Marine heatwaves cause an annual decline of almost 20% in fish biomass

This study reveals a dynamic that should concern us from a governance perspective: long-term warming reduces fish biomass, while heat waves can generate temporary increases that mask the underlying trend. This combination introduces a clear risk of misinterpretation in decision-making.

Temporary gains may encourage increases in fishing effort or delay management measures, when in reality the system is losing structural productive capacity. The problem is not only extreme variability, but also confusing specific episodes with lasting changes.

In a context of accelerated climate change, policies cannot react solely to extreme events or be based on short-term signals. They need consistency between science, planning and governance, especially in shared ecosystems or on the high seas.

Ultimately, the challenge is not only to measure biomass better, but to prevent transient signals from distracting us from the structural transformations of the ocean-climate system.

I believe this is a methodologically sound and valuable study that provides valuable evidence on how different components of ocean warming affect fish biomass. The work is particularly important because it confirms a message already well supported by the literature: chronic ocean warming is associated with persistent declines in biomass.

These results are consistent with existing physiological knowledge. Rising ocean temperatures and their associated effects (such as reduced dissolved oxygen, stratification, and changes in productivity) have well-documented impacts on the metabolism, growth, and reproductive success of many marine species.

That said, there are important nuances that require further investigation. Changes in observed biomass may reflect not only actual variations in abundance, but also spatial redistributions of populations across their range, a process widely documented in warming marine systems. Distinguishing between displacement and net changes in biomass will remain a key scientific challenge, and I believe the spatial modelling tools exist to attempt to predict these distributional shifts.

It is positive and rigorous that the authors themselves acknowledge that ‘fully disentangling the impacts of fishing from those of climate drivers is beyond the scope of this study.’ At the same time, it underscores the need to avoid a simplistic narrative that attributes biomass changes exclusively to climate change. Although this is not the authors' objective, there is a risk, in my opinion, that climate change will become the main explanation for changes in marine species biomass, leaving aside overfishing. Historically, overfishing has been the main determinant of biomass declines in many fisheries around the world. According to the FAO, the proportion of overexploited stocks globally continues to increase, indicating that fishing pressure remains a dominant risk factor. The current challenge is that this overfishing crisis is being further exacerbated by ocean warming and deoxygenation.

In terms of public policy, the study is highly relevant because it emphasises that fisheries management systems must become more climate-adaptive. I agree with the authors that there is a real risk of overexploiting temporary increases in biomass associated with warm events if catch limits do not explicitly incorporate climate variability. However, any management reform must simultaneously address both drivers of change: climate and fisheries. Adjusting quotas solely on the basis of climate without reducing overcapacity and the impact of high-impact gear, such as bottom trawling, is likely to be insufficient to recover stocks.

In conclusion, I believe this study represents an important contribution to advancing more dynamic, ecosystem-based approaches to fisheries management in a changing ocean. Its results should be interpreted within the broader context of the global overfishing crisis, which remains and will likely continue to be a determining factor in the health of marine populations in an increasingly warmer and less oxygenated ocean.

This study synthesises a huge amount of data collected during decades of research on fishery resources in the northern half of the Atlantic Ocean. The results are presented clearly and conceptually through a series of highly illustrative figures, although behind this apparent simplicity lies a remarkable effort of data collection, standardisation and analysis: more than 123,000 samples taken between 1993 and 2021.

Obtaining robust and comparable time series on a large scale is extraordinarily complex in marine ecology, so the value of this work lies not only in the magnitude of the dataset, but also in its methodological consistency over time. This provides solid support for its conclusions on how progressive warming reduces total fish biomass, while marine heatwaves have varying effects depending on the region and species in question. Furthermore, its results are consistent with trends previously described on a smaller scale and consolidate the accumulated evidence on the effects of climate change on marine ecosystems.

As with any scientific study, there are limitations that must be taken into account. In the case of predictive models, it is not always possible to incorporate other natural and anthropogenic factors that also influence biomass, such as fishing pressure or other ecosystem changes. Furthermore, the data comes from scientific trawling campaigns, which means that the conclusions mainly refer to the fish communities associated with the sampled habitats, leaving out part of the coastal communities and other environments not covered by this methodology.

Overall, this is a highly reliable study that clearly contributes to unravelling the changing trends of numerous fish stocks in the face of rising sea temperatures. Its implications for management are clear: the authorities responsible for regulating fishing effort should incorporate this type of evidence into their decision-making frameworks, promoting more flexible management models with greater capacity to respond to climatic anomalies such as marine heatwaves, the frequency and intensity of which are expected to increase in the coming decades. Persisting with static exploitation schemes in the face of such a dynamic climate system would pose a growing risk to long-term sustainability.