When we look at how climate change is affecting the ocean, the narrative can sometimes feel contradictory. On one hand, we hear alarming reports of widespread biomass decline and mass mortality events. On the other hand, fisheries in certain regions suddenly report high catches during unusually warm years. How can the ocean be slowly emptying, yet simultaneously bursting with fish in specific areas? Read the full paper here.

This paradox was the starting point for our team—myself, Juan David González-Trujillo, and Miguel B. Araújo at the Museo Nacional de Ciencias Naturales (MNCN-CSIC). We wanted to disentangle the mess of ocean warming. We suspected that the confusion stemmed from how we measure "warming". Are we talking about the slow, relentless, decadal rise in temperatures, or are we talking about the acute shocks of marine heatwaves (MHWs)?

To answer this, we knew we couldn't just look at a handful of species or a single region. We needed a global, macroecological perspective. We utilized massive data sources, combining high-resolution bottom temperature reanalysis data with three decades of scientific bottom trawl surveys. In the end, we analyzed 702,037 estimates of biomass change across 33,990 fish populations, covering 1,566 species across the major Northern Hemisphere basins.

 The "Aha!" Moment: Two Different Threats

When we finally ran the models isolating the effects of long-term warming from short-term temperature spikes, the results were intriguing. We had uncovered a "double threat" that operates on completely different temporal rules.

First, the long-term reality is grim. We found that long-term warming was associated with an annual biomass decline of up to 19.8%. This chronic warming acts as a persistent and dominant stressor, silently eroding fish biomass regardless of where the fish live within their geographic range. This aligns with previous large-scale studies focusing on global simulation models.

But then came the twist. When we looked at short-term thermal stressors—marine heatwaves and unusually warm years—a mixed landscape of winners and losers emerged.

The Climate Trap

The key to predicting these short-term responses lies in a population's thermal position. For fish populations living at the warm edge of their species' distribution, heatwaves are devastating. These populations experienced sharp biomass losses of up to 43.4%.

However, populations living at the cold edge of their range experienced the exact opposite. During extreme marine heatwaves, these populations showed biomass increases of up to 176%. For these fish, a sudden spike in temperature temporarily transformed a hostile, cold environment into a highly hospitable one. We saw this clearly in species like the European sprat (Sprattus sprattus), which exhibited declining biomass at its warm edge while simultaneously increasing at its cold edge during warmer years. This effect actually aligned with more than 750 of the examined species.

Why This Matters for the Real World?