Well, what exactly is a farm-raised fish? Farm-raised fish are those raised in pens in the sea, ponds, or tanks on land. Some farmed fish like tuna and salmon may be fed wild-caught fish (sardines, anchovies) as feed, while others such as tilapia can be fed diets of more grains and plant proteins.
There are a variety of fish farming (aka Aquaculture) systems, but the safest for the environment and wild populations are closed and/or treated systems. These systems minimize the risk of releasing pollutants into the environment, escaped fish, and disease/parasite transfer to wild fish. An example of such a system is a Recirculating Tank. Recirculating Tanks are completely contained, as the water that flows through the tanks is treated, and then returned to the tanks. An example of a more risky system is a Net Pen, which is often used to raise salmon and tuna. Net pen systems are suspended in the open ocean with a wooden, mesh, or net screens. Because net pens are in direct contact with the wild environment, wild populations of fish can be exposed to diseases and parasites from the farmed fish.
Over time, wild populations for many species of fish have declined due to over-fishing, environmental changes, and environmental degradation. It is important to avoid fish caught by damaging methods when possible - bottom trawling and dredging are among the most detrimental fishing processes because they damage the seafloor and have high rates of bycatch. Wild fishery management is improving and helping to maintain and recover populations, but fisheries are still subject to illegal fishing in some areas. New certifications are being introduced to help people understand where their fish is coming from and if it was sustainably-caught. The Marine Stewardship Council, for example, developed standards for sustainably managed and traceable wild-caught seafood.
When shopping for fish of the farmed or wild-caught variety, ask questions. Ask the market if the fish are sustainably raised. Ask where they come from. Seafood Watch’s Consumer Guide has up-to-date fish recommendations on “Best Choices”, “Good Alternatives”, and fish to “Avoid” for farmed and wild-caught fish. Check it out here.
Seafood Watch, “Fish & Farming Methods” http://www.seafoodwatch.org/ocean-issues/fishing-and-farming-methods
Seafood Watch, Wild Seafood, https://www.seafoodwatch.org/ocean-issues/wild-seafood
Seafood Watch, Aquaculture, http://www.seafoodwatch.org/ocean-issues/aquaculture