As chronic ocean warming relentlessly reshapes marine ecosystems, a groundbreaking study by the Museo Nacional de Ciencias Naturales (MNCN-CSIC) and the National University of Colombia reveals a nearly 20% annual decline in fish biomass across key Northern Hemisphere waters. This monumental research disentangles the nuanced interplay between long-term thermal stress and transient, temperature-extreme marine heatwaves, showcasing how these dynamics collectively alter fish populations in the Mediterranean Sea, the North Atlantic Ocean, and the Northeastern Pacific Ocean.
Between 1993 and 2021, the study meticulously analyzed an unprecedented dataset comprising over 700,000 biomass estimates derived from close to 34,000 distinct fish populations. This deep reservoir of empirical evidence highlights the perilous trajectory of marine vertebrates under a warming climate. Significantly, it illustrates that the ongoing increase in ocean temperature imposes a constant physiological strain on fish species, depressing their population sizes year over year, despite the episodic boosts associated with heatwaves.
Marine heatwaves, which have surged in frequency and intensity in recent decades, inject considerable complexity into the biomass narrative by temporarily altering local thermal environments. Rather than uniformly impacting fish, these abrupt thermal anomalies trigger species-specific responses dictated by each population’s unique thermal comfort zone—the temperature range within which optimal physiological growth, metabolic function, and reproductive success occur. Fish residing at warmer margins, already near or beyond their optimal range, suffer steep biomass declines during heatwaves, with reductions reported as severe as 43.4%.
Conversely, populations inhabiting colder regions often experience a paradoxical biomass surge in response to acute warming episodes, sometimes expanding by as much as 176%. This phenomenon arises because heatwaves push water temperatures temporarily into their ideal thermal window, boosting growth and survival rates in the short term. However, these biomass increases are fleeting and can mislead fisheries management, which if based on such transitory data, risks overharvesting and precipitating local stock collapses once temperatures stabilize or decline, or the chronic warming effect reasserts itself.
The researchers emphasize that while these anomalous heatwave-induced biomass upticks might appear encouraging for fishery stakeholders in cooler latitudes, long-term sustainability demands a sober understanding that these are ephemeral phenomena. Shahar Chaikin, a principal investigator from the MNCN, warns against the dangers of adjusting catch limits upward in response to temporary abundance increases. Failing to account for the underlying long-term warming trend could exacerbate population declines and threaten marine biodiversity over succeeding decades.
Disentangling the noise of extreme short-term thermal events from the persistent stress of chronic ocean warming was a critical analytical focus of the study. By filtering out the episodic fluctuations, the authors elucidated a sustained annual biomass decline hovering near 19.8%. This pervasive decrement acts as a “silent killer” that gradually erodes the resilience and productivity of fish populations, signaling a warning bell for marine ecosystem health and food security worldwide.
The geographic scope of this decline underscores the global scale of climate-induced fish biomass loss, affecting temperate marine environments integral to global fisheries. The persistent warming pressure is not restricted to isolated hotspots but manifests broadly across oceanic regions long exploited for commercial fishing, with the Mediterranean, North Atlantic, and Northeastern Pacific standing out due to their economic and ecological importance.
Conventional fisheries management frameworks, often predicated on static historical baselines, now face unprecedented challenges in accommodating the dynamic realities of climate change. To navigate this shifting landscape, the study’s authors advocate a multifaceted management approach structured around three critical axes: rapid, heatwave-responsive protections; strategic, long-term sustainable planning; and enhanced international cooperation for transboundary species.
Immediate interventions—dubbed ‘climate-ready plans’—aim to address the short-term volatility induced by marine heatwaves. These measures are designed to be deployed swiftly as extreme thermal anomalies arise, safeguarding stressed populations concentrated at their warm range boundaries from precipitous declines or local extinctions triggered by sudden thermal shocks that exceed their physiological capacities.
Yet, short-term measures alone cannot counteract the pervasive downward pressure from gradual ocean warming. Sustainable fisheries management must prioritize adaptive strategies that incorporate predicted biomass declines, adjusting catch quotas and conservation targets accordingly to ensure long-term ecosystem stability. This shift requires recalibrating management objectives to a warming baseline rather than outdated historical norms.
Given that species’ distributions shift poleward or to deeper waters in response to temperature gradients, the study highlights the critical need for multinational coordination in fisheries governance. Populations may dwindle in one nation’s waters while flourishing in neighboring jurisdictions, rendering static political management unfit for a transboundary environmental challenge. International cooperation and joint resource management agreements become essential for conserving migratory and range-shifting species under climate change pressures.
The transient fishing opportunities presented by biomass increases at cold edges, while economically tempting, must be approached with caution. Overexploitation driven by a misreading of ephemeral trends risks undermining long-term resilience. Miguel B. Araújo of the MNCN-CSIC stresses that management frameworks must prioritize resilience building, integrating projected warming impacts to safeguard fish stocks amid escalating climate uncertainty.
This study’s findings carry profound implications for global food security, ecosystem services, and biodiversity conservation. The compounding effects of chronic warming and increasing marine heatwaves necessitate a rapid evolution in marine resource management, predicated on robust scientific monitoring and climate-adaptive governance. As ocean temperatures continue their upward march, embracing flexible, responsive, and collaborative strategies is paramount to sustaining the marine species that millions rely upon for nourishment and livelihoods.
In essence, the research underscores a pressing ecological truth: while heatwaves temporarily distort the fabric of marine biomass, the relentless tide of ocean warming exerts an inexorable pressure driving widespread declines. To preserve marine biodiversity and fisheries into the future, science-informed, climate-resilient management is not merely advisable—it is imperative.
Subject of Research: Animals
Article Title: Long‑term warming reduces fish biomass, but heatwaves shift it
News Publication Date: 25-Feb-2026
Web References: 10.1038/s41559-026-03013-5
Keywords: Marine fishes
