Fish farming was supposed to be sustainable. But there’s a giant catch.

Earlier this summer, the United Nations reported that humanity now consumes more fish raised in farms than taken from the ocean. 

The milestone was the culmination of a decades-long growth spurt in aquaculture, or fish farming, an industry that produces more than four times as much fish today than it did 30 years ago. Fish farming’s growth was spurred primarily by government subsidies around the world, as the world’s wild fish catch peaked in the 1990s and countries sought another source of seafood.   

Aquaculture has also been boosted by academic institutions, philanthropic foundations, nonprofit organizations, and the United Nations on the belief that fish farming can give overexploited oceans a break and more sustainably improve food security.

But fish farming comes with — forgive the pun — some major catches. Some of the most valuable farmed species, like salmon and trout, are carnivorous and must be fed wild-caught fish when farmed. Farmed shrimp, along with a number of omnivorous fish species, are also fed wild-caught fish. All told, some 17 million of the 91 million metric tons of wild-caught fish are diverted to the aquaculture industry annually. 

In other words, what was supposed to relieve pressure from overexploited oceans has become a new source of its exploitation. According to a new study published in Science Advances by a team of researchers from the University of Miami, New York University, and conservation group Oceana, fish farming might kill far more wild-caught fish than previously thought — a finding that throws the aquaculture industry’s sustainable branding into question. 

The researchers found that the amount of wild-caught fish — usually from small species like anchovies and sardines — to feed the top 11 farmed fish and crustacean species could be 27 to 307 percent higher than current estimates, or even higher, depending on how it’s calculated. (The high degree of variability and uncertainty is due to a lack of validated industry data on what farmed fish are fed.) 

“The extraction of wild fish to manufacture aquaculture feed is likely far higher than we’ve been told,” Spencer Roberts, a PhD researcher at the University of Miami and lead author of the study, told me. “The story about fish farming feeding the world is very optimistic, but it’s based on incomplete data. So what we’re trying to do is portray a more realistic and comprehensive picture.”

The aquaculture industry now uses almost one-fifth of the global wild fish catch just to feed farmed fish, adding pressure to already taxed oceans and threatening the food sources of some coastal communities in the Global South. It has also created a new realm of animal suffering: Fish farms, sometimes called “underwater factory farms” by animal advocates, often keep fish in conditions similar to the crowded industrial farms that confine pigs, chickens, and cows raised on land.

“There is a lot of hype in not just media but in governance conversations about aquaculture or blue foods more broadly as a sustainable source of food and a way to combat hunger or reduce food insecurity, but there are so many things ignored,” Roberts said. “I hope that [the new research] prompts other academics, but especially policymakers, to question some of the narratives.”

Fish farming might waste more fish meat than it produces

The aquaculture industry measures the amount of wild-caught fish required to produce one unit of farmed fish with what it calls the Fish In:Fish Out (FIFO) ratio. In 1997, the early days of the fish farming boom, the industry had a FIFO ratio of 1.9, meaning that for every kilogram of fish it produced, it had to catch and kill almost two kilograms of fish used for feed. 

By 2017, according to a team of aquaculture experts, that figure dropped to .28, an all-time low, largely because the industry switched much of its feed from wild-caught fish to crops like soy and corn, along with vegetable oils, minerals, and vitamins. 

Those findings were published in Nature, and it’s since been widely cited in food systems research. The fish farming industry puts its FIFO ratio at a similarly low rate, claiming a major sustainability win. (It’s worth noting that some of the Nature paper’s authors hold close ties to the aquaculture and livestock feed industries.)

But the model used in that paper was incomplete, according to Roberts. For instance, it didn’t include trimmings, the parts of a fish considered byproducts that do end up in fish feed, nor fish that were unintentionally killed and turned into fish feed. The model also used industry data reporting that, on average, only 7 percent of its farmed fish feed consisted of wild-caught fish; the rest consisted of crops. Other data sources, from the Monterey Bay Aquarium, the United Nations’ Food and Agriculture Organization (FAO), and another team of researchers, reported much higher rates of wild-caught fish in farmed fish diets. 

When correcting for these factors from the original model, Roberts and his co-authors found that for the top 11 farmed species, the global aquaculture industry’s Fish In:Fish Out ratio is much higher than the original model’s estimate of 0.28, ranging from 0.36 to 1.15, or 27 to 307 percent higher.

Then the researchers ran the numbers again, adding in other fish and other marine animals killed unintentionally by commercial fishing vessels, and removed fish farms that don’t feed their animals at all. That adjustment brought up the industry’s FIFO ratio to between 0.57 and 1.78, or 103 to 535 percent higher than the original model. That means that at the upper estimate of 1.78, the industry still generates a net loss of fish, just as it did in the 1990s.

For carnivorous farmed species like salmon and trout, the aquaculture sector’s demand for wild-caught fish is especially high. By Roberts and his co-authors’ upper-bound estimate, it could take up to 6.24 kilograms of wild-caught fish to produce just one kilogram of salmon — 230 percent more than previously estimated.

“It appears this current paper replaced [earlier studies’] simplifying assumptions with better sources of data or better estimates,” said David C. Love, an aquaculture and fisheries research professor at Johns Hopkins Bloomberg School of Public Health, who was not involved in the study. “What they found was that more fish are being used in the diet than what was previously thought.”

But looking at the FIFO ratio for the entire industry obscures major differences in feeding requirements across species. 

“It’s hard to say, ‘Well, aquaculture is just one thing.’ It’s not. It’s lots and lots of different species with different needs,” Love said. The biggest difference is between herbivorous species, like carp and tilapia with a FIFO ratio up to .83 at the upper bound of the adjusted model, and carnivorous species like salmon and trout, with a FIFO ratio up to 5.57 — a near sevenfold gap. Shrimp, freshwater crustaceans, and catfish also require more wild-caught fish than they produce at the upper bounds.

Paul Zajicek, executive director of the National Aquaculture Association, dismissed the study’s findings in an email to Vox. 

“As noted by the authors, these types of analyses are very challenging and we suspect a rival analysis will show differences as well,” Zajicek wrote.

But the massive amounts of wild-caught fish fed to farmed fish is only one piece of the bigger picture on fish farming’s unsustainability. 

Fish farming’s environmental, social, and animal welfare costs

Although the fish farming industry over time has lowered its reliance on wild-caught fish on a per-kilogram basis, it has replaced it with corn and soy. 

“Every bit of fishmeal that you [remove from fish diets] has to still be substituted with something from land,” said Jennifer Jacquet, a co-author of the study and professor of atmospheric and earth science at the University of Miami. “We’re already concerned with deforestation for [feeding] land animals, and now farmed salmon are also contributing to the deforestation of our world.”

This explosion in crop use — about a fivefold increase in recent decades — doesn’t just mean more potential deforestation. Those crops are grown using a lot of synthetic fertilizer, which in turn pollutes waterways and harms wild fish. It’s also a significant source of greenhouse gas emissions and displaces land that could be used to otherwise grow food directly for humans.

“What we’re talking about is not so much increasing efficiency as much as a shift in pressure from ecosystems like the Humboldt Current [in Peru], where the anchovies come from, to ecosystems like the Amazon rainforest where the soy comes from,” said Roberts.

What we need, Love of Johns Hopkins University told me, are holistic life-cycle assessments that cover not just a species’ FIFO ratio but other metrics, too, such as carbon footprint, water use, land use, and pollution, to give us a more accurate picture of aquaculture’s environmental impact.

One such assessment, published Nature in 2021, found that seaweed and bivalves, like mussels and oysters, have the lowest environmental footprint of all aquaculture foods. But it also illustrates the complex range of trade-offs between different species and whether they’re wild-caught or farmed. For example, farmed salmon use little land and water but use a lot of wild-caught fish and generate a lot of pollution. By comparison, farmed carp eat almost no wild-caught fish but require much more land and water.

When we farm or catch fish at scale, much like animals raised on land, we tend to overexploit one ecosystem or another, making it an inefficient way of producing protein relative to plant-based agriculture.

The rapid growth of fish farming has also come with grave ethical implications. 

Animal rights advocates have lambasted fish farm conditions, where fish often suffer from many of the same issues as animals raised on land, like overcrowding and disease. Slave labor on commercial fishing vessels and inside fish processing plants has long plagued the industry.

Catching wild fish for fish feed also undermines food security in some regions. For example, many of the fish caught for the aquaculture industry come from West Africa and “could be part of the West African diet, but are instead being sold to fish meal plants” as food to be used on fish farms, said Love. Many of those fish end up in wealthier markets, like Europe and North America.

“While the aquaculture industry regularly uses the narrative of food security, their top products, salmon and shrimp, are prized not for their nutritional value but for their export value,” wrote Patricia Majluf of Oceana, a biologist and co-author of the study, in a separate analysis of the aquaculture feed industry. 

Much of the conversation among governments, philanthropies, nonprofits, and academics around the future of seafood — which is anticipated to grow some 30 percent by 2050 — aims to balance conservation and economic development. But famed ecologist and author Carl Safina, in a recent commentary, called for something grander: a clear-eyed look at aquaculture’s environmental and social harms — one that would require us to fundamentally rethink aquaculture. “Problems in animal aquaculture stem from failures of care and conscience,” Safina wrote. “Solutions require not ‘balanced’ goals but moral reckonings overhauling economic valuations and policies.”