This story was originally published by Inside Climate News and is reproduced here as part of the Climate Desk collaboration.

In 2022, fish farms produced an unprecedented 131 million tons of seafood, officially surpassing the global wild-caught fishing industry for the first time, according to a report released in July. Also known as aquaculture, the fish farming sector is often touted as a sustainable way to rapidly scale up the production of crucial fish protein sources without pulling them directly from wild habitats. 

But there’s a catch—literally. Some of the main ingredients that farmers feed their fish are, ironically, wild-caught fish. And a new study suggests that the aquaculture industry uses far more wild fish than previously estimated. The research is the latest in a wave of criticism against fish farming, which a group of scientists and conservationists say is fueling environmental degradation.

However, the global demand for fish is expected to skyrocket in the coming decades. Some experts say that despite its shortfalls, aquaculture is improving, and will be a crucial part of the sustainable food supply chain. 

While certain species like mussels dine mostly on algae, omnivorous and carnivorous fish require a certain amount of fish in their diets to thrive on farms. To quantify aquaculture’s reliance on wild-caught fish, researchers rely on a seemingly straightforward equation: How much fish goes into the food to produce a certain amount of farmed fish—otherwise known as the “fish-in: fish-out” (FIFO) metric.  

In 1997, aquaculturists were using a staggering amount of fish in their feeds to produce relatively low quantities of farmed fish across the board, with a global FIFO of about 1.9, according to a 2021 study. That’s almost two fish in for every fish out, by weight. In some cases, it took as much as 3.16 kilograms of wild-caught fish to produce a single kilogram of salmon. That research found that the FIFO ratio sharply decreased by 2017 as the aquaculture industry sought alternative feed ingredients.

“The metrics used to assess the sustainability of manufacturing aquaculture feed have left out large aspects of its environmental impacts.”

However, there are a variety of ways to calculate this metric. A new study shows how different the results can be if you broaden the definition of the “fish in” side of the equation. Using data from four sources of industry-reported feed composition during 2017, researchers calculated fish inputs to farmed outputs at a range of 0.36 to 1.15. That high end is roughly four times the previous study’s estimate.

One of the main reasons for this discrepancy is that the researchers accounted for several additional factors in their equation, including updated values for fish oil and something called wild fish trimmings. Those are the parts of marine animals’ bodies that are removed during wild-caught fish processing because they are undesirable to many consumers (think heads and tails).

These parts are often used in fish feeds but rarely accounted for in FIFO equations since they are considered waste byproducts. In a separate calculation, the authors also factored in estimates for some of the unintended animal deaths involved in the fishing process, including the accidental catch of non-target species known as bycatch. That pushed the FIFO figures even higher.

“The main recommendation that emerges from the work is to have a closer look at the data,” study co-author Jennifer Jacquet, a professor of environmental science and policy at the University of Miami’s Rosenstiel School of Marine, Atmospheric and Earth Science, told me over email. “When that happens what is clear is that the picture is not as rosy as the aquaculture industry or the fishery industry wants us to believe.”

Small fish such as anchovies and sardines are among the main species targeted for aquaculture fishmeal. The issue is that wild animals depend on these fish for food as well. Studies show that depleting these stocks could be particularly bad for seabirds. For example, penguins in Cape Town are declining largely due to the intense fishing pressure on sardines and anchovies, which I wrote about last year. 

“One of the take-homes that I really liked of this paper was [its] underscoring that we need better transparency and data availability to really have a good understanding” of the different proportions and species of wild-caught fish being used in aquaculture, said Halley Froehlich, an assistant professor at the University of California, Santa Barbara, who studies the industry and was not involved in the study. 

However, Froehlich noted that the study’s findings may not be as bad for wild ocean fish populations as they seem because the use of fish trimmings in feed is seen by many as a sustainable option.  

“It creates a circular economy,” she told me in a phone interview. “Otherwise, [fish trimmings] would just be thrown out.” 

The tricky part is that fishers can make additional income selling their trimmings, study author ​​Matthew Hayek, an assistant professor in environmental studies at New York University, told me over email. 

This “provides further incentive for fisheries to continue contributing to this value chain,” he said. The study also notes that whole fish from species that are less desirable on the market—dubbed “trash” fish—are sometimes added into that mix as well. 

To help mitigate aquaculture’s wild-fish problem, scientists and companies are formulating plant-based alternatives, which have been increasingly integrated into carnivorous fish diets, particularly salmon. This option comes with its own set of risks, according to the new study. For example, they say soy and maize feed options can increase the generation of agricultural-based emissions as well as freshwater consumption. 

“Our takeaway is that the metrics used to assess the sustainability of manufacturing aquaculture feed have left out large aspects of its environmental impacts, both at sea and on land,” study author Spencer Roberts, a doctoral student at the University of Miami’s Rosenstiel School. “These omissions have helped to portray fish and crustacean farming as uniquely efficient or sustainable. Our research shows that it is more similar to other forms of animal farming, albeit with a uniquely high reliance on wild fish extraction.”

Despite these impacts, research shows that our appetite for seafood is expected to double by 2050. As a result, the demand for aquaculture is rising as well. Froehlich stressed that the industry has to find a way to feed fish somehow, and that plant-based or other alternative feeds—particularly microalgae—are the most sustainable options at the moment. In the end, she said, “there is no free lunch.”