Wild fish catch must inexorably decline. Caught seafood is increasingly being replaced by fish farming, which began in the 1950s. In 2015, 80 million tonnes of seafood came from aquaculture. That figure is bound to grow.
Farming fish does not relieve pressure on wild fish stocks. To the contrary. While some smaller farmed fish are fed corn and soy, the most popular species, such as salmon and tuna, require a fish-based diet. Hence, aquaculture largely relies upon a steady supply of caught fish.
Aquaculture is industrialized. The environmental and nutritional results have been predictable.
Caged fish foul the sea around their pens. Fish waste and uneaten feed litter the sea floor beneath aquafarms, generating bacteria that consume oxygen vital to shellfish and other bottom dwellers.
Fish farm waste promotes algal growth that reduces oxygen in the water, posing risks to nearby coral reefs and other aquatic life. But this is just the beginning.
As concentrated targets for viruses, fish farms spread disease and sea lice. To counteract this, antibiotics and pesticides are dumped into the water, poisoning the area around aquaculture farms.
The pesticide that kills sea lice is lethal to other marine invertebrates. This toxic blanket persists in the water for hours and may diffuse for a kilometer around the farm.
All told, the environmental costs of aquaculture have been estimated to be half their production value. Like most other capitalist enterprises, these are costs that fish farmers do not have to pay, but societies will eventually.
Fish regularly escape their pens. Many interbreed with wild varieties, producing hybrids that can be less capable of survival.
Farmed fish are typically less nutritious than their wild cousins and are stocked with antibiotics and pesticides. Farmed salmon have less than half the healthy omega-3 fats that are a principal benefit of eating fish.