Study: Ocean Warming Puts "Constant Negative Pressure" on Fish Populations

A new meta-study published in Nature Ecology & Evolution suggests that ocean warming has an outsize impact on the total amount of fish in the water, enough to have major implications for global fisheries.

The study, led by researchers at Spain's Museo Nacional de Ciencias Naturales, looked at hundreds of thousands of studies of fish populations in the Northern Hemisphere over a period of nearly 30 years, spanning 1993-2021. The vast data set covered more than 1,500 fish species and nearly 34,000 distinct populations, primarily in the Mediterranean, North Atlantic and Mediterranean.

Focusing on fish populations of interest to fishermen and policymakers in the U.S. and Europe, the team determined that long-term warming trends drove an annual biomass decline of as much as 20 percent. However, the overall decline could easily be masked year to year by short-term drops and spikes linked to marine heatwaves. The study found that different fish win or lose during a heat wave depending upon their place within the species' thermal cumfort zone, the amount of temperature rise that the species can tolerate. During a heat wave, biomass loss could fall by as much as 43 percent in the hottest areas of the species' range, and could jump upwards by up to 176 percent on the coldest edge of the species' range - indicating that individuals in previously-cold areas were doing better as they warmed up, and those in already-warm areas were doing worse as they overheated. This has implications for fisheries managers who detect increases in fish populations: it's not as simple as allowing more fishing to catch a rising number of fish, the authors said.

"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 heat wave, 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," warned MNCN researcher Shahar Chaikin.

The real concern for the long term, the researchers said, is the long-term sustained decline in biomass caused by warming waters. "Chronic warming exerts a constant negative pressure on fish populations in the Mediterranean Sea, the North Atlantic Ocean, and the Northeastern Pacific Ocean," said National University of Colombia researcher Juan David González Trujillo, a coauthor of the study.

Fisheries managers will have to bear this in mind when setting quotas, the authors said, even when a population looks suddenly-healthy. Fish populations are moving around to escape the pressure of warming, and when they cross national boundaries, they will enter new regulatory regimes - increasing the need for shared control of the resource base.

"As ocean warming continues, the only viable strategy is to prioritize long-term resilience. Management measures must plan for the biomass decline expected in an increasingly warm ocean," coauthor Miguel B. Araújo said in a statement.