Ocean Warming Linked to Nearly 20% Annual Fish Biomass Decline, Global Study Finds

Chronic ocean warming is driving a sustained annual decline of nearly 20% in fish biomass across major marine regions, according to new research published in Nature Ecology & Evolution.

The study, conducted by scientists from the Museo Nacional de Ciencias Naturales (MNCN-CSIC) and the National University of Colombia, analyzed 702,037 biomass change estimates from 33,990 fish populations recorded between 1993 and 2021 across the Northern Hemisphere.

The findings confirm that long-term ocean temperature rise, rather than short-lived marine heat waves, is the dominant driver behind declining fish stocks in the Mediterranean Sea, the North Atlantic Ocean, and the Northeastern Pacific Ocean.

Chronic Warming vs. Marine Heat Waves

While extreme marine heat waves can temporarily increase fish biomass in colder regions, researchers found these short-term gains often mask the deeper, long-term decline caused by persistent warming.

When heat waves push fish populations beyond their optimal thermal comfort zone, the temperature range where species grow and reproduce best ,biomass can drop by as much as 43.4% in already warm waters.

In contrast, fish populations located in colder areas may temporarily expand during warming events, with biomass increasing by up to 176%. However, scientists warn these spikes are short-lived.

“Although this sudden increase in biomass in cold waters may seem like good news for fisheries, these are transient increases,” said MNCN researcher Shahar Chaikin. Raising catch quotas based on heat-wave-driven population surges could lead to stock collapse once temperatures normalize or chronic warming reasserts its pressure.

Up to 19.8% Sustained Annual Decline

After filtering out short-term extreme weather variability, the data revealed that chronic warming is associated with a sustained annual biomass decline of up to 19.8%.

Researchers emphasize that unlike short-lived heat waves, ongoing ocean warming exerts continuous stress on fish populations. The Mediterranean, North Atlantic and Northeastern Pacific show consistent downward biomass trends tied directly to rising sea temperatures.

The decline presents a significant risk to fisheries management and marine ecosystem stability  both critical for global food security.

Fisheries Management Must Adapt to Climate Reality

The study highlights that traditional fisheries management frameworks are no longer sufficient under accelerating climate change.

To address this, researchers propose a three-level management strategy:

1. Rapid Response to Marine Heat Waves
Short-term climate-ready measures should be implemented immediately when extreme thermal events occur, particularly in regions where species are already near their upper thermal limits.

2. Long-Term Planning Around Biomass Decline
Management strategies must account for the documented sustained biomass reduction linked to chronic warming, rather than relying on temporary increases caused by short-term anomalies.

3. International Cooperation
As warming shifts species distribution across borders, static national management models become outdated. Coordinated international agreements are needed to manage shifting fish stocks effectively.

“A species population may be declining in one country but increasing in another,” Chaikin noted. “Effective conservation requires international coordination and joint resource-management agreements.”

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Miguel B. Araújo of MNCN-CSIC warned that temporary fishing opportunities at the cold edges of species’ ranges must not overshadow the broader long-term decline.

“As ocean warming continues, the only viable strategy is to prioritize long-term resilience,” he said, emphasizing that management must plan for continued biomass reduction in an increasingly warm ocean.

Implications for Global Food Security

The large-scale dataset, covering nearly three decades, underscores how climate-driven ocean warming is reshaping marine ecosystems at a systemic level. With fish stocks playing a central role in feeding millions worldwide, sustained biomass decline poses economic, ecological and food-security risks.

The findings reinforce growing scientific consensus that climate adaptation in fisheries governance is no longer optional but essential.

As marine heat waves grow more frequent and ocean temperatures continue to rise, the study makes clear that short-term fluctuations should not distract policymakers from the persistent, long-term erosion of global fish biomass.