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Aquaculture is often promoted as a sustainable alternative to catching wild fish—a way to reduce pressure on overexploited stocks while providing affordable and necessary protein for people’s diets.

It’s an argument put forward by major international organizations like the World Bank and the intergovernmental Organisation for Economic Co-operation and Development. But it’s an argument that doesn’t hold up, according to new research.

“Our findings suggest that aquaculture is just adding to seafood production, not having any conservation effect,” says Stefano Longo, a social scientist who studies the interactions between human and ecological systems at North Carolina State University.

Longo and his colleagues used statistical models to analyze global aquaculture production and wild fish harvests from 1970 to 2014. They compared the total weight of aquaculture production with the total weight of wild-caught fish per capita. They found that increases in aquaculture production did not result in fewer wild fish being caught, and may have contributed to an increased demand for seafood.

The effect is similar to how the introduction of energy-efficient LED light bulbs did not result in the expected reduction in total energy use—instead, people simply used more light bulbs, as they were cheaper to run.

“The notion that aquaculture is sustainable, and helps conserve marine species, is disingenuous,” says Longo. “That’s not what they’re doing. They’re producing a commodity for consumption.”

A major reason for the lack of any substitution effect with aquaculture, says Longo, is that many farmed species, such as salmon and bluefin tuna, are top predators so farming them requires feed made from other fish. Even if farming salmon means fewer wild salmon are being caught—an idea that Longo did not investigate, but which is by no means certain—salmon production still results in more baitfish, such as herring, being caught for feed.

“It’s possible that we are not eating as much wild salmon, but, at a societal level, the production of salmon becomes an avenue for increased seafood consumption,” says Longo.

Melanie Wiber, an anthropologist who studies aquaculture at the University of New Brunswick, says these results are not surprising for those who know a lot about how aquaculture works.

“There is increasing awareness that aquaculture may not be a sustainable protein production source, let alone that it should replace better sources of wild protein,” she says. But, she adds, the idea is not well understood by the public, or indeed by many international organizations.

Aquaculture companies are aware of this critique, Wiber says, and are trying to develop more sustainable food sources for their farmed fish. “But it’s hard to take a top marine predator and feed it soy meal,” Wiber says.

Longo stresses that it is not the technology of aquaculture itself that is problematic, but how it has been used.

“The goal is to produce and sell as much as possible, that’s how the economy operates. It’s based on growth, not conservation,” he says. “Aquaculture can be an aspect of creating a more sustainable food system, it just hasn’t been employed in that way.”