Ocean Warming Drains Fish Biomass Up to 19.8% Per Year - While Heatwaves Hide the Trend

The fish are disappearing from warming waters, slowly and consistently, at rates that standard fisheries assessments may be systematically underestimating. And the extreme weather events that generate headlines - marine heatwaves - are complicating the picture by causing temporary population spikes that can look like good news for fisheries management while masking a worsening underlying trend.

A study from Spain's Museo Nacional de Ciencias Naturales (MNCN-CSIC) and the National University of Colombia analyzed 702,037 estimates of biomass change from 33,990 fish populations recorded between 1993 and 2021 across the Mediterranean, North Atlantic, and Northeastern Pacific.

Chronic warming versus acute heatwaves

The researchers drew a distinction that turns out to be analytically critical: the effects of sustained long-term ocean temperature rise versus the effects of episodic marine heatwaves.

When the short-term noise of heatwave events is removed from the signal, the underlying relationship between chronic warming and fish biomass becomes clear. Long-term temperature rise is associated with a sustained annual decline in biomass of up to 19.8% in populations living near the warm edge of their tolerance range.

"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%," said MNCN researcher Shahar Chaikin.

The heatwave illusion

Marine heatwaves affect different populations in opposite ways depending on where those populations sit relative to their thermal comfort zone. Species pushed beyond their thermal optimum by a heatwave experience acute stress - biomass can fall by up to 43.4%. Species in colder waters may temporarily find conditions more favorable, and their biomass can increase by as much as 176%.

That temporary increase creates a dangerous illusion. If a manager observes a large population increase following a heatwave and raises catch quotas accordingly, the quota will be calibrated to a boom that is inherently temporary. When temperatures normalize or long-term warming continues to push the thermal optimum northward, the population collapses.

"Although this sudden increase in biomass in cold waters may seem like good news for fisheries, these are transient increases. 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," Chaikin warned.

A three-level management framework

The researchers propose an approach addressing both the episodic and chronic dimensions of climate impact. In the short term, they argue for "climate-ready plans" - pre-defined protective measures that activate automatically when marine heatwave events occur, reducing fishing pressure during acute stress phases.

Over longer timescales, management needs to be structured around the documented chronic decline rather than historical baselines that no longer reflect current conditions. The third dimension is international: as species shift their ranges in response to changing temperatures, a population declining in one country may be expanding in another.

"A species population may be declining in one country but increasing in another. Effective conservation requires international coordination and joint resource-management agreements," Chaikin concluded. The study covers the Northern Hemisphere only - populations in data-poor tropical and Southern Ocean regions are not represented.