If you have the chance to see a whole salmon at the fish market, observe its tail: It will likely be small and withered. This is because most salmon have never used their tail fins—have never swum upriver or propelled themselves through an ocean. In fact, they have never left the nets they are grown in.

Most commercial salmon is produced by corporate aquaculture giants in Chile, Norway, Scotland, and elsewhere. The farms suspend metal nets in the sea, pack them with thousands of fish, then quickly fatten the salmon with fish-meal pellets. But aquaculturists had to face an inconvenient fact: