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Aquaculture’s Impact Obstacle: Phosphorus discharge from aquaculture The race to put fish on the table is in lockstep with the world’s rising food demand
But fish farms have been criticized for producing concentrated levels of nitrogen and phosphorus, creating a growing source of nutrient pollution in waterways and oceans. Critics claim that rising levels of phosphorus and other pollutants from excess excrement, uneaten food, and other organic waste could plague the world’s waterways. Phosphorus is necessary for a number of metabolic functions in fish, such as bone development and scales, said Gary Fornshell, an aquaculture extension educator with the University of Idaho, but too much phosphorus can clog waterways with algae and excessive plant growth. Such eutrophication uses up oxygen in the water, suffocating aquatic life and creating impaired waterways. Fornshell uses Idaho’s Snake River Valley to explain how producers are meeting phosphorus discharge guidelines and maintaining the state’s healthy waterways. Idaho accounts
Photos: @John Mollison
The Food and Agriculture Organisation of the United Nations (FAO) reports that aquaculture grew about 4.5 percent in 2017, with a total harvest of 83.6 million metric tons.
for about 68 percent of all trout production in the United States, making it the nation’s largest commercial producer of rainbow trout. In the early 1990s, Idaho’s Department of Environmental Quality (DEQ) and federal Environmental Protection Agency (EPA) cited issues with excessive plant growth, finding the mid-Snake River water-quality limited. Fornshell explains that background levels of phosphorus already exist in the area, as well as nonpoint runoff from surrounding ag land and cities. At the time, “the DEQ stated that
point sources (such as aquaculture) were contributing 15 percent of the phosphorus waste,” he said, adding that point sources often are the target of emission guidelines because they offer identifiable levels of target pollutants and are regulated under the Clean Water Act. The “water-quality limited” designation triggered requirements under the Clean Water Act and a Total Maximum Daily Load (TMDL) was calculated, showing allowable levels of phosphorus in water being cycled through from flow-through aquaculture systems (raceways) to the Snake River or its tributaries.
