The primary potential water quality effects from fish farms include dissolved nitrogen and phosphorus, turbidity, lipids and dissolved oxygen fluxes. When farms are sited in well-flushed water, enrichment to the water column is generally not detectable. Nutrient spikes and declines in dissolved oxygen are sometimes seen following feeding, but there are few reports of long-term risk to water quality. Better feed formulation and feeding efficiency have resulted in decreased nutrient loading at fish farms.
Phytoplankton response to nutrient loading at fish farms is generally considered to be low risk. Other factors besides nutrients, such as light availability and water temperature, often control natural variability in phytoplankton productivity. Naturally occurring nutrient fluxes from coastal ocean upwelling, or from land- and ocean-based sources, are often high relative to loads from aquaculture.
Benthic impacts result where organic nutrients in uneaten feed and fish waste accumulate on the seafloor and do not decompose quickly enough to keep up with the supply. Accumulation of waste is unlikely at farms over erosional seafloors, and those sited in well-flushed areas. Under these more dispersive conditions, wastes are spread away from the immediate farm area, aerobically decomposed, and assimilated by benthic organisms. Conversely, depositional sites tend to accumulate waste. In this case, fallowing allows chemical and biological recovery of sediments on a regular schedule. Fallowing generally takes months but can take a couple of years under extreme conditions for bottoms to return to pre-farm status.
Fish farms in the United States must monitor discharges to both the benthic environment and the water column according to the Clean Water Act, and follow effluent limitations set by the Environmental Protection Agency. Environmental impact models now allow regulators to assess the suitability of sites, to understand the risks and benefits of potential fish farms, and to estimate the limits of fish biomass for sites.
Good site selection is key in minimizing the impacts of fish farms nutrients to the water column and benthic environment. Fallowing and integrated multitrophic aquaculture (IMTA) are two other tools that can be used to further reduce environmental impacts. Fallowing is the practice of relocating or not re-stocking fish pens or cages to allow the sediment below to undergo natural recovery. Under ideal conditions, farms should not require fallowing.
IMTA is the practice of culturing species from multiple trophic levels that allow for assimilating of fish wastes, thus reducing environmental discharge. The most commonly selected species for IMTA with marine fish are seaweeds, oysters and mussels.
Price, C. S., and J. A. Morris, Jr. 2013. Marine cage culture and the environment: Twenty-first century science informing a sustainable industry. NOAA Technical Memorandum NOS-NCCOS -164, National Oceanic and Atmospheric Administration, Silver Spring, MD. 158 p.
Black, K. D., P. K. Hansen, and M. Holmer. 2008.Salmon aquaculture dialogue: working group report on benthic impacts and farm siting. World Wildlife Fund, Washington DC. 54 p.
Rensel, J. E., and J. R. M. Forster. 2007. Beneficial environmental effects of marine finfish mariculture. Final report to the National Oceanic and Atmospheric Administration, NOAA Award #NA040AR4170130, Washington, D.C.
Brooks, K. M., A. R. Stierns, C. V. W. Mahnken, and D. B. Blackburn. 2003. Chemical and biological remediation of the benthos near Atlantic salmon farms. Aquaculture 219:355-377.
Pearson T. H., and R. Rosenberg. 1978.Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanography and Marine Biology: Annual Review 16:229-311