheel aerator design among manufacturers
is not great. Most manufacturers use paddles with a triangular cross
section, but paddles rectangular in cross section (channel iron) have
been successful. Paddle wheel length and diameter vary slightly among
manufacturers. For 7.5-kw units, length varies from 3.05 to 3.66 m and
diameter from 80 to 90 cm. Paddle wheel speed varies from 78 to 90 rpm
among different brands of steel, electrically powered aerators. Of
course, the paddle depth must be adjusted so that the electric motor is
properly loaded. It is important to check the amount of current
(amperes) used during paddle wheel operation; it is wasteful to run the
31
motor much below the rated current Ndraw, and a current overload will
cause the motor to overheat and "burn out." Use of plastic or metal
barrels for flotation permits the use of turnbuckles to adjust paddle
depth. However, some manufacturers use styrofoam blocks for flotation
and attach the blocks directly to the aerator frame. This arrangement
necessitates the use of bearings with vertical adjustment, "take-up
bearings" so that paddle depth can be adjusted. Gear motors are
convenient, but some manufacturers use a separate motor and gear
reducer. The gear reducer is driven by a belt and pulley system, and
the output shaft from the gear reducer housing is attached by flexible
coupling to the aerator shaft.
It is possible to mount the power unit for paddle wheel aerators on
the pond bank, figure 23. This paddle wheel is mounted on floats, but
the motor and jackshafts for speed reduction are placed on the pond
bank. The paddle wheel is driven by a long shaft which is connected by
universal joints to the second jackshaft and to the aerator shaft. This
is an excellent design, provided it is not necessary to move the paddle
wheel aerator within the pond. This driver mechanism would be
especially useful for gasoline or diesel power units.
Paddle wheel aerators may be powered by hydraulic pumps. A hydrau-
lic pump positioned on the pond bank is connected to aerators through
hydraulic lines. Several aerators can be operated from a single power
unit. Unfortunately, because of excessive maintenance costs, hydraulic
systems have not been popular.
Paddle wheel aerators may be constructed of plastic. The popular
"Taiwanese paddle wheel" is familiar to most aquaculturists, figure 24.
A Japanese firm manufactures a paddle wheel aerator similar to the
Taiwanese model.
A company in the United States has been working with Auburn Univer-
sity to develop a paddle wheel aerator constructed of polyurethane
plastic. The prototype aerator is shown in figure 25. The hub was made
from a solid piece of polyurethane (20 cm in diameter by 1 m long) with
stainless steel shafts attached. The polyurethane paddles were attached
to threaded polyurethane rods and screwed into tapped holes in the hub.
Paddle wheel diameter was 60 cm. Hubs were inserted into bearings made
of solid polyurethane. Power was provided by a 0.75-kw gear motor, and
the paddle wheel turned at 88 rpm. The frame was made of polyurethane
bars attached with brass bolts and screws. The flotation system
consisted of plastic barrels.
Where electricity is not available at ponds, aerators may be pow-
ered by PTOs of farm tractors. These aerators are not highly energy
efficient, but they are useful in an emergency. A typical PTO paddle
wheel aerator is shown in figure 26. The aerator was mounted on a
trailer fabricated from 9-cm and 10-cm-diameter steel pipe. The paddle
wheel hubs were 46-cm-diameter by 46-cm-long pieces of steel tubing.
Twelve 36-cm-long by 15-cm-wide by 0.32-cm-thick paddles of slightly
concave cross section were welded to each hub. There were four paddles,
each 900 apart, in each row of paddles around the hub. The middle row
of paddles was rotated 450 on the circumference of a hub to the other
32
two rows to provide a staggered arrangement of paddles. The hubs and
attached paddles were attached on the axles of a truck differential. A
1.95-cm-diameter drive shaft, supported by bearings which were attached
to the trailer, was connected at one end to the differential and fitted
at the other end with a PTO shaft. Speed reduction by the differential
was. 4.5:1; this provided a paddle wheel speed of 120 rpm when the
tractor PTO was operated at 540 rpm. The overall length of the trailer
was 6.86 m and the center of the paddle wheel was 1 m above ground
level. This particular paddle wheel can be operated by tractors of 30
kw and 
larger.
Considerable variation in PTO paddle wheel design has been used.
Paddle width has varied from 5 cm to 20 cm; paddles have been flat,
triangular, rectangular or flat in cross section. Various patterns of
paddle positions around the hub have been used. Diameters of paddle
wheels (paddle tip to paddle tip) have varied from 60 to 150 cm. Hub
lengths and rows of paddles per hub have varied tremendously. Gear
boxes have been used instead of automotive differentials for speed
reduction. Hydraulic cylinders have been inserted into the trailer
frame to permit the paddle wheel to be raised or lowered without
changing the position of the trailer.
Aeration of Ponds
It is common to aerate ponds when DO concentrations are low; this
practice is called emergency aeration. Pond managers check DO
concentrations during the night, and when DO concentrations are expected
to fall below 2 to 4 mg/l, emergency aeration is initiated (13). At
feeding rates above 40 or 50 kg/ha per day, aeration may be needed
frequently. Practical experience indicates that shrimp stress and
mortality can be prevented by emergency aeration (25).
It is well known that aeration will increase shrimp production, but
there are few data on aeration of shrimp ponds. There is, however, much
information on aeration of channel catfish ponds. The author has
visited many shrimp farms in Panama, Guatemala, Australia, Ecuador, and
Thailand. Relationships among feeding, dissolved oxygen, and shrimp
production are almost identical in general features to these
relationships in catfish production. Therefore, information that
follows on catfish pond aeration should be applicable to shrimp ponds.
However, there are major differences which must be considered. Some
shrimp ponds are large (20 ha or more); catfish ponds seldom exceed 8 ha
in area. Catfish readily swim to the oxygenated zone around aerators.
It is not known if shrimp will respond in like manner. Shrimp spend
much time at pond bottoms, so DO concentrations at pond bottoms are more
critical in shrimp ponds than in catfish ponds.
Aeration may also be applied on a continuous or on a nighttime ba-
sis. In an experiment with continuous diffused-air aeration, catfish
production was increased from 2,700 kg/ha in control ponds to 5,500
kg/ha in aerated ponds (45). Parker (48) used several types of aeration
in small ponds to increase channel catfish production from 3,000 kg/ha
in control ponds to 15,000 kg/ha in aerated ponds. In another
experiment, the combination of continuous aeration with vertical pump
33
aerators at 2.5 kw/ha, emergency aeration with PTO aerators, 
and water
exchange permitted a channel catfish yield of 12,800 kg/ha (51).
Production of over 10,000 kg/ha of catfish annually in ponds is an
unrealistic expectation in commercial production of this species. Such
high production requires considerable water exchange and frequent
emergency aeration above the supplemental aeration applied on a daily
basis. However, production of 10,000 to 20,000 kg/ha of various species
of fish and of shrimp is achieved in commercial ponds in Israel and in
Asia. Such high production is possible in small ponds (usually less
than 1 ha) when aeration is applied at or above 5 kw/ha and there is
considerable water exchange (10 to 30% pond volume per day) employed.
Unfortunately, only a few studies of the use of aeration in high
intensity aquaculture have been conducted, and these studies have not
been sufficiently quantitative to permit 
generalization.
Channel catfish fingerlings were stocked in ponds at 1,200, 4,300,
8,600, 17,300, 26,000, and 34,600 per hectare, and maximum daily feeding
rates of 0, 28, 56, 84, 112, 168, and 224 kg/ha, respectively, were
established (32). Aeration was applied with vertical pump aerators at
6.1 kw/ha when DO concentration was expected to fall below 2 mg/l.
Aerators were seldom operated in ponds with feeding rates of 0 to 56
kg/ha per day. Aeration was applied almost constantly at night in ponds
with feeding rates of 112 kg/ha per day and above. Even though aeration
prevented extremely low DO concentrations in all ponds, net fish
production increased with feeding rate only up to 112 kg/ha per day.
Feed conversion values were between 1.6 and 1.8 for daily feeding rates
of 0 to 112 kg/ha. Feed conversion values were 
2.5 and 16.5 for daily
feeding rates of 168 and 224 kg/ha, respectively. Maximum net
production of 6,000 kg/ha was achieved at a feeding rate of 112 kg/ha
per day. Ammonia nitrogen accumulated in ponds, and high ammonia
concentrations apparently limited production at high feeding rates.
This experiment shows that production cannot be increased without limit.
Some other water quality variable, probably ammonia, will impose limits
on production even though there is adequate DO. Of course, water
exchange can sometimes be used to improve water quality by reducing
concentrations of ammonia 
and other toxic metabolites.
In an experiment conducted in 1986, channel catfish ponds were
stocked at 10,000 fingerlings/ha and fed to a maximum daily rate of 53.2
kg/ha (42). Three ponds were aerated 6 hours per night from May 30 to
October 12 at a rate equivalent to 6.25 kw/ha with vertical pump
aerators that had an SAE of 1.34 kg 02/kw'hr. Three ponds served as
un-aerated controls, but emergency aeration was occasionally applied in
these ponds. An example of DO concentrations during the night in
aerated and un-aerated ponds is illustrated in figure 27. Dissolved
oxygen concentrations were always above 4 mg/l in aerated ponds, but DO
concentrations below 2 mg/l often were recorded in un-aerated ponds.
Harvest weight of fish averaged 4,813 kg/ha in aerated ponds and 3,659
kg/ha in un-aerated ponds. The same quantity of feed was applied to all
ponds. Feed conversion values were 1.32 and 1.75 in aerated and
un-aerated ponds, respectively. Production data were expanded to larger
ponds for a budget analysis. Net returns to land, management, and
equity capital were $1,719/ha for aerated ponds and $896/ha for
un-aerated ponds. The experiment was repeated in 1987 with similar
34
results. Production in aerated ponds was 4,475 kg/ha as compared to
3,551 kg/ha in control ponds. Feed conversion values were 1.58 and 2.04
in aerated and unaerated ponds, respectively. These findings suggest
that nightly aeration of ponds stocked and fed at moderate rates may be
more profitable than aeration of heavily stocked and fed ponds.
Aerators for nightly aeration may be attached to an inexpensive
timer and turned on and off at any desired time. Electric service
panels for aerators which incorporate a motor starter, a timer, and an
ammeter in a single weatherproof box may be purchased at a reasonable
cost.
The results of the experiments summarized above suggest that in
ponds where a large degree of water exchange is impractical, aeration
can best be used with moderate feeding rates (50 to 60 kg/ha per day) to
eliminate nighttime DO problems and increase production through
enhancement of feed conversion efficiency. Extremely high feeding rates
(60 to 100+ kg/ha per day) are necessary for production of 5,000 to
20,000 kg/ha. Such high levels of production necessitate water exchange
or some type of water treatment to reduce concentrations of toxic
metabolites. Aeration alone usually is not sufficient to permit the
highest levels of production. Little is known about feed conversion
values, and above a certain point, feed conversion probably begins to
deteriorate even though net production continues to increase. Much
additional research on relationships among feeding rate, water quality,
aeration, water exchange, net production, and feed conversion efficiency
is needed.
Aerator Placement
Water circulation tests show that aerators mix water throughout
ponds, and paddle wheel aerators are especially efficient in circulating
pond water. A recent study was conducted to ascertain the best position
for a paddle wheel aerator in a rectangular pond. Sixty 1-1 plastic
bottles were filled with water so that they floated with only their caps
above the water. These bottles were released at once in front of a
2.25-kw aerator in a 0.4-ha pond, and the paths of the bottles recorded.
The best place to mount a paddle wheel aerator is at the middle of one
of the long sides of the pond, figure 28. The aerator should direct
water parallel to the short sides of the pond. The worst arrangement is
to mount the aerator in one corner of the pond to direct water
diagonally across the pond, figure 28. Although this arrangement was
best for the particular pond, it would likely be necessary to use some
other aerator placement pattern to achieve the best water circulation in
ponds of other sizes or shapes. The technique using bottles that is
given above could be used by shrimp farmers to decide where to place
aerators in ponds.
Many people think that most aerators only mix surface water. This
opinion is untrue. Tests with water current meters demonstrated that
surface aerators propel water away from the aerator, and a return
current of water is established at the pond bottom (Boyd and Moore,
unpublished data).
35
Operating Cost and Service Life
The initial prices of PTO aerators range from $2,000 to $4,000 U.S.
per unit. Of course, a tractor must be available to power each unit.
Operating cost will vary with tractor size and aerator efficiency.
There are not sufficient data to estimate the amount of oxygen
transferred by PTO aerators per liter of fuel, so the cost per kilogram
of oxygen cannot be estimated.
Electric aerators vary tremendously in cost per unit. The usual
range in cost is $350 to $1,200 U.S. per kilowatt. Small aerators are
more expensive than large ones. For example, a 0.5-kw vertical pump
aerator costs about $500 ($1,000/kw); a 2.25-kw paddle wheel aerator
costs about $1,500 ($711/kw); a 7.5-kw paddle wheel aerator costs about
$2,800 ($373/kw).
The only reliable way of determining the operating cost of a par-
ticular electric aerator is to attach a watt hour meter to the aerator.
The power use per hour can be determined, and the operating cost per
hour estimated by multiplying the power use in kilowatt* hours by the
cost of electricity per kilowatt. hour. An approximate method for
estimating operating cost is to calculate power use from current and
voltage. For a single phase motor, power may be approximated as:
(I)(V)
p 
=
1,000
where P = power use (kw)
I = current (amp)
V = voltage (volt)
The equation for three-phase motors is:
(I)(v)(')
1,000
Of course, the above equations are approximate because the power factor
of the motor will be less than unity. For example, a three-phase,
electric paddle aerator was operated at 225 volts and 27 amps. The
actual power consumption as measured with a wattehour meter was 9.44 kw.
The power consumption estimated was 10.52 kw.
The cost of electricity is highly variable and depends upon many
factors. The average cost of electricity in the United States is around
$0.075 per kilowatt'hour. Using this price of electricity, the hourly
operating cost for the 7.5-kw electric paddle wheel aerator mentioned
above would be $0.708/hour (9.44 kw x $0.075/kw'hr). In standard tests,
the aerator had a standard oxygen transfer rate (SOTR) of 21.81 kg
02/hour. However, under pond conditions the actual oxygen transfer 
rate
(AOTR) would seldom be more than 50% of the SOTR. Assuming an AOTR of
10.90 kg 0
2
/hr, the cost of electricity per kilogram of oxygen would be
36
$0.065. The DO concentration in ponds is constantly changing, so it is
more reasonable to express the cost of electricity for aerators in terms
of the cost per hour of operation.
The service life of aerators is highly variable and depends to a
great extent upon care and maintenance. Lubrication of PTO aerators is
essential. With proper lubrication, PTO aerators will last for many
years.
Electric aerators must not be operated in a manner to overload the
motor. Once the aerator is positioned in a pond, it should be verified
that the current draw does not exceed the rated current for full load of
the motor. Most electric motors have a service factor of 1.10 to 1.20,
and can be operated at an overload of 110% or 120% for an extended
period. However, this practice is not advisable, and it is best to run
motors at a slight underload (95% full load is ideal). The current draw
of a motor can change, so it should be checked periodically. Motor
controllers are essential for aerators larger than 1 or 2 kw. These
controllers have fuses (heaters) that protect the motor against an
overload, but the overheating of the motor that occurs before the fuses
break the controller circuit is harmful to the motor and reduces its
service life. Motor controllers are built directly into small electric
motors.
Care also must be taken to lubricate electric aerators and to main-
tain paint to resist corrosion. In saline water, corrosion is a major
problem, and the use of corrosion resistant material is recommended.
Experience indicates that small aerators (?2.25 kw) can be operated
for about 2 years before major repairs are necessary. Larger aerators
usually do not require major repairs for 4 or 5 years.
WATER CIRCULATION
There is a consensus among aquaculturists that water circulation 
in
ponds is beneficial (28, 29, 30, 31, 38, 55). Water circulation
prevents thermal and chemical stratification. This makes the entire
pond volume habitable and eliminates oxygen depletion at the mud-water
interface. High concentration of oxygen at the pond bottom is
especially important in shrimp ponds, because shrimp spend a lot of time
at the bottom.
Shrimp farmers often exchange water in ponds on a daily basis.
Water exchange causes some circulation. There are many ideas about how
to situate entrance gates and exit gates to achieve the best water
circulation, but definitive answers are not available.
Aeration of pond water causes water circulation, and paddle wheel
aerators are more efficient than other types in circulating pond water.
There have been some studies of devices designed to circulate pond water
(28, 38). Water circulation devices create surface turbulence and this
effects a small degree of aeration. However, water circulators should
not be considered aerators. The greatest influence of water circulators
on oxygen concentration results because these devices blend surface
37
water with subsurface water. During daylight hours, surface waters in
ponds often are supersaturated with DO, and water at greater depths may
have low DO concentrations. By mixing pond water, a uniform DO profile
can be established (31, 38, 55). Oxygen produced by phytoplankton is
possibly conserved by water mixing, because the high degree of DO
supersaturation normally found at pond surfaces during daylight is
eliminated by mixing. Preliminary evidence suggests that the total DO
content of a pond can be increased by mixing (30, 31, 38). An extensive
research effort on water circulation in ponds would be worthwhile.
An air-lift pump designed by Parker (49) is illustrated in figure
29. The pump is constructed of a length of PVC pipe and a PVC elbow.
Air from an air blower is released through a 900 hose adaptor into the
PVC pipe. If desired, an air diffuser that releases smaller 
bubbles of
air can be placed in the pipe. The rising air bubbles lift water
through the pipe and discharge it at the pond surface. A pump holder 
is
attached between the anchor post and the pump. This holder contains a
flotation device and it permits the pump to pivot. Ballast 
must be
provided at the bottom of the pump.
Parker (49) demonstrated that two 10-cm-diameter air-lift pumps
with 122-cm vertical risers would ump the entire volume of water in a
0.2-ha pond in 3 days if 0.14 mJ/min of air was injected into each
vertical riser at a depth of 76 cm. Obviously, if a high degree of
mixing is desired, a lot of air lift pumps would be required. For
example, to achieve a pumping rate of one pond volume per day, a 1-ha
pond with a depth of approximately 1 m would require thirty
10-cm-diameter air-lift pumps. Total air flow to the pumps would be 4.2
m
3
/min.
Researchers in Hawaii (38) designed, fabricated, and tested a
device which they called a water circulator, figure 30. It consisted of
a 61-cm-diameter fan (turbine impeller) attached to a shaft which was
connected to a 0.19-kw electric gearmotor that provided an impeller
speed of 60 rpm. The device was mounted on a small cart to facilitate
mobility. Discharge was estimated at 5.7 m
3
/min.
The water circulator (38) was tested in 0.2-ha prawn ponds. In
calm conditions without artificial circulation, the pond developed
thermal stratification and DO concentrations in bottom water were often
less than 5 mg/l DO during late afternoon. With artificial circulation,
DO concentrations in bottom water sometimes exceeded 12 mg/l during late
afternoon. Minimum daily DO concentrations in bottom water averaged 1.0
mg/l higher and maximum daily DO concentration in bottom water averaged
4.0 mg/1 higher during artificial circulation. These findings suggested
that artificial circulation increased the potential for prawn
production.
TOXIC METABOLITES
As a result of metabolic activity by organisms in ponds, carbon
dioxide, ammonia, and hydrogen sulfide sometimes may reach harmful
concentrations.
38
Carbon Dioxide
High concentrations of C002 can be tolerated by aquacultural spe-
cies, although fish are known to avoid C002 concentrations as low as 5
mg/l. Most aquacultural species will survive in waters containing up to
60 mg/l 002 , provided DO concentrations are high. When DO
concentrations are low, the presence of appreciable CO2 hinders uptake
of oxygen. Unfortunately, CO02 concentrations normally are high when DO
concentrations are low. This results because carbon dioxide is released
in respiration and utilized in photosynthesis. Dissolved oxygen
concentration declines when photosynthesis is not proceeding as rapidly
as respiration; thus, CO2 accumulates because it is not removed for use
in photosynthesis.
Because of the necessity of light for photosynthesis, CO2 concen-
trations increase at night and decrease during the day. High concen-
trations of CO2 also occur in ponds during cloudy weather and following
die-offs of phytoplankton or of filamentous algae.
It seldom is practical to remove CO02 from pond waters. However, it
sometimes is necessary to remove CO02 from tanks or other containers in
which shrimp or fish are reared or held. Removal may be effected by
applying calcium hydroxide (CaOH2) or calcium oxide (Ca0). For calcium
hydroxide, apply 0.84 mg/l to remove 1 mg/l CO2 (13). Less calcium
oxide is needed; 0.64 mg/l of calcium 
oxide will remove 1 mg/l of CO2.
For example, water in a 25-m
3 
tank contains 20 mg/l CO2. To remove
20 mg/l C02 with calcium oxide will require 20 mg/l x 0.64 = 12.8 mg/l
calcium oxide. On a cubic meter basis, 12.8 mg/l calcium oxide will
require 12.8 g/m
3 
of the substance. Hence, 320 g (25 m
3 
x 12.8 g/m
3
) of
calcium oxide must be applied to the tank.
Ammnonia
Ammonia reaches pond water as a by-product of metabolism by animals
and by decomposition of organic matter by bacteria (13). In water,
ammonia nitrogen +occurs in two forms, un-ionized ammonia (NH
3
) and
ammonium ion (NH
4 
),in a pH and temperature dependent equilibrium:
+
NH
3 
+ H20 = NH + OH.
As pH rises, un-ionized ammonia increases relative to ammonium ion.
Water temperature also causes an increase in the proportion of
un-ionized ammonia, but the effect of temperature is less than that of
pH. Analytical procedures for ammonia nitrogen measure both un-ionized
and ionized ammonia. Percentages of un-ionized ammonia at different
temperature and pH values are available in the literature (13, 36, 58).
The toxicity of ammonia nitrogen is attributed primarily to the un-
ionized form. As ammonia concentrations in water increase, ammonia
excretion by aquatic organisms diminishes, and levels of ammonia in
blood and other tissue increases. The result is an elevation in blood
pH and adverse effects on enzyme-catalyzed reactions and membrane
stability. Ammonia increases oxygen consumption by tissues, damages
39
gills, and reduces the ability of blood to transport oxygen. Disease
susceptibility also increases in organisms exposed to sublethal
concentrations of ammonia (52, 60).
The tolerance of aquatic organisms to ammonia varies with species,
p rsiological condition, 
and environmental factors. 
Lethal
c ncentrations for short-term exposure (24 to 72 hours) are between 0.4
a d 2.0 mg/l of un-ionized ammonia (13). Concentrations of total
ammonia nitrogen necessary to give 0.4 mg/l un-ionized ammonia at 30
0
C
and different pH values follow:
Concentration of total
ammonia nitrogen to
pH give 0.4 mg/l NH
3
7.0 49.38
7.5 15.62
8.0 5.32
8.5 1.93
9.0 0.89
9.5 0.56
10.0 0.45
Ponds seldom contain more than 2 or 3 mg/1 of total ammonia nitrogen
(13). Obviously, ammonia toxicity will be a greater problem at high pH.
It is difficult to evaluate ammonia concentrations in ponds. Because of
the daily cycle in pH, figure 1, un-ionized ammonia concentrations
change continuously. Ammonia toxicity usually is expressed by reduced
growth rate instead of mortality.
High ammonia concentrations are most common in ponds with high
feeding rates. The only feasible means of reducing ammonia
concentration is water exchange.
Hydrogen Sulfide
Under anaerobic conditions, certain heterotrophic bacteria can use
sulfate and other oxidized sulfur compounds as terminal electron
acceptors in metabolism and excrete sulfide (13) as illustrated below:
so42- + 8H+ = S
2- 
+ 4H
2
0.
Sulfide is an ionization product of hydrogen sulfide and participates in
the following equilibria:
- +
H2S = HS + H
HS = S2- + H
+ .
The pH regulates the distribution of total sulfide among its forms
(H
2
S, HS~, and S2-). Un-ionized hydrogen sulfide is toxic to aquatic
organisms; the ionic forms, however, have no appreciable toxicity.
Analytical procedures measure total sulfide. Values given below show
the percentages of un-ionized hydrogen sulfide at different pH values at
25
0
C
?
40
pH Hydrogen sulfide, pct.
5.0 99.0
5.5 97.0
6.0 91.1
6.5 76.4
7.0 50.6
7.5 24.4
8.0 9.3
8.5 3.1
9.0 1.0
The percentage of hydrogen sulfide decreases as the pH increases. A
water containing 0.01 mg/1 total sulfide would have an H
2
S concentration
of 0.009 mg/1 at pH 6 (0.01 x 0.911 = 0.009); the same total sulfide
concentration at pH 8.5 would contain only 0.0003 mg/l H
2
S (0.01 x 0.031
= 0.0003).
Concentrations of 0.01 to 0.05 mg/l of H
2
S may be lethal to aquatic
organisms (13, 57). Any detectable concentration of hydrogen sulfide is
considered undesirable. The presence of hydrogen sulfide may be
recognized without water analysis, for the "rotten-egg" smell of
hydrogen sulfide is detectable at low concentration.
If water contains hydrogen sulfide, water exchange will reduce its
concentration. Application of lime to raise the pH of the water will
reduce the proportion of the total sulfide that is comprised of hydrogen
sulfide.
WATER SUPPLY
Manufacturers of standard pumps provide data on power-head-dis-
charge relationships for their pumps. These data are used to produce
pump performance graphs which allow one to estimate pump discharges for
different pumping heads. The pumping head is the vertical distance that
water is lifted plus friction losses. Pump performance graphs also show
the power required for each head-discharge possibility 
and pump
efficiency for each head-discharge combination. Unfortunately, many
shrimp farmers do not have performance data for their pumps. The best
way of measuring pump discharge is to determine the length of time
required to fill a basin of known volume. For example, suppose that a
pond with an area of 1,000 m
2 
and a depth of 1.3 m is filled by a pump
in 4 hours. The total volume of the pond is 1,300 m
3
. Pump discharge
is calculated as follows:
1,300 m
3  
(6 hr x 60 min/hr) = 3.61 m1
3 / m i n .
For smaller pumps, the time to fill a barrel or tank of known volume can
be used to estimate discharge. Dimensions of the jet of water flowing
from a pipe may be used to estimate pump discharge; a nomograph for
calculating pipe discharge is provided, figure 31. This nomograph has
English dimensions. Gallons per minute may be converted to liters per
minute by multiplying gallons by 3.785.
41
Water budgets for shrimp ponds are relatively simple. Inputs are
rain falling directly into ponds and water pumped into ponds. Runoff
from levees will not contribute a significant amount of water. Normal
monthly rainfall data can usually be obtained from governmental weather
services. Pump discharge and time of pumping may be used to estimate
the amount of water pumped into water supply canals or ponds.
Where water enters ponds through gates, the water usually flows
over a rectangular weir-type structure. This structure may be used to
measure inflow (44). The equation for the discharge of a rectangular
weir is: /
weir is 
Q = 1.84(L
w 
- 0.2h)(h)32(60)
3
where Q = discharge in m /min
Lw= width of weir crest (the width of the structure
over which water flows) in m
h = head (elevation difference between the weir crest
and the water surface) in m.
To obtain an accurate measurement of discharge, the elevation of the
water surface should be measured a distance equal to 4 times the head
upstream from the weir crest. Therefore, a transit or level should be
used to establish the elevation of the weir crest relative to a
benchmark. Then, a staff gauge should be placed at an appropriate
distance upstream from the weir. A staff gauge should be installed so
that the exact reading of the staff gauge which corresponds to the
elevation of the weir crest is known. This reading on the staff gauge
corresponds to the water level that is exactly equal to the elevation of
the weir crest. When the water level is higher than the weir crest, the
staff gauge reading will afford an estimate of the depth (head) of water
flowing over the weir.
To illustrate the calculation of water flow over a rectangular
weir, suppose that a weir has a crest width of 1.00 m and the crest
elevation corresponds to a staff gauge reading of 1.00 m. On a
particular day, the staff gauge reads 0.71 m. Therefore, the head is
1.00 - 0.71 
f 
0.29 m. The discharge of the weir is:
Q = 1.84[1.O0 - 0.2(0.29)](0.29)3/2(60)
Q = 16.2 m3/min.
if the head for a weir changes over time, it will be necessary to
make several readings of the head during the water exchange period and
average these readings.
Outputs of water are seepage, evaporation, and overflow during
water exchange. Seepage is difficult to measure. Soils for shrimp
ponds usually are heavy clays that do not seep appreciably. Seepage
rates probably will not exceed 0.2 cm/day in most ponds. Evaporation
usually is high year round in shrimp-farming regions. Pan evaporation
data for Bangkok, Thailand, provided below show that evaporation rates
are high in tropical areas.
42
Pan evaporation, mm
135
141
183
188
169
151
Month
July
August
September
October
November
December
Pan evaporation, mm
147
145
129
126
125
130
Evaporation pans are commonly used by meteorologists to estimate
potential evaporation. Pond evaporation may be calculated by
multiplying 0.8 by pan evaporation (15). Discharge from ponds can be
measured by weirs, but it is more convenient to estimate discharge by
difference when the volumes of all other variables are known (14).
WATER EXCHANGE
Water exchange serves several purposes: it flushes nutrients and
phytoplankton from ponds to prevent excessive phytoplankton blooms; it
removes toxic metabolic wastes such as ammonia; it dilutes pond water,
so that salinity does not become excessive during the dry season.
Water pumped into ponds should be applied at the pond surface.
Water should be drained from near the pond bottom and at the opposite
side from which it is introduced. The most beneficial means of
exchanging water in a pond is to first drain out the volume to be
exchanged and then pump in an equal volume of replacement water.
However, many farmers just pump water into a pond and allow the pond to
overflow.
Water exchange rates used by farmers in various countries range
from 5 to 30% per day. The water exchange rate in shrimp farming
probably averages about 10% per day, which appears to be more than
necessary for ponds with stocking densities less than 8 or 10 shrimp per
square meter. If water quality in ponds is good, there is no reason to
exchange water. Additionally, water quality in supply canals or
reservoirs sometimes may be of lower quality than waters in ponds.
The water exchange rate in a pond may be estimated as follows, if
the pond is full of water:
[(PR x T) + P1 - (S + E)
ER = x 100
where ER
PR
T
P
S
E
V
exchange rate, % pond volume/day
pumping rate, m
3
/hr
time of pumping per day, hr
precipitation, m
3
/day
seepage, m
3
/day
evaporation, m
3
/day
pond volume, M
3
43
Month
January
February
March
April
May
June
Suppose that a pond has an area of 1 ha and an average depth of 80 cm.
Water will be pumped into the pond at a rate of 5 m
3
/min for 5 hours per
day. The evaporation rate is 0.5 cm/day, and seepage is 0.2 cm/day.
Calculations will be made assuming that there is no rain.
(i) Pond volume 
= 10,000 m
2 
x 0.8 m 8000 
m
(ii) Pump rate = 5 m3/min x 60 min/hr = 300 
m3/hr
23
(iii) Evaporation = 10,000 m x 0.005 m/day = 50 m3/day
(iv) Seepage = 10,000 m
2 
x 0.002 m/day = 20 m3/day
(300 m
3
/hr x 5 hr/day) - (50 m3/day + 20 m3/day)
ER = 
x 100 = 17.9%.
8,000 m
The influence of evaporation on salinity may be quite drastic. To
illustrate, suppose that the water level in a pond is maintained at a
constant depth by pumping in just enough water to replace evaporation
and seepage losses. Seepage will not affect salinity, because 
salts
leave the pond with the Seeping water. However, salts are not lost when
water evaporates. If the evaporation rate in an 80-cm-deep pond 
is 0.5
cm/day, water lost to evaporation is equal to 0.62% of pond volume per
day [(0.5 + 80) x 100 = 0.62%]. Thus, salinity will increase by 0.62%
per day. Suppose that salinity was 27 ppt at the beginning of a 
month.
By the end of the month, salinity would have increased to 32.02 
ppt [27
ppt + 30(27 x 0.0062) = 32.02 ppt]. Even with water exchange, salinity
will increase, but at a lesser rate; it will be in direct proportion 
to
the water exchange rate.
In estimations of water exchange, it is necessary to know pond vol-
ume. One also must know pond volumes to calculate chemical treatments
of ponds, e.g., application of chemicals to combat diseases. Pond
volume is estimated as area multiplied by average depth. For
rectangular or square ponds, one only need measure the length and width
and multiply the two dimensions to obtain area. For irregular shaped
ponds, surveying techniques must be used to obtain areas. Average depth
may be obtained by making depth soundings at a number of places in a
pond with the aid of a calibrated rod. Depths are then averaged. The
best way of obtaining soundings is to travel across the pond in an
S-shaped pattern and make soundings at frequent intervals.
Suppose that a pond is 100 m long and 40 m wide; results of sound-
ings, in meters of depth, are: 0.25, 0.75, 0.85, 0.95, 0.78, 0.91,
0.82, 0.77, 0.91, 0.91, 1.05, 1.10, 1.01, 0.96, 0.89, 0.75, 0.68, 0.70,
0.72, 0.65, 0.66, 0.58, 0.48, and 0.16. Calculations of pond volume
fo llow:
44
(i) Area = 100 m x 40 m 
=
4,000m
0.25 + 0.75 +......+ 0.16 m 18.29
(ii) Average depth = 24fi = 0.76m
(iii) Pond volume 
= 4,000 m2 0.76 
m = 3040 
3
POND BOTTOMS
Pond bottom soils react with water and influence water quality.
Furthermore, shrimp spend much of their time on pond bottoms, and pond
bottom conditions are more critical for shrimp than for most other
aquacultural species.
Waters in coastal areas often are heavily laden with silt, and sed-
iment ponds should be employed to remove silt. However, if sediment
ponds are not used, silt should be removed when ponds are drained. Pond
bottoms also should be dried. This insures that all contaminating
organisms are killed and that soil is aerated. For maximum benefit,
pond bottoms should be worked with a plow. The top 10 to 15 cm of soil
should be tilled. If pond soils are acidic, application of about 500
kg/ha of calcium oxide over dry pond bottoms at the time tilling is done
would be beneficial.
Bottoms of ponds should be smoothed and sloped to facilitate drain-
ing. In some places, ponds have been constructed with a central pond
area that is more shallow than the pond bottom just inside the levees.
This practice should be avoided, because water will not flow well across
the shallow area. It will flow across the deeper areas. If water forms
puddles in bottoms after draining, puddles should be treated with tea
seed cake, rotenone, or chlorine to kill wild organisms.
Pond managers often wonder about black mud in pond bottoms. The
black color of mud usually is caused by an accumulation of ferrous iron
which results when the mud is depleted of oxygen. When the mud is
oxidized (contains oxygen), the ferrous iron changes to ferric iron and
the mud will no longer be black. A brown crust at the mud-water
interface suggests that there is oxygen at the mud surface. However, if
this crust is disturbed, the mud beneath usually will be black.
Some readers will have heard of the redox potential of pond water
and mud. In simplest terms, the redox or oxidation-reduction potential
is a measure of the proportion of oxidized to reduced substances in a
solution or mixture. In practice, the redox potential is measured with
respect to a hydrogen electrode and is called the E.. If electrons flow
from the solution to the electrode, reducing conditions are said to
exist in the solution and the electron flow in volts is assigned a
negative sign. Oxidizing conditions are said to exist in the solution
when electrons flow from the electrode to the solution and the voltage
is given a positive sign. The opposite convention regarding signs is
used by some workers.
45
The redox potential of oxygenated water is the result of the oxygen
potential,
02 + 2H20 + 4e=40H .
At oxygen saturation and pH 7, the redox potential of water (E
h ) 
at 250C
should be 0.80 volt. However, because of phenomena associated with the
measurement of the oxygen potential, the Eh of oxygenated natural waters
ranges from 0.45 to 0.52 volt. A change in Eh of 0.059 volt occurs per
unit change in pH. This change is negative for pH values below 7 and
positive for values above 7. Values for Eh are often corrected to pH 7.
Temperature has little influence on Eh, as long as DO is present, the
redox potential remains at about 
0.5 volt, but once DO is depleted 
Eh
drops rapidly.
In a shrimp pond, the water above the mud usually contains dis-
solved oxygen and the water has an E
h 
near 0.5 volt. Water cannot
exchange freely between the pond and the pore spaces in the mud, so
microbial activity depletes DO concentrations in mud. Thus, the Eh
drops rapidly with depth beneath the mud surface. Reducing conditions
(anaerobic conditions) usually occur at depths greater than 1 or 2 cm
into mud. The Eh tends to decline with depth in the mud. Values of Eh
associated with the appearance of various reduced substances follow:
NO
2 
, 0.4 volt; Mn
2
+ and Fe
2
+, 0.2 volt; H
2
S, 0.1 volt. The appearance
of ferrous iron (Fe
2
+) and manganous manganese (Mn
2
+) coincide closely
with the point where DO just becomes depleted. Values for Eh as low as
-0.1 volt have been recorded in mud.
In shrimp ponds, the development of reducing conditions at the sur-
face of the pond mud is a highly undesirable event. Water circulation
in ponds caused by water exchange, wind, or aeration tends to move water
across the mud surface and prevent the development of reduced
conditions. As mentioned before, reduced mud has a deep black color.
HEAVY METALS
Fairly high concentrations of heavy metals have been reported in
estuarine waters in many nations. Thus, there is interest in the
toxicity of heavy metals to shrimp and other aquacultural species. The
toxicity of heavy metals to a variety of species of freshwater and
marine animals, mostly fish, were obtained from various publications and
summarized below:
Safe level recommended
Range of 96-hr by U.S. Environmental
Metal LC-50 (pg/l) Protection Agency (g/1)
Cadmium 80-420 10
Chromium 2,000-20,000 100
Copper 300-1,000 25
Lead 1,000-40,000 100
Mercury 10-40 0.10
Zinc 1,000-10,000 100
1
The 96-hr LC-50 is the concentration of a substance which will kill
50% of the organisms in a laboratory toxicity test within a 96-hr ex-
posure time.
46
The safe levels recommended by the U.S. Environmental Protection Agency
are conservative estimates, which are 10 to 100 times lower than the
lowest concentrations which have been reported to harm organisms in
laboratory toxicity tests.
The procedure for heavy metal analysis (atomic absorption spectro-
photometry) measures the total concentration of a particular metal.
Waters for shrimp ponds contain suspended clay particles and organic
matter. Heavy metals are adsorbed onto clay particles and chelated by
organic matter. Some of the heavy metals also form complexes with
oxides, hydroxides, and carbonates in water. The toxicity of heavy
metals is related primarily to the dissolved, ionic form of the metal,
e.g., Cu
2
+ or Zn
2
+ , rather than to adsorbed, chelated, or complexed
forms. A small percentage of the heavy metals in most waters are in
ionic form, so toxicity of heavy metals should not be a problem in
brackishwater ponds.
PESTICIDES
A number of pesticides are used on agricultural crops in tropical
nations, and pesticides enter rivers in runoff. Chlorinated hydrocarbon
insecticides have the greatest potential for harming shrimp and fish.
Toxicities of some selected insecticides to a wide array of freshwater
and marine animals are summarized below:
Safe level recommended
Range of 96-hr by U.S. Environmental
Metal LC-50 (pg/l) Protection 
Agency (Ug/l)
Aldrin/Dieldrin 0.20-16 0.003
BHC 0.17-240 4
Chlordane 5-3,000 0.01
DDT, 0.24-2 0.001
Endrin 0.13-12 0.004
Heptachlor 0.10-230 0.001
Toxaphene 1-6 0.005
Again, the recommended safe level is well below the lowest concentration
of an insecticide reported to harm aquatic organisms in laboratory
toxicity tests.
Apparently, the use of pesticides with long residual lives is de-
clining in most nations, and many pesticides that are used today degrade
to non-toxic forms within a few days. However, pesticides are
potentially harmful until they are degraded. The use of pesticides in
shrimp farming areas should be discouraged. Pesticides sprayed onto
fields may drift over considerable areas and reach ponds or canals. Key
factors for protecting ponds from pesticides are as follow: put shrimp
farm a considerable distance from pesticide-treated fields; plant trees
or other high-growing vegetative cover between ponds and
pesticide-treated fields to intercept airborne drift of pesticides;
construct topographic barriers (ditches or terraces) to prevent runoff
from fields from entering ponds; finally, use proper methods of
pesticide application to fields. The disposal of pesticides and
47
pesticide containers should be done in such a way that pesticides do not
contaminate waterways.
MEASUREMENT OF WATER QUALITY
For research purposes, standard laboratory procedures are available
for making water analyses. However, such procedures are not practical
for use by shrimp farmers. Hach 
water analysis kits provide relatively
accurate measurements of water quality in waters of salinities of 0 to
30 ppt (20). The Hach Fish Farmer's Water Quality Test Kits Models FF-2
and FF-1A (Hach Company, Loveland, Colorado) can provide reliable
estimates of dissolved oxygen, carbon dioxide, pH, total alkalinity,
total hardness, chloride, total ammonia nitrogen, and nitrite nitrogen
for use in shrimp pond management. However, when purchasing these kits,
one should indicate that the kits will be used in salt water so that the
titrating agents provided in the kit will be of sufficient strength.
Suitable water analysis kits are manufactured by other companies; Hach
kits were mentioned only because a study of their performance has been
published.
Salinity may be measured with a hand-held refractometer (usually
called a salinometer). The salinity of water affects the refractive
index, and the salinometer translates this effect into a reading of
salinity in parts per thousand. Salinometers are expensive and
delicate; they should be handled carefully. However, as mentioned
earlier, salinometers are not accurate when salinity is low. For
salinities below 4 or 5 ppt, chloride should be measured by titration
using a water analysis kit and the following equation from Boyd and
Lichtkippler (21) used to estimate salinity:
30 + (1.805 x Cl)
Salinity (ppt) = 
1,000
where C1 = chloride concentration, mg/l.
Although dissolved oxygen can be measured with water analysis kits,
it is tedious and costly to make dissolved oxygen measurements with
water analysis kits when many ponds must be checked each day.
Therefore, the use of dissolved oxygen meters is suggested. When using
oxygen meters, always check the membrane for tears and to ascertain that
an air bubble is not beneath the membrane. The accuracy of oxygen
meters should be verified often. To do this, saturate the water in a
container by pouring the water back and forth between two containers 40
or 50 times. Check the temperature and salinity of the water and
determine the dissolved oxygen concentration at saturation from table 7.
Read the dissolved oxygen in the container with the oxygen meter. The
meter should give a reading of + 0.5 mg/l of the expected saturation
concentration.
The amount of suspended matter in water that will settle out
quickly is termed settleable solids. The settleable solids are
determined with an Imhoff cone, figure 32. The cone holds 1 1 of water
and the lower end of the cone is graduated so that the volume of
48
sediment may be read directly. To
sample is thoroughly mixed and 1 1
the Imhoff cone. The cone is set
and then the volume of sediment
directly.
measure settleable solids, the water
of sample is poured immediately into
aside, left to stand for 60 minutes,
in milliliters per liter is read
ACKNOWLEDGMENTS
The preparation of this manual was supported by the Asian Develop-
ment Bank, Manila, Philippines, and by the U.S. Agency for International
Development through AID Cooperative Agreement (DAN-4180- A-00-8008-00),
AID Program Support Grant(DAN-5058-G-55-6073-00), and AID (CRSP)
(DAN-4023-G-SS-706600). C. R. "Corny" Mock kindly read the manuscript
and made several helpful suggestions.
49

pH
10 ...
81-
7
6 a.m. Noon 6 p.m.
Time of day
Midnight
FIG. 1. Daily fluctuation in pH in a fish culture pond.
51
6 a.m.
I
I I r
91-
FIG. 2. Sediment pond installed-'in 
an existing shrimp culture pond.
52
Supply canal and
settling basin
Canal
FIG. 3. Use of water supply canal as a sediment pond.
53
Drain
ditch
I I~ P.nd C~~X
Turbidity, NTU
1,000
800
600
400 -
200 -
0
o 0 ppt
* 15ppt
0
0 30 60
90
Time (minutes)
I
FIG. 4. Sedimentation rates for two soils in waters of 0 ppt and 15 ppt
salinity.
54
A
FIG. 5. A Secchi disk.
FIG. 6. The food web in a shrimp pond.
supplement natural food.
Feed may be applied to
55
2 eye bolt
20 cm
weight
graduated rope
I
I ~_
r r
Depth, meters
0
0.5-
1.0
* Light plankton
1.5 
bloom
* Moderate plankton
bloom
2.0 
o Heavy 
plankton
bloom
2.5
0 4 8 12 16 20 24
Dissolved oxygen, mg/liter
FIG. 7. Influence of depth on dissolved oxygen concentrations in ponds
with different amounts of plankton.
56
FIG. 8. Effect of time of day and plankton density on concentrations of
dissolved oxygen in surface water.
57
FIG. 9. Graphical representation of a 
procedure for predicting the
nighttime decline in dissolved oxygen 
in a pond.
58
Dissolved oxygen,
mg/liter
clear omplete ompletel
overcas overcas
10
5
6 a.m. 6 a.m. 6 a.m.
Time of day
FIG. 10. Effect of cloudy weather on dissolved oxygen concentrations in
a pond.
59
a.m.
Total phytoplankton,
thousand/mi
38 -
36
12
10
8
6
4
2
0
18
* Surface
0 1 meter
0 2 meters
22 26 30 4 8 12
April.
16
May
FIG. 11. Changes in phytoplankton abundance before, during, and after a
massive phytoplankton die-off in a pond.
60
Dissolved
mg/liter
12 -
10 -
8 Os
6 -
4 "
2-
0-
18 22 26 30
April
4 8 12
May
FIG. 12. Dissolved oxygen concentrations before, during, and after a
massive phytoplankton die-off in a pond.
61
oxygen,
16
0
Secchi 
disk
Secchi disk
visibility,cm
-110
Dissolved
oxygen, mg/I
1oIQ0
Secchi disk visibilit
Dissolved oxygen
80
60
40
20
10 50
0 20 30 40
iFeed, kg/ ha per day
FIG. 13. Effect of feeding rate on dissolved oxygen concentrations at
dawn and Secchi disk visibilities in ponds.
62
8
6
4
21-
0
0
50
10
Dissolved
oxygen, mg/I
101-
* 30 kg/ha/day
o 50 kg/ha/day
o 70 kg/ha/day
81-
6-
4-
2-
0
June July Aug. Sept.
FIG. 14. Effect of feeding at three rates on dissolved oxygen
concentrations at dawn in ponds.
63
Oct.
- - mm 0 OMAN- m
~.
FIG. 15. A vertical pump aerator.
40K
?. -.. ' ?
FIG. 16. A um pryr eatr
FIG. 17. A paddle wheel aerator.
~----UI~P"~--- ~-~:~~
*~c ~
i -
-p;
4n
14A
2'a
SBlower
FIG. 18. A diffused-air aeration system.
FIG. 19. A propeller-aspirator 
pump aerator.
)
Porous pipe
I _ ~
FIG. 20. Design for a highly 
efficient paddle wheel.
FIG. 21. A 7.5-kw, floating, 
electric, paddle wheel aerator.
_ I _ ~1~1 
__
FIG. 22. A 2.25-kw, floating, electric, paddle wheel
aerator without floats attached.
.~.,. ~v
FIG. 23. A floating, paddle wheel aerator with gear reducer and motor
on the pond bank.
I UVA" :
k'd I , ,
FIG. 24. A "Taiwanese" paddle wheel aerator.
~----; (
'I~PL-
c
*
BBDrZ "'F"~=
I;:;P~
,. ,,
Oil(
FIG. 25. A paddle wheel aerator constructed of polyurethane.
$,
FIG. 26. A paddle wheel aerator which is powered by the 
power-take-off
(PTO) of a farm tractor.
Dissolved
oxygen,mg/liter
14
12 
f Aeration4)
10
8
6
4
2--
0
8 a.m. Noon 4 p.m. 8 p.m. Midnight 4 a.m. 8 a.m.
Time of day
FIG. 27. Effect of nighttime aeration on dissolved oxygen
concentrations in a pond.
72
~,
4,
~ yr ~
'~ ~.
0~
~-~-~-
-'7
T1
pI
w:
0
H-
a:J
moo$
a:-
0
H-
a:i
VP....''
4 ~*
4 ~v~' t+ t
U.
'1'
~
$ ~~&**i 4~4
E
~4
FIG. 28. Effect of paddle wheel aerator placement on water circulation
patterns in ponds.
73
x:
0
H-
Lid
w\-
a:
0
E
.PUMP HOLDER
PIVOT
POINT
SUPPORT
BALLAST
FIG. 29. An air-lift pump.
74
FLOAT
I I I I
/ ,
AXONOMETRIC VIEW
FIG. 30. A water circulator.
75
I / ~ r _IT - / \11111111
FIG. 31. A nomograph for estimating pipe discharge.
76
U
FIG. 32. An Imhoff cone.

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83
abama's Agric " ' .- ,""m-
? Main Agricultural Experiment
Station, Auburn
>t E.V. Smith Research Center,
Shorter
1. Tennessee Valley Substation, Belle Mina
2. Sand Mountain Substation, Crossville
3. North Alabama Horticulture Substation, Cullman
4. Upper Coastal Plain Substation, Winfield
5. Forestry Unit, Fayette County
6. Chilton Area Horticulture Substation, Clanton
7. Forestry Unit, Coosa County
8. Piedmont Substation, Camp Hill
9. Plant Breeding Unit, Tallassee
10. Forestry Unit, Autauga County
11. Prattville Experiment Field, Prattville
12. Black Belt Substation, Marion Junction
13. The Turnipseed-lkenberry 
Place,
Union Springs
14. Lower Coastal Plain Substation, Camden
15. Forestry Unit, Barbour County
16. Monroeville Experiment Field, Monroeville
17. Wiregrass Substaton, Headland
18. Brewton Experiment Field, Brewton
Solon Dixon Forestry Education 
Center,
19. Covington and Escambia counties
20. Ornamental Horticulture Substation, Spring Hill
21. Gulf Coast Substation, Fairhope