Survival, growth and gonad development of two hermatypic corals subjected to in situ fish-farm nutrient enrichment

Nutrient enrichment in oligotrophic tropical waters is considered one of the main causes for coral reef degradation. In the present study, the impacts of net-pen fish-farming in the Gulf of Eilat, Red Sea, on coral biology were evaluated by investigating survival, growth and gonad development in 2 common branching coral species. From 10 Acropora eurystoma genotypes (colonies) 200 branches were collected and suspended for 7 mo from PVC plates adjacent to a commercial fish farm. They grew 3 times faster than a similar set of branches taken from the same 10 colonies and suspended on PVC plates at a non-enriched reference site. The increase in branch weight and volume was 2.7 and 4.3 times greater, respectively, at the fish farm than at the reference site. Survival rates were 100% at both sites. Nubbins of Stylophora pistillata deployed on plates at the 2 sites initially (first 4 mo) grew more rapidly at the reference site, but after 13 mo, the vertical extension levels were significantly larger at the fish-farm site (19.2 +/- 6.1 vs 16.3 +/- 4.6 mm, respectively). The average number of oocytes polyp(-1) and the number of polyps with developing testes in mature S. pistillata colonies were significantly higher at the fish farm than at the reference site. It is suggested that nutrients released from intensive mariculture may not necessarily lead to the demise of coral reefs, as is commonly presumed.