Long-term warming reduces fish biomass, but heatwaves shift it

Long-term ocean warming, interannual temperature variability and marine heatwaves pose serious but poorly quantified threats to marine species. Here, to quantify and isolate their individual effects, we analysed 702,037 estimates of biomass change across 33,990 fish populations (1,566 species) between 1993 and 2021, covering major Northern Hemisphere basins. Long-term warming was associated with an annual biomass decline of up to 19.8%. However, on shorter timescales, warmer years and marine heatwaves were linked to sharp biomass losses of up to 43.4% in populations at the warm edge of the species' range and biomass increases of up to 176% at the cold edge. Accounting for these edge-dependent 'winners and losers' in response to long- and short-term warming will be essential to avoid overexploiting transient biomass gains. Ultimately, management strategies must plan for the biomass loss expected under continued ocean warming.