A few years ago, while buying some fish for dinner, I shied away from wild salmon in favor of farm-raised. It sounded like a good way to lessen my footprint on nature, and wild salmon are rather scarce in New England rivers.
Now I know better. In our local estuary, Cobscook Bay, one of that nation’s largest aquaculture companies, Cooke Aquaculture, has proposed expanding its fish-farming operations. This, of course, means feeding fish, mainly salmon, in pens suspended in the waters of the estuary. The feed is largely delivered from outside sources; it is exogenous to the Bay.
It is instructive to calculate the impact of fish feeding on the excretion of nitrogen in the Bay. The State of Maine allows an array of 15 to 20 pens containing about 3 million kilograms of fish, each excreting about 0.035 g of nitrogen per gram of fish each year, for a total of 105 million grams of nitrogen per year. The nitrogen is largely excreted as ammonia, which is readily soluble in water.
By comparison, humans excrete about 11 grams of nitrogen per capita per day, so the annual excretion from a fish farm is equivalent to a city of about 25,000 people discharging sewage into Cobscook Bay without any water treatment. Similar results stem from a calculation based on the excretion of phosphorus. Nitrogen and phosphorus are primary water pollutants, contaminating the habitat for lobster, clams, and scallops.
Cooke’s operations in Cobscook Bay contribute just a small fraction, < 1%, of the national production of farmed salmon, close to 3 million tons/year. The excrement of farmed salmon may contribute as much nitrogen as 2,500,000 people without sewage treatment—hardly acceptable in an era when the water quality of estuaries is essential for the health of both humans and fisheries.
Consuming farm-raised salmon poses questions for consumers. Wild-caught salmon are generally regarded as more nutritious and with lower content of pesticides and drugs, including antibiotics. But, wild-caught salmon are scarce, and the fishery is not sustainable. Raising salmon on land would allow some of the pollutant effluent to be captured. Producing artificial salmon meat in laboratory conditions may be the wave of the future.
References
Rose, A., et al. 2015. The characterization of feces and urine: A review of the literature to inform advanced treatment technology. Crit. Rev. Environ. Sci and Techno. 17: 1827-1879
Wang, X., et al. 2013. Discharge of nutrient wastes from salmon farms: environmental effects, and potential for integrated multi-trophic aquaculture. Aquaculture Environ. Interactions 2: 267-283.
