Negative effects of fish cages on coral reefs through nutrient enrichment and eutrophication in Nha Trang Bay, Viet Nam

Abstract

The potential impact of cage farming activities on nutrient enrichment of the coastal waters of Nha Trang bay, Viet Nam as investigated within a project financed by the Vietnam Academy of Science and Technology (VAST). Results show a very high level of nitrogen (N) in the studied area: the average content of dissolved inorganic nitrogen (DIN) was 177.4 g/l (range from 169.9 to 188.9g/l), in which NO3-N accounts for the major proportion (average 152.9 g/l). The Redfield ratio (N/P) also showed values larger than 16 in almost all studied sites, indicating that phosphorus may be the limiting nutritional factor in these coastal waters. The TRIX indexes recorded for both the surface and bottom waters are within a range of about 5–6 (at high nutrient level); especially at sites under the cages and near the cage farming area. Cross-transplantation experiments with corals showed significant differences at different sites, particularly in relation to distance to fish cages. In comparison, the zooxanthellae densities of Acopora formosa in coral polyps indicated a rapidly reduced value after 31 days of exposure in the polluted areas. Strong side effects such as increased sedimentation as a result of fish farm activities can be detected on nearby coral reef. It is concluded that fish cage farming is negatively impacting coral reefs by being a point source of surplus nutrients.