Assessment of the effects of cage fish-farming on damselfish-associated food chains using stable-isotope analyses

- Damselfishes living at sites near a cage farm bore lower δ13C and higher δ15N.
- Similar trends occurred in zooplankton and detritus, major foods for damselfishes.
- δ15N enrichment in fish may have arisen from the uptake of excess feed and prey.
- Farm wastes were documented entering the ecosystem through the pelagic food chain.
- No clear evidence of the effects of cage farming on stable isotopes in macroalgae.

To assess the effect of cage fish-farming on the coral reef ecosystem off Xiaoliuchiu Island, southern Taiwan, geographical differences in the food chain of each of two damselfishes, Pomacentrus vaiuli and Chromis margaritifer, were examined using a stable-isotope approach. For each damselfish, individuals were found to consume similar foods at all sites. However, specimens collected at sites near the cage farm (as the experimental sites) exhibited lower δ13C and higher δ15N signatures compared to those from reference sites. Similar trends also occurred in the zooplankton and detritus, two major food sources for both damselfishes. This finding indicates that particulate organic matter released by the farm may have entered the coral reef ecosystem through the pelagic food chain. Artificial reef emplacement is recommended to provide extra habitats under cage farms to support additional pelagic-feeding fish populations, thereby reducing environmental impacts of cage farming on coral reefs.