Fish Farming's Impact on Ocean Environments
Fish farming is often promoted as a solution to “overfishing” and a “sustainable” way to feed the planet. Industrial aquaculture promises high-value seafood for markets in North America, Europe, and Asia, while claiming to reduce pressure on wild fish. In reality, the industry depends heavily on wild-caught fish for feed, destabilizing marine ecosystems and decimating wild fish populations, while recent research shows that shifts toward plant-based feed are amplifying other environmental impacts rather than reducing them.
Understanding fish farming’s hidden costs requires looking beyond marketing campaigns and selective metrics. It’s not just about salmon and trout on supermarket shelves; it’s about small coastal fish pulled from the Global South to feed farmed fish, flawed industry metrics that mask real impacts, and the redistribution of environmental pressures from oceans to land.
The Small Fish Behind Big Seafood
The irony of fish farming is that, rather than reducing pressure on wild fish, it increases it. Many of the species we consume on our plates, like salmon and trout, are carnivorous. That means they need diets rich in fishmeal and fish oil made from small coastal fish such as anchovies, sardines, herring, and menhaden. These fish might seem insignificant, but they are the foundation of marine ecosystems. They feed larger predators, recycle nutrients, and support seabird populations.
Yet today, these small fish are being captured in huge quantities to feed farmed salmon headed for Europe, North America, and China. In West Africa, industrial fisheries in Mauritania, Senegal, and The Gambia process small fish into fishmeal and fish oil, exporting them for profit. A 2019 Greenpeace report shows that these wild populations are now “overexploited.” Still, the industry continues to pull vast numbers of these fish from the ocean, often without strong regulatory oversight or enforcement. In South America, Peru’s anchoveta fishery—the world’s largest source of fishmeal—has faced repeated early closures due to low adult populations and high juvenile catches, a direct result of industrial fishing.
Decimating small coastal fish populations doesn’t just affect the fish themselves. In southern Africa, the exploitation of Benguela sardines has contributed to the endangered status of African penguins and Cape cormorants. In Peru and Chile, seabird populations have also declined, and top predators like sharks struggle as their food supply diminishes.
Communities Paying the Price
The environmental toll is matched by social costs. Many coastal communities in the Global South rely on small fish for subsistence and local markets. When industrial aquaculture diverts these fish to feed high-value salmon for wealthier markets, local diets and food security suffer.
Norway’s salmon industry is a prime example. In 2019, the world’s largest salmon producer, Mowi, used 880,000 tonnes of wild fish—much of it sourced from the same West African fisheries described above—to produce 436,000 tonnes of salmon, just over twice has much wild fish, by weight, as salmon produced. Left in local food systems instead, that fish could have fed millions of people in West Africa. Meanwhile, the company markets itself as a solution to food insecurity, highlighting the gulf between marketing and reality.
A new Harvard-led study found that if wild fish diverted to aquaculture feed remained in local food systems in the Global South, they could meet the nutritional needs of up to 31 million undernourished people. Instead, these fish feed farmed salmon and other luxury species destined for wealthy consumers. The industry’s “feeding the world” narrative is not just misleading; it’s the opposite of what happens on the ground.
The Metrics That Mask the Truth
To reassure consumers and policymakers, the aquaculture industry relies on metrics like Maximum Sustainable Yield (MSY) and the Fish In:Fish Out (FIFO) ratio. On paper, these make fish farming look responsible and efficient—but they hide the real story.
MSY measures how much of a fish population can be harvested without causing collapse. But it assumes populations are healthiest when kept at half their natural size, ignoring the ecological role of older fish and predator-prey dynamics. In practice, some fisheries are considered “sustainable” even when populations are depleted by 70 percent—well beyond MSY’s own threshold. Over decades, the proportion of “underfished stocks” has plummeted from 34 percent in 1980 to just 7 percent today, while “overfished” populations have tripled.
FIFO, developed in the early 2000s, measures how many kilograms of wild fish are needed to produce one kilogram of farmed fish. Industry reports suggest aquaculture is becoming more efficient, sometimes even claiming that farmed fish “produce” more protein than they consume. But a 2024 Science Advances study tells a different story. Accounting for bycatch and fish-processing trimmings, researchers found that farmed salmon and trout often consume more than twice their weight in wild fish before harvest and sometimes nearly six times their weight. In other words, the farmed fish we eat are a net drain on ocean protein.
A Shift to Crops Doesn’t Solve the Problem
Facing scrutiny over its reliance on wild fish, the aquaculture industry has shifted toward plant-based feeds like soy protein and rapeseed oil. At first glance, this seems like a win. But new research shows that this shift has not reduced environmental harm but instead redistributed and magnified it.
Between 2000 and 2020, European aquaculture nearly doubled production. At the same time, environmental impacts linked to feed production rose even faster. Greenhouse gas emissions from feed production jumped 314 percent, land use grew by 594 percent, and water use climbed 236 percent. Fertilizers for soy and rapeseed also increased nutrient runoff into rivers, lakes, and coastal waters.
Even when measured per kilogram of farmed fish—a way to control for industry growth—the impacts worsened. Emissions per kilogram more than doubled, and land use per kilogram increased 336 percent. In short, swapping marine ingredients for crops didn’t make aquaculture greener; it transferred pressure from oceans to forests and farmland, especially in the Amazon. As Dr. Spencer Roberts from the University of Miami notes, “We’re not increasing efficiency; we’re shifting pressure from the Humboldt Current, where anchovies come from, to ecosystems like the Amazon rainforest, where soy comes from.”
What This Means for “Sustainable Seafood”
Industrial aquaculture depends on wild fish, destabilizes ecosystems, and shifts environmental burdens onto land. Its metrics are designed to reassure, not to reflect reality. Even when the industry moves toward plant-based feeds, it merely transports its harms, often to vulnerable ecosystems and communities far from the fish farms themselves.
Taking seriously the question of the “sustainability” of fish farming means considering the full environmental and social impacts of every kilogram of fish produced, including the human communities that rely on wild fish and the ecosystems that support marine life. Expanding the production of farmed fish for wealthy markets is, then, not a solution, but a part of the problem.
To learn more about the environmental costs of fish farming, as well as our debunking of five core myths of aquaculture, read our 2026 report.