Research Update
k 1989
CATFISH
FIRST IN RESEARCH UPDATE
SERIES ON CATFISH
This is the first catfish re-
search report published in a new
publication series, entitled "Re-
search Update," inaugurated in
1989 by the Alabama Agricultural
Experiment Station. The new
series is meant to promote timely
reporting of research results deal-
ing with a specific crop or com-
modity, with distribution to all pro-
ducers of that particular commod-
ity. In this case, the target audi-:
ence is all Alabama catfish pro.
ducers.
Other information about cat-
fish production and latest recom-
mendations are available from
each county Extension Service
office in Alabama . ..
EDITOR'S NOTE
Mention of company or trade names
does not indicate endorsement by the Ala-
bama Agricultural Experiment Station or
Auburn University of one brand over an-
other. Any mention of non-label uses or
applications in excess of labeled rates of pes-
ticides or other chemicals does not constitute
a recommendation. Such use in research is
simply part of the scientific investigation
necessary to fully evaluate materials and
treatments.
Information contained herein is avail-
able to all persons without regard to race,
color, sex, or national origin.
Channel X Blue Hybrid Catfish Show
Production Advantages
Twenty-eight interspecific hy-
brids from seven species of catfish
have been produced and evaluated
for growth rate. Channel catfish
female X blue catfish male hybrids
showed superior growth in the pond
environment in AAES research.
This hybrid had an average increase
in body weight of 20 percent above
that of channel catfish. Furthermore,
the feed conversion of channel X
blue hybrids was 11-14 percent more
efficient than that of channel catfish.
These growth and efficiency advan-
tages could make a big difference in
profitability of production, but the
hybrid also exhibited other desir-
able traits in AAES tests.
Culture of the channel X blue
hybrid could reduce losses of catfish
due to oxygen depletion because this
catfish proved resistant to critically
low oxygen levels. When 90 percent
of a channel catfish population suc-
cumbed to low dissolved 
oxygen,
only 50 percent of the hybrids died.
When 50 percent of a channel catfish
population expired from deficiency
of dissolved oxygen, only 10 percent
of the hybrids died.
Channel X blue hybrid finger-
lings were also more resistant to co-
lumnaris than channel catfish. How-
ever, there was no increased resis-
tance to channel catfish virus injected
into the test fish.
Fishing success in fee-fishing
catfish ponds could be improved by
stocking the channel catfish X blue
catfish hybrid. These hybrids were
more catchableby hook and line than
their parent species, which did not
differ. The channel X blue was also
much easier to catch by seining than
channel catfish.
The average dressout percent-
age for the channel X blue hybrid
was higher, 64.5, than for the
channel catfish, 61.2. The higher
dressout percentage of the channel
X blue hybrid may be related to its
deep body conformation and small
head.
The hybridization rate between
the parent species 
has been variable,
0-100 percent in pens and 30 percent
A LABAMA AGRICULTURAL EXPERIMENT STATION AUBURN UNIVERSITY
LOWELL T. FROBISH, DIRECTOR, AUBURN UNIVERSITY, ALABAMA
I
in ponds. This remains the major ob-
stacle to the commercial production
of channel Xblue hybrid fingerlings.
The use of crossbred channel catfish
females increased the hybridization
rate with blue catfish. Hatchability
of hybrid eggs and viability of hy-
brid fry are as high as those of paren-
tal species.
Recent research has focused on
overcoming these obstacles to hy-
bridization. Various ovulating
agents and inhibitors, such as hu-
man chorionic gonadotropin, carp
pituitary extract, luteinizing
hormone releasing hormone
(LHRH), and pimozide, have been
evaluated. LHRH in combination
with pimozide resulted in the high-
est hybridization rates.
Several strains of blue catfish
and channel catfish have been ex-
amined for their propensity for hy-
bridization. Significant strain effects
for hybridization rate and perform-
ance of the resulting hybrid were ob-
served. Hopefully, this beneficial
hybrid will soon be ready for utiliza-
tion by commercial catfish farms.
Rex A. Dunham and
R. O. Smitherman
New Vitamin C Source for Fish Feeds
While most animals don't re-
quire vitamin C in their diet, fish are
extremely sensitive to a deficiency.
Without vitamin C, fish show re-
duced growth rate, physical deformi-
ties (crooked backs, etc.), slow
wound healing, and reduced resis-
tance to infections, environmental
contaminants (nitrites, chlorinated
hydrocarbons, etc.), and other
stresses.
L-ascorbic acid is the vitamin C
source used in commercial fish feeds,
but there is a serious problem with
this compound. It is sensitive to
oxidation, and heat and moisture in
feed processing destroy 40 to 60
percent of the amount put into the
feed. Also, its half-life (time required
for 50 percent to be lost) in fish feeds
during storage is less than 90 days.
Because of the large losses of ascor-
bic acid during processing and stor-
age, new sources of vitamin C are
needed for use in aquaculture feeds.
Phosphate and sulfate deriva-
tives of ascorbic acid, which are rela-
tively stable against oxidative dete-
rioration, were examined in AAES
research as potential sources of vita-
min C for fish feeds. Ascorbic acid
phosphate (AAP) and ascorbic acid
sulfate (AAS) were compared with
L-ascorbic acid (AA) for vitamin C
activity for channel catfish. Each
source of vitamin C was fed at four
levels in purified diets under con-
trolled environmental conditions.
As shown by the graph, fish fed
no ascorbic acid (control) grew
poorly. Those fed ascorbic acid sul-
fate also grew poorly and although
growth improved as the dietary level
increased, it never reached the
growth rate of the fish fed L-ascorbic
acid. Growth rate of fish fed ascor-
bic acid phosphate was equal to that
of the fish
fed L-as-
cor bic 
Wt. gain, 
grams
acid. This 
200
was true
even at the
lowest
level fed,
which is
near the
lower 100
limitof the
channel
catfish's
vitamin C
require-
ment.
There 0
were no 0 11
deformi-
Dieta
ties in the
fish fed any of the dietary levels of L-
ascorbic acid or ascorbic acid phos-
phate; however, over 50 percent of
fish fed the control diet or diets
containing 44 milligrams per kilo-
gram' (mg/kg) or less of ascorbic
acid sulfate had crooked backs. Only
the highest dose of ascorbic acid
sulfate (132 mg/kg) prevented de-
formities. The results show that
ascorbic acid phosphate is equal to
L-ascorbic acid in meeting the vita-
min C requirements of channel cat-
fish, but ascorbic acid sulfate has
less vitamin C activity than the other
compounds.
During extrusion processing of
catfish feed, only 10 to 20 percent of
ascorbic acid phosphate was lost,
while 50 to 60 percent of L-ascorbic
acid was lost. Because of increased
stability during processing and high
vitamin C activity, ascorbic acid
phosphate will likely be a major
source of vitamin C in aquaculture
feeds in the future. This will allow
moreprecise supplementation of fish
feeds with vitamin C, providing a
cost savings and increased assur-
ance that the feed is sufficient in
vitamin C.
' 1,000 milligrams = 1 gram = 1/28 ounce;
1 kilogram = 2.2 pounds.
R. T. Lovell
o Control
AA
O AAP
[ AAS
22
ry ascorbic
44 132
acid, mg/kg
r
AAES Catfish Study
Explains Winter-Kill
"Winter-kill" is a major cause of
losses of commercially reared cat-
fish. The name implies a causative
relationship with the winter months;
however, there may be a number of
factors involved, including the lower
temperatures and subsequent effect
on the immune system of fish with
inability to fight off invading patho-
gens. One manifestation of winter-
kill is dark coloration and fungus
infections of the fish's skin.
An AAES study that examined
skin of infected fish gave an insight
into how the mucus of the skin was
produced and the nature of fungus
infections. Goblet cells, which pro-
duce the mucus, are located at the
surface and mid layers of the epider-
mis with openings distributed on
the surface of the epidermis. The
goblet cells produce mucin, which is
released from the cells in tiny drop-
lets. The mucin combines with water
to form mucus. This mucus is a first
line of defense against invading
pathogens.
Light and electron microscopy
were used to study fungus infec-
tions of catfish skin. Infected skin
lacked the mucus layer that was
normally present on the skin of
healthy fish. Most of the epidermal
cells in fungalions were necrotic. In
some lesions the epidermis was
completely lost and the dermis layer
was exposed. Two species of Sap-
rolegnia (water mold) were involved
in the lesions and both penetrated
the dermis layer. Pigment cells were
decomposed and the pigment was
scattered among the 
cell debris,
which may account for the dark
coloration associated with winter-
kill.
In addition to the fungus infec-
tion, loss of the mucus layer and epi-
dermis of catfish may result in a loss
of body fluids. The inability to regu-
late osmotic pressure within the body
may be the primary cause of the
extensive mortalities associated with
winter-kill.
W. A. Rogers and
Dehai Xu
Omega-3 Fatty Acids
in Catfish Increased
by Diet Manipulation
There is convincing evidence
that omega-3 polyunsaturated fatty
acids (n-3 PUFA) in fish oils reduce
risk of heart disease in humans by
lowering cholesterol levels and pro-
longing clotting time of the blood.
Marine fish have traditionally been
associated with oils high in the n-3
PUFA. Cultured fish fed grain-soy-
bean meal based feeds, however,
have a fatty acid composition that is
much lower in n-3 PUFA's than sea-
caught fish; in fact, the fatty acid
profile of farm-raised catfish is simi-
lar to that of grain-fed farm animals.
Because the fatty acid composi-
tion of fat in animals is influenced by
the diet, a study was conducted to
determine if the amount of n-3
PUFA's in catfish could be changed
by manipulating the diet.
Four test diets were fed to chan-
nel catfish in 0.1-acre earthen ponds
for a 12-week summer period. The
control diet was a practical type cat-
fish feed containing 50 percent soy-
bean meal,40 percent corn, 8 percent
I
fish meal, and
2 percent vi-
2 percent vi- Comparison of Polyunsaturated Fatty Acid (PUFA)
tamins and Contents of Catfish 
Fed Various Supplements of
minerals. Menhaden Fish Oil and of Sea-caught Salmon
Three per-
centages of 
Catfish
menhaden PUFA Salmon
Control Fish oil, Fish 
oil, Fish oil,
fish oil, 2, 4, feed 2% 4% 
6%
and 6, were
added to the 
Pct. Pct. Pct. 
Pct. Pct.
basal diet re- Total ............ 17.0 19.0 
20.5 21.5 21
placing corn. n-3 ............... 3.0 5.7 8.4 
10.1 15
M enhadenoil n-6 ............... 
12.3 10.4 
9.8 9.0 5
is high in n-3 n-3/n-6 ratio .2 .5 .9 1.1 3.0
PUFA, containing approximately 15
percent. At the end of the feeding
period, the fish were analyzed for
fatty acid composition and evalu-
ated for taste and potential oxida-
tive flavor deterioration in frozen
storage.
Comparisons of catfish from the
four test feeds with sea-caught
salmon are given in the table. Cul-
tured catfish fed a commercial type
(control) feed contained much less
n-3 PUFA than sea-caught salmon.
However, adding 2 to 6 percent fish
oil to a catfish feed increased the n-3
PUFA content to 38, 56, and 67 per-
cent of that of wild salmon. This
indicates that n-3 PUFA in cultured
catfish can be significantly increased
by dietary manipulation. Although
n-3 PUFA content was increased, n-
6 PUFA content was not reduced as
much. The n-3/n-6 ratio in the cat-
fish was increased to only 36 percent
of that in the salmon.
Taste tests revealed that the cat-
fish fed menhaden oil had a "fishy"
flavor which intensified withamount
of fish oil in the feed. This was
considered undesirable for the nor-
mally mild-flavored cultured catfish.
Fat content in the flesh of the catfish
ranged from 6.6 percent in the con-
trol fish to 12.1 percent in the fish fed
6 percent fish oil. Iodine number,
which indicates amount of unsatu-
ration of fat, increased from 85 for
the control fish to 103 for those fed 6
percent fish oil. The increase in body
fat and fat unsaturation may increase
the susceptibility of the fish to oxida-
tive rancidity in frozen storage.
Although n-3 content of farm-
raised catfish can be increased by
adding fish oil to feeds, the adverse
effect on sensory quality of the fish
makes this questionable at present.
More information is needed, such as
the amount of n-3 PUFA that must
be consumed by various individuals
to significantly reduce the risk of
heart disease, before considering a
feeding program to increase n-3
PUFA in catfish.
R. T. Lovell
Nighttime Pond Aeration Boosts
Catfish Production
An AAES experiment com-
pared the effects of emergency aera-
tion and nighttime aeration on wa-
ter quality and fish production.
Results showed a considerable ad-
vantage in both production and effi-
ciency with nighttime aeration. The
bottom line was about double the
return over emergency aeration man-
agement.
Six ponds were stocked with
4,000 channel catfish per acre and
fed a commercial feed. A maximum
feeding rate of 47 pounds per day
was attained on July 21 and contin-
ued until fish were harvested Octo-
ber24. Emergencyaeration was used
a few times in three ponds when dis-
solved oxygen concentrations were
low. In the other three ponds, aera-
tion was applied from midnight until
6 a.m. every night between May 30
and October 12. The aeration rate
was equivalent to 1.5 horsepower
per acre of aeration with a highly
efficient, floating, electric paddle-
wheelaerator. Aeratorswereturned
on and off by timers.
Dissolved oxygen concentra-
tions in the early morning were
much higher in the ponds with
nightly aeration than in the ponds
with emergency aeration only. The
average dissolved oxygen concen-
tration at 6 a.m. for all dates was 2.8
p.p.m. in emergency-aerated ponds
and 4.5 p.p.m. in nightly aerated
ponds.
Fish production data are sum-
marized in the table. Harvest weight
of fish in ponds with nighttime aera-
tion averaged about 1,000 pounds
per acre more than in ponds with
emergency aeration. The stocking
and feeding rates were identical for
the two treatments. Greater produc-
tion in ponds with nighttime aera-
tion resulted from a better feed con-
version ratio in these ponds (1.32 vs.
1.75). Better feed conversionin ponds
with nighttime aeration resulted
from higher dissolved oxygen con-
centrations between midnight and
dawn. An economic analysis of the
data gave net returns to land, man-
agement, and equity capital of $696
per acre for ponds with nighttime
aeration and $363 per acre for ponds
with emergency aeration only.
C. E. Boyd
Effects of Nighttime Aeration of Ponds
on Catfish Performance
Variable Nighttime Emergency
aeration aeration
Fish stocked/acre, no ........................................ 4,000 4,000
Fish harvested/acre, no ..................................... 3,939 3,808
Harvest weight/acre, lb ................................... 4,288 3,258
Average weight/fish, Ib. .................................. 1.09 0.86
Feed applied/acre, Ib ............... ................ 5,550 5,550
Feed conversion ratio' .......... 1.32 1.75
Pounds of feed per pound of fish production.
I
I -
Gypsum Improves
Water Quality for
Catfish Farming
Ponds in certain areas of the
catfish-farming region of west-cen-
tral Alabama have high concentra-
tions of total alkalinity and dissolved
phosphate but low concentrations
of calcium and total hardness. For
example, a water sample from a
well used to fill catfish ponds near
Boligee had a total alkalinity of 189
p.p.m., a total hardness of 3 p.p.m., 1
p.p.m. calcium, and 0.7 p.p.m. dis-
solved phosphate. Excessive phyto-
plankton often develops in such
ponds and dangerously high pH
may result. Performance of eggs
and larvae may not be good in cat-
fish hatcheries supplied with water
of low calcium concentration.
Such water quality problems
may be solved by use of agricultural
gypsum, AAES research results
reveal. Gypsum treatment to in-
crease total hardness to roughly the
same concentration as total alkalin-
ity lowered phosphate concentra-
tions and reduced phytoplankton
blooms and high pH in ponds which
naturally had low hardness but high
alkalinity. A gypsum concentration
of 2 p.p.m. was necessary to increase
total hardness by 1 p.p.m. Gypsum
treatment also proved useful to in-
crease calcium concentrations in
water supplies for hatcheries.
Gypsum can be used to remove
suspended clay particles from
"muddy" ponds. Treatment 
rates of
250 to 500 p.p.m. of gypsum often re-
duced turbidity of pond water to
less than one-fourth of original tur-
bidity. Gypsum was less effective
and slightly more expensive than
alum for removing turbidity from
pond water. However, it is safer to
use than alum because it will not
depress pH and it also has a lower
residual action than alum.
Gypsum is fairly soluble in
water, but best results came from a
slurry made of 1 part gypsum to 10
parts water splashed or pumped over
pond surfaces. Gypsum is not a
"cure-all," 
but it 
did improve 
the
specific water quality problems men-
tioned. Although it contains cal-
cium, gypsum is not a substitute for
agricultural limestone because it will
not neutralize acidity.
C. E. Boyd
Disease Outbreaks
Show Seasonal Nature
Infectious disease outbreaks are
related to the presence of pathogenic
organisms and a susceptible host,
and are usually related to some
stressful condition. Many disease-
causing organisms are common in
ponds. Several diseases affect chan-
nel catfish and may cause high
mortalities.
The seasonality of disease de-
velopment was established byevalu-
ation of cases received by the South-
eastern Cooperative Fish Disease
Laboratory at Auburn. Asillustrated
by the graph for the 1988 calendar
year, the greatest number of cases
occurred in June, but there were large
numbers of cases
from April through
October. Major bac-
terial and parasitic
diseases identified
are described here:
Motile aero-
monas septicemia
(MNS), a bacterial
disease caused by
Aeromonas hydro-
phila affectschannel
catfish and other
warmwater species.
The optimum tem-
perature for MNS is
68 to 86?F, and out-
breaks usually occur
when water tem-
peratures are in this
range.
Columnaris disease is caused by
the bacterium Flexibacter columnaris,
and its optimum temperature range
isalso 68 to 86?F. This disease occurs
in spring, summer, and fall.
Edwardsiellosis, caused by Ed-
wardsiella tarda, and enteric septice-
mia of catfish, caused by Edwards-
iella ictaluri, occur when tempera-
tures are 860 and 72-82?F, respec-
tively.
Among the parasitic infections,
Ichthyopthirius multifiliis, commonly
called "ich," is considered a "cool
water" pathogen and occurs in
spring when temperatures range
from 68 to 76?F. Anchor worm
(Lernaea elegans) is common during
warmer temperatures of late spring
through summer.
Viral diseases also occur in a
specific temperature range. Chan-
nel catfish virus disease affects cat-
fish fry and fingerlings and is most
devastating in summer when water
temperatures average 82?F. High
nitrite levels may be detected in
culture ponds in spring and fall. This
causes a condition called methemo-
globinemia, or "brown blood" dis-
ease, and may predispose fish to
bacterial infections.
Y. J. Brady
Reported bacterial and parasite cases for 1988
No. of cases
100
80
60-
40m
20
0r INAM
J MAM JASO0N D
Poor Pond
Environment Promotes
Catfish Diseases
Many of the common infectious
diseases of catfish are closely related
to physical and chemical features of
the pond. Bacteria and parasites
that cause catfish diseases are usu-
ally present in the water, mud, 
or
fish in the pond. Typically, how-
ever, a disease outbreak does 
not
occur unless something happens to
trigger the disease. Moving catfish
to a different pond is an example of
a common "trigger" event that can
result in a disease outbreak.
Nitrite is a natural product that
occurs in fish ponds because of the
bacterial metabolism of ammonia
excreted from fish. Nitrite toxicity
causes a disease which can be easily
diagnosed by the brown color 
of the
blood in affected fish, but the toxic-
ity of nitrite varies depending on
characteristics of the water, such 
as
pH, salinity, and temperature. In
AAES laboratory experiments, cat-
fish exposed to nitrite at a concentra-
tion of 6 milligrams per liter (mg/l)
were killed with a lower dose of
Aeromonas hydrophila than that re-
quired to kill control fish. Flexibacter
columnaris infections occurred spon-
taneously in catfish exposed to 
5
mg/1 nitrite for 7 days, while none 
of
the control fish became 
infected.
These results indicate that sublethal
concentrations of nitrite can cause
increased susceptibility to bacterial
diseases.
Perhaps the most common envi-
ronmental problem leading to 
dis-
eases of catfish is low dissolved
oxygen (DO) concentrations. Un-
less adequate supplemental aeration
is available, low DO will often result
in the sudden death of most catfish
in an intensively managed pond.
This is why most catfish farmers
using high stocking rates have aera-
tors available. A lesser known prob-
lem caused by low DO is the greater
chance that bacterial diseases will
occur if DO becomes low. Labora-
tory experiments at Auburn in which
catfish were exposed to low concen-
trations of dissolved oxygen have
demonstrated that bacterial infec-
tions in catfish are triggered by low
DO. The same result was evident
from monitoring of fish in ponds
after DO depletions. Even if the DO
concentration is not low enough to
cause fish deaths, the low DO can
cause catfish to be weakened and
become infected by the bacteria
during the period of low DO.
Perhaps the overwhelming fac-
tor influencing bacterial diseases of
catfish is water temperature. Cat-
fish have a body temperature that is
the same as the water 
temperature,
so they are greatly influenced by
temperature. In the spring, bacterial
infections are more common than at
other times, at least partly because
winter temperatures slow the im-
mune system of the fish. During the
summer, diseases develop faster
than at other times of the year be-
cause the metabolic rate of both fish
and pathogens is increased. Conse-
quently, rapid diagnosis and timely
corrective measures are important
because of the rapid progression of
diseases.
John M. Grizzle
Vaccination offers
Encouragement
for Control of Enteric
Septicemia
During the last 10 years, enteric
septicemia (ESC) has become the
most serious infectious disease prob-
lem in cultured channel catfish. Also
known as "hole-in-the-head," ESC
is caused by the bacterium Edwards-
iella ictaluri. In recent years, ESC has
been diagnosed in over 30 percent of
all catfish disease cases. It is esti-
mated that the disease costs catfish
farmers millions of dollars each year.
November 1989 4M
Research at the AAES has em-
phasized control of ESC by vaccina-
tion. Results show that channel cat-
fish exposed to the killed bacterium
for 1 to 2 minutes in a bath will
develop immunity. Channel catfish
were vaccinated (immersed) in late
October and stocked into 22-square-
foot net pens in a pond for overwin-
tering. Thelevel of antibody, a meas-
ure of immunity, was determined at
30-day intervals. It was found to
increase throughout the winter and
decline in the spring. A natural ex-
posure of the vaccinated fish in May
following primary vaccination
served as a booster vaccination and
increased the immune response four-
fold.
Although an ESC vaccine is not
yet available for catfish growers to
use, results are encouraging and in-
dicate that such a vaccine can be
used as a disease management tool.
John A. Plumb
S UPPORT CATFISH RESEARCH
Funds appropriated by the Alabama U. 
S. Department of the Interior,
Legislature provide the major financial 
Fish and Wildlife Service
support for Alabama Agricultural Ex- 
Tennessee Valley Authority
periment Station research. Hatch funds 
States of Alabama, Arkansas, Florida,
from the U.S. Government also repre- 
Georgia, Kentucky, Louisiana, Missouri,
sent an important funding source. Since 
and Tennessee as part of the Southeastern
these funds are limited, however, 
many Cooperative Fish Parasite 
and Disease Proj-
areas of research would 
go unsupported 
ect
except for financial support 
from vari- States 
of Alabama, Arkansas, 
Florida,
ous' granting agencies, commodity 
Georgia, Kentucky, North Carolina, 
South
groups, and other friends of the Experi- 
Carolina,andTennesseeaspartof theSouth-
ment Station. Contributions of these
supporters to the AAES programs of eastern Cooperative Sportfish Genetics and
supporters to the AAES programs 
of Breeding Project
research are acknowledged with grati- Breeding Project
tude. Catfish Farmers of America
Among these supporters 
of AAES Alabama 
Soybean Assoc.
research, the following are 
acknowl- Fats and 
Proteins Foundation
edged and thanked for their 
contribu- Abbott 
Laboratories
tions to research on catfish production: H. J. Baker and Bro., Inc.
U. S. Department of Agriculture, Hoffman-LaRoche, Inc.
Cooperative State Research Service Showa Denko America, Inc.
Alabama Agricultural Experiment Station NON-PROFIT ORG.
Auburn University POSTAGE 
& FEES PAID
Auburn University, Alabama 36849-0520 PERMIT No. 9
AUBURN, ALA.
I