This silent threat that is decimating the fish in our oceans

The ocean is warming, and fish are disappearing

The ocean, which covers most of our planet, is home to vast populations of fish. These resources are vital: they feed millions of people and support countless jobs around the world. Yet this balance is now threatened by an unrelenting phenomenon: rising water temperatures.

A new study conducted by Spain’s Higher Council for Scientific Research (CSIC) is sounding the alarm. The researchers’ conclusions are unequivocal: ocean warming is causing a drastic decline in fish populations. This profound change could not only disrupt the marine food chain but also directly impact what ends up on our plates and the survival of fishing communities.

The Constant Pressure of Chronic Warming

To reach this conclusion, the team of scientists conducted a large-scale analysis. They studied changes in fish populations across three key regions: the Mediterranean Sea, the North Atlantic Ocean, and the northeastern Pacific Ocean. In total, 702,037 estimates of biomass change, drawn from 33,990 distinct fish populations, were examined over a period spanning 1993 to 2021.

Biomass is the total weight of living fish within a population. It is a key indicator for experts assessing the health of fish stocks. After a careful analysis of this data, scientists identified a direct link: chronic ocean warming is associated with an annual decline in biomass of up to 19.8%. In some regions, this means that nearly one-fifth of the fish disappear each year.

This warming acts as a slow but steady stressor. Shahar Chaikin, of the National Museum of Natural Sciences, explains: “When we filter out the noise from short-term extreme weather events, the data show that this warming is associated with a sustained annual decline in biomass of up to 19.8%.” Unlike passing storms, this temperature increase persists year after year. Juan David González Trujillo, of the National University of Colombia, adds: “Unlike extreme and brief meteorological fluctuations, which can vary dramatically, this chronic warming exerts constant negative pressure on fish populations in the Mediterranean Sea, the North Atlantic Ocean, and the northeastern Pacific Ocean.”

Heat Waves: A Double-Edged Sword

The phenomenon of marine heatwaves—periods when ocean temperatures rise well above normal levels—is becoming increasingly frequent. But not all fish react to them in the same way. Each species has what scientists call a “thermal comfort zone”—an ideal temperature range for its growth and survival. When the water temperature strays outside this zone, problems begin.

If a heat wave hits waters that are already warm, the effect can be devastating. Fish populations can collapse, with biomass declines reaching up to 43.4%. Fish in these regions struggle to survive in water that has become too warm for them. Conversely, a different scenario unfolds in colder areas. A sudden rise in temperature can bring conditions closer to their thermal ideal. In these specific cases, biomass can experience a dramatic increase of up to 176%.

This explosion of life might seem like excellent news. Scientists warn, however, that this is a short-term illusion. This beneficial effect is only temporary.

The Trap of Short-Term Gains and Food Security

“Although this sudden increase in biomass in cold waters may seem like good news for fisheries, these are fleeting increases,” explains Shahar Chaikin. The researcher cautions against jumping to conclusions: “If managers increase catch quotas based on biomass increases caused by a heat wave, they risk triggering a collapse in populations when temperatures return to normal or when the effects of long-term warming take hold, since these are short-lived increases.” In short, a temporary boom can cause irreversible damage if fishing limits are raised too quickly.

Fish are not merely a component of marine ecosystems. They are a source of food and income for millions of people around the world. The health of fish populations is therefore crucial to global food security. When biomass declines year after year, the fishing industry faces major risks. A sudden surge during a heat wave could lead to increased quotas, resulting in overfishing that could wipe out already vulnerable populations.

“Managers must balance localized increases and long-term declines with extreme caution to avoid overexploitation,” emphasizes Miguel B. Araújo, also of the National Museum of Natural Sciences. “As the oceans continue to warm, the only viable strategy is to prioritize long-term resilience. Management measures must account for the expected decline in biomass in an increasingly warmer ocean.”

Rethinking Fisheries Management in the Era of Climate Change

For the authors of the study, published in the journal Nature Ecology & Evolution, traditional fisheries management models are no longer adequate. Ocean conditions are changing too rapidly. They propose a new approach. First, governments must develop “climate-compatible” plans. When a marine heatwave occurs, immediate protective measures should be triggered to help populations recover.

Second, long-term planning must account for the ongoing decline caused by background warming. Fishing limits should reflect this long-term trend, rather than short-term fluctuations. Finally, international cooperation is considered essential. As temperatures rise, fish migrate to remain within their thermal comfort zone, often crossing national borders.

“The population of a species may decline in one country but increase in another. In this context, static management models are outdated. Effective conservation requires international coordination and joint resource management agreements,” concludes Shahar Chaikin. Countries must collaborate rather than act in isolation. While cold regions may see temporary fishing opportunities arise, focusing on these short-term gains must not distract from the global crisis: chronic warming continues to reduce global fish biomass.

Source: earth.com

This silent threat that is decimating the fish in our oceans