A shift in farmed salmon's diet is having significant downstream impacts on the environment and climate. Hayek noted it's difficult to attribute soy consumption to individual fish species, though other research shows a rise in crop-based ingredients fed to farmed salmon. In Norway, the world's biggest producer of farmed salmon, 90 percent of feed ingredients were marine-based in 1990, dropping to 23 percent by 2020. Advocacy groups found that 90 percent of the soy fed to farmed salmon in Norway comes from three Brazilian companies, each linked to soy ingredients sourced from illegally deforested patches of the Amazon rainforest.

Other research found the shift to crop-based ingredients doubled greenhouse gas emissions, per kilogram, of farmed fish in Europe and the United Kingdom from 2000 to 2020. "It's trading one problem—marine exploitation—for another—land exploitation," Cascada said.

Some environmental groups note the surge in salmon demand has disproportionately come from wealthy countries without nutritional deficits. "The claim is that the salmon industry is here to feed the world," Cascada said, "but what it's really doing is flooding the Global North with protein that is not necessarily needed at the expense of the Global South."

In the United States, lawmakers are advancing the Marine Aquaculture Research for America (MARA) Act to expand offshore salmon farming, though environmental advocates and fishermen oppose the legislation.