Marine cage culture and the environment: effects on water quality and primary production

Increasing human population and reliance on aquaculture for seafood will lead to expansion of the industry in the open ocean. To guide environmentally sustainable expansion, coastal stakeholders require tools to evaluate the risks that marine aquaculture poses and to craft science-based policies and practices which safeguard marine ecosystems. We summarized current knowledge regarding dissolved nutrient loading from marine fish farms around the world, direct impacts on water quality and secondary impacts on primary production, including formation of harmful algal blooms. We found that modern operating conditions have minimized impacts of individual fish farms on marine water quality. Effects on dissolved oxygen and turbidity are largely eliminated through better management. Nutrient enrichment of the near-field water column is not detectable beyond 100 m of a farm when formulated feeds are used, and feed waste is minimized. We highlight the role of siting fish farms in deep waters with sufficient current to disperse nutrients and prevent water quality impacts. We extensively discuss the potential for advances in integrated multi-trophic aquaculture (IMTA) to assimilate waste nutrients. Although modern farm management practices have decreased environmental effects of marine fish farms, we conclude that questions remain about the additive impacts of discharge from multiple farms potentially leading to increased primary production and eutrophication. Research results on secondary effects upon primary production are highly variable. In some locations, nutrient loading has little or no trophic impact, while at others there is evidence that nutrients are assimilated by primary producers. Research on far-field and regional processes, especially in intensively farmed areas and over longer time scales, will refine understanding of the full ecological role of fish farms in marine environments.