Indirect Impacts of Recreational Scuba Diving: Patterns of Growth and Predation in Branching Stony Corals
The highest recorded rates of recreational SCUBA diving occur on coral reefs at Eilat in the northern Red Sea, where many scleractinian corals are damaged by physical contact with divers. We compared coral skeletal growth and damage at sites that were protected vs unprotected from diving tourism, and identified tissue mortality through monthly monitoring of the branching stony coral Acropora hemprichii (Ehrenberg, 1834). We also monitored several genera of branching stony corals at sites that varied in levels of recreational diving. Major causes of tissue mortality in A. hemprichii were unknown (possibly bleaching and disease) and predation by the corallivorous snail Drupella cornus (Röding, 1798), both of which were much more frequent at heavily-dived than at undived sites. High frequencies of coral damage led to significantly slower coral growth at heavily-dived than undived sites. Annual rates of damage to branching corals were approximately twice as high at a site open to divers than at a closed site, and were intermediate at a site with restricted diving. The proportions of colonies infested by D. cornus increased significantly during the study year at sites open to divers but not at closed and restricted sites. We conclude that frequent mechanical injury by divers to coral colonies leads to enhanced susceptibility to predation and possibly to coral disease. Current levels of recreational diving at Eilat severely compromise the growth and survival of major reef-building corals, not only via direct mechanical damage, but also through a positive feedback loop of indirect effects on coral predators.