Warming Seafloors Are Quietly Draining Fish From Northern Seas, Major Study Finds

From the deck of a research vessel in the North Atlantic, the ocean looks much as it always has: gray, cold and seemingly endless. But the nets coming up from the seafloor are telling a different story.

A quantifiable link between warming and declining fish

A major study published Feb. 25 in Nature Ecology & Evolution finds that chronic warming of the ocean floor is steadily erasing fish from northern seas. Analyzing nearly three decades of scientific surveys, researchers report that for every additional 0.1°C of seabed warming per decade, regional fish biomass drops by about 7.2%. In the most affected areas, long-term warming is associated with annual losses of up to 19.8%.

“To put it simply, the faster the ocean floor warms, the faster we lose fish,” said lead author Shahar Chaikin, a marine ecologist at Spain’s National Museum of Natural Sciences, part of the Spanish National Research Council in Madrid.

The study offers one of the clearest numerical links yet between climate change and declines in marine life that sustains major fisheries. It also warns that marine heatwaves—short bursts of extreme warming—can produce temporary booms in some northern stocks, potentially misleading managers into raising catch limits just as the underlying trend is headed down.

What the researchers analyzed

The research team, which also includes Colombian biologist Juan David González‑Trujillo and Portuguese-based climate-biodiversity expert Miguel B. Araújo, assembled an unusually large dataset: 702,037 estimates of year-to-year biomass change from 33,990 fish populations representing 1,566 species. The records come from standardized scientific bottom-trawl and related surveys conducted between 1993 and 2021 in the Mediterranean Sea, the North Atlantic Ocean and the Northeastern Pacific.

Those surveys, often carried out by government research vessels, are designed to monitor fish stocks independently of commercial fishing. Trawl nets are towed along the seafloor along fixed routes, allowing scientists to track changes in the abundance and size of fish communities over time.

To connect those changes to climate, the authors used a global ocean reanalysis known as GLORYS12, produced under Europe’s Copernicus Marine Service, to reconstruct how seabed temperatures evolved in each area. They then separated three kinds of temperature effects: the long-term trend of warming at the bottom, year-to-year variability, and discrete marine heatwaves.

“When we remove the noise of extreme short-term weather events, the data show that this warming is associated with a sustained annual decline in biomass of up to 19.8 percent,” Chaikin said in a statement released by the museum.

Crucially, the paper looks specifically at fish, not all marine animals. While bottom-trawl nets also pull up invertebrates such as crabs, starfish and mollusks, the analysis is restricted to fish biomass. The regions covered—the Mediterranean, North Atlantic and Northeastern Pacific—represent the Northern Hemisphere seas with the longest and most consistent bottom-trawl records, rather than every shelf sea globally.

Why warming reduces biomass

Within that scope, the pattern is stark: as decadal rates of seabed warming rise, fish biomass falls.

“A 7.2 percent decline for every tenth of a degree per decade might sound small,” Chaikin said. “But compounded over time, across entire ocean basins, it represents a staggering and deeply concerning loss of marine life.”

Scientists say the mechanisms behind the trend are well understood. Warmer water increases metabolic demands on fish, making it harder for them to maintain body condition if food supplies do not rise in step. Warming can alter plankton communities at the base of the food web and reduce oxygen levels, further stressing fish. Many species respond by shifting toward cooler waters—often poleward or into deeper layers—leaving traditional fishing grounds depleted.

“What this paper adds is robust, basin-scale evidence that these processes are now translating into significant, measurable declines in fish biomass in the real world,” said Carlos García‑Soto, an ocean scientist at Spain’s National Research Council who contributed to the United Nations World Ocean Assessment but was not involved in the study.

Heatwaves can create misleading “booms”

Marine heatwaves, which have drawn increasing attention in recent years as record sea surface temperatures have scorched coastal ecosystems from the Mediterranean to the Pacific, play a more complicated role in the analysis.

At the warm edge of their ranges—lower-latitude or shallower limits where populations already live close to their thermal tolerance—fish hit by bottom heatwaves saw biomass plunge by as much as 43.4% in a single year. At the cold edge, where temperatures are generally below the optimum, the same events could produce short-lived booms, with biomass rising by as much as 176%.

Those spikes, the authors say, are dangerous if they are mistaken for a new normal.

“If managers raise catch quotas based on biomass increases caused by a heatwave, they risk causing the collapse of populations when temperatures return to normal or when the effect of long-term warming prevails, because these are short-lived increases,” Chaikin said.

García‑Soto described the dynamic as a governance challenge.

“Overall warming reduces fish biomass, while heatwaves can generate temporary increases that mask the underlying trend,” he said. “This introduces a clear risk of poor interpretation when taking decisions.”

Climate change and fishing pressure collide

The findings arrive amid long-running debates over the relative roles of climate change and fishing pressure in global stock declines. International agencies such as the U.N. Food and Agriculture Organization have documented a steady rise in the share of fish stocks classified as overfished over recent decades, driven largely by excess fishing capacity and weak regulation.

“Historically, overfishing has been the main driver of biomass declines in many of the world’s fisheries,” said Guillermo Ortuño Crespo, a marine biologist and adviser to the International Union for Conservation of Nature who was not involved in the new paper. “The current challenge is that this overfishing crisis is being further exacerbated by ocean warming and deoxygenation.”

The authors do not dispute that. Instead, they argue that climate change has moved the baseline on which traditional fisheries management was built.

“Static management models are outdated,” Chaikin said. “A species population may be declining in one country but increasing in another. In this context, effective conservation requires international coordination and joint resource-management agreements.”

Araújo, a research professor at the National Museum of Natural Sciences and chair of biodiversity at the University of Évora in Portugal, said the results underscore the need for “climate-ready” fisheries policies built on three pillars:

1. Rapid-response plans for marine heatwaves, including temporary closures and emergency quota reductions where warm-edge populations face sudden crashes.
2. Long-term management that explicitly accounts for chronic biomass declines under continued warming, integrating them into harvest rules and stock rebuilding timelines.
3. Stronger cross-border cooperation as fish shift their ranges across exclusive economic zones.

“As ocean warming continues, the only viable strategy is to prioritize long-term resilience,” Araújo said. “Management measures must plan for the biomass decline expected in an increasingly warm ocean.”

What’s at stake

The stakes extend beyond the seafood industry. Fish and seafood provide essential protein and micronutrients for hundreds of millions of people, and support jobs in coastal communities from the Gulf of Alaska to the Bay of Biscay. Many regions covered in the study underpin some of the world’s most valuable fisheries, including cod, hake, flatfish and a variety of small pelagic species.

“The collected data are essential for fisheries management and for the conservation of marine ecosystems, which are crucial for feeding the global population,” the Spanish research council said in a statement.

The study also adds weight to a message echoed in recent assessments by the U.N. Intergovernmental Panel on Climate Change: every fraction of a degree of warming matters. Global climate agreements, including the 2015 Paris accord, commit nations to holding temperature rise “well below” 2°C above preindustrial levels and to pursue efforts to limit it to 1.5°C. While those targets are framed in terms of air temperature, the new work illustrates what incremental warming means at the seabed.

“Our research proves exactly what that biological cost looks like underwater,” Chaikin said.

From the deck, the ocean’s surface may still appear unchanged. But along the continental shelves and slopes that ring the Northern Hemisphere, the water at the bottom is getting warmer, slowly but relentlessly. The study suggests that as it does, the nets will keep coming up a little lighter—not in a sudden collapse, but in a steady thinning that will test the resilience of ecosystems, fishing communities and the rules written to manage the life beneath the waves.