Warmer oceans are redistributing fish biomass

Ocean warming does not reduce every fish stock uniformly or at the same time. An analysis published in Nature Ecology & Evolution distinguishes two dynamics: a background erosion of biomass associated with long-term warming and much faster shifts during exceptionally warm years. Populations near the warm edge of their species’ range tend to lose biomass during those events, while populations near the cold edge may gain it temporarily.

Evidence across the Northern Hemisphere

Shahar Chaikin and colleagues assembled 702,037 estimates of biomass change covering 33,990 populations, 1,566 species and the period from 1993 to 2021. The time series span major Northern Hemisphere basins. They draw on sources including the FISHGLOB database and fishery-independent surveys in the Mediterranean and Black Seas. Temperature fields came from the Copernicus Marine Service, while species occurrence records helped locate thermal range edges.

This scale enables comparisons among populations exposed to very different climates. It does not make every observation a complete census. Biomass estimates still depend on survey gear, sampled habitats and species detectability. The study identifies broad patterns across many series; it does not replace local stock knowledge.

The authors separated three components that are often combined: multi-decadal warming, year-to-year temperature departures and marine heatwaves. The distinction matters. Gradual warming changes average conditions, whereas an extreme event can rapidly alter where fish are found or how available they are to a survey.

Long-term warming and hot years tell different stories

In the models, sustained warming was associated with annual biomass declines of up to 19.8% under the most unfavourable combinations. “Up to” is crucial: this is the upper end of an estimated relationship, not an annual decline expected for every species or every sea.

At shorter timescales, a population’s position within the species’ thermal range became decisive. During warmer years and marine heatwaves, biomass losses reached as much as 43.4% near warm range edges. By contrast, increases of up to 176% appeared near cold range edges. Such gains may reflect arrivals, improved local survival or greater survey availability; they do not prove sustained growth of the stock across its full range.

The same thermal anomaly can therefore produce local “winners” and “losers”. This is consistent with redistribution. As conditions become less suitable in southern or shallow areas, some populations move towards cooler water, often poleward or deeper. Biomass may rise in a receiving region while falling elsewhere.

A local gain may not last

For managers, a central risk is interpreting a temporary abundance increase as a permanent rise in productivity. A newly arriving population may remain dependent on a short climatic window, suitable prey or spawning habitat. If warming continues, today’s receiving area may eventually approach the species’ warm edge.

Harvest decisions therefore need to distinguish three questions: how many fish are observed, where did they come from, and can local production support that level? Higher catches or survey indices alone do not describe recruitment, age structure or mortality. Coordination across jurisdictions also becomes more important as stocks cross administrative boundaries.

The same reasoning prevents the opposite error: attributing every local decline to direct heat mortality. Temperature affects physiology, but it can also alter migration, prey, reproduction, dissolved oxygen and pathogen distributions. A falling index may combine emigration with a real reduction in production.

Monitoring rapid ecological change

Robust monitoring should preserve standardised time series while adding thermal information at a relevant resolution. Annual mean temperature cannot represent the duration, intensity and depth of a heatwave. Measurements are more useful when matched to survey dates, sampled depths, fish sizes and life stages.

Managers can also track early indicators: movement of the distribution centre, a different arrival season, unusual recruitment, changing body condition or the appearance of warm-affinity species. These signals require interpretation alongside fishing effort, oxygen, productivity and habitat. A presence map is not yet a measure of demographic viability.

Aquatic health records should place temperature in the chronology of events. Warmer water changes metabolic demand and can reduce safety margins when oxygen falls. It may also shift the timing of some diseases. That supports targeted vigilance, not an assumption that every clinical episode has a climatic cause.

Limits prevent overgeneralisation

The dataset is large, but dominated by Northern Hemisphere basins and by the series available through the selected repositories. Tropical waters, some coastal habitats and many data-poor fisheries are not represented equally. Statistical associations integrate several sources of uncertainty and cannot capture every species-specific mechanism.

The reported maxima should therefore never become universal correction factors. Their value lies in showing how strongly the outcome depends on thermal position and timescale. The authors have released processed data and code, allowing other teams to test robustness and compare the framework with additional regions.

Observational coverage also varies through time. Survey continuity, gear changes and uneven species records can influence estimates despite statistical controls. A local application should review those metadata before treating a broad relationship as a forecast for one population.

Conclusion: managing biomass in motion

The message is not that all fish populations will decline at the same rate. Long-term warming applies a persistent pressure, while hot years redistribute gains and losses rapidly according to thermal range edges. A local increase can therefore precede another shift rather than signal a permanently more productive resource.

Vetofish can support monitoring programmes by standardising observations, integrating temperature and oxygen data, and examining ecological and health signals together. The goal is to preserve comparable time series while detecting distribution shifts early enough to adapt surveillance and management.

References

• Chaikin, S., González-Trujillo, J. D. & Araújo, M. B. (2026). “Long-term warming reduces fish biomass, but heatwaves shift it.” Nature Ecology & Evolution, 10, 932–941. https://doi.org/10.1038/s41559-026-03013-5
• Chaikin, S., González-Trujillo, J. D. & Araújo, M. B. (2026). Processed data and code associated with the study. Figshare. https://doi.org/10.6084/m9.figshare.30061603