Overseas
Who works at sea already knows that something has changed. The waters feel different, the seasons are less predictable, and some species are behaving in unusual ways. Science is now putting numbers to that perception.
A study by researchers from the Institute of Marine Sciences of the CSIC in Barcelona and the LOCEAN laboratory in Paris, published in Scientific Reports, examines nearly three decades of data from the Western Mediterranean and reaches conclusions the fisheries sector should not ignore. At the heart of the research are marine heatwaves, episodes in which sea temperatures rise abnormally above seasonal averages and remain elevated for days or even weeks.
The research team built a computerized model of the entire Western Mediterranean food web, from microorganisms to top predators, and ran the simulations twice: once including the marine heatwaves recorded between 1995 and 2022, and once without them. Comparing the two scenarios made it possible to isolate the specific effect of these extreme events, separating them from gradual background warming and fishing pressure.
Waters are warming ever deeper
Until a few years ago, marine heatwaves mainly affected the sea surface. Since 2016, however, things have changed. Thermal anomalies have begun reaching depths of 150 meters and even the seabed, often affecting more than 40% of the basin’s total surface area. The year 2022 was the worst in the entire time series. According to the authors, the full effects of that event have not yet fully emerged: fish with long life cycles and species at the top of the food chain may take years to show the consequences of thermal stress.
The south is losing, the north is holding on — for now
One of the study’s most significant findings is the geographic contrast between the northern and southern parts of the basin. Waters in the Gulf of Lion, the Ligurian Sea, and the northern Tyrrhenian Sea show neutral or slightly positive responses to extreme thermal events. Moving south toward the Alboran Sea and the Algerian coast, biomass losses become far more severe across every level of the food chain.
The explanation is not that the north is immune. Rather, species in the southern Mediterranean are already living close to their upper thermal tolerance limits, so every additional heat peak hits them harder. The authors warn, however, that under more extreme scenarios even northern waters could reach temperatures that are currently typical of the south. In other words, the climatic boundary is moving north faster than previously expected.
Benthos, fish and crustaceans: who pays the highest price
Among the hardest-hit categories are benthic producers, including seagrasses, corals and algae, which show an almost continuous decline along much of the western basin’s coastline. For the fishing industry, however, the most concerning issue is the condition of commercially important species.
Pelagic and demersal fish, crustaceans and invertebrates all show reductions in catches that in some areas exceed 10% compared with a scenario without marine heatwaves. Top predators respond more slowly, but in southern areas they have also shown significant declines in the past five years. These impacts do not replace those of gradual warming or fishing pressure. They add to them.
What changes for people who work at sea
Less biomass means less catch. In some parts of the basin, the study reports losses in catches of demersal species, pelagic fish and invertebrates that exceed 10%. These are numbers that can make the difference between a profitable season and a loss-making one.
The authors stress the value of this kind of modelling for management decisions. Knowing in advance which areas and which species are more exposed makes it possible to intervene before the damage becomes irreversible. This is not only a scientific matter. With marine heatwaves becoming more frequent and more intense, reliable ecosystem forecasting tools are becoming essential for the planning of fishing activities across the Mediterranean.
The study is freely accessible in Scientific Reports.
What you need to know
What are marine heatwaves?
They are prolonged periods in which sea surface temperatures rise abnormally above historical seasonal averages for at least five consecutive days. In the Mediterranean, they have increased in frequency, intensity and depth of penetration over the past thirty years.
Which marine species are most affected in the Western Mediterranean?
Benthic producers such as seagrasses and corals show the most consistent decline. Among commercially important species, pelagic and demersal fish, crustaceans and invertebrates record the largest losses, with catch reductions exceeding 10% in southern areas.
Why is the southern Mediterranean more vulnerable than the north?
Species in southern waters are already living close to their maximum thermal tolerance threshold. Each new marine heatwave therefore affects them more severely than species in northern waters, which still retain a wider thermal safety margin.
Do marine heatwaves really affect fisheries?
Yes. The study estimates commercial catch reductions of more than 10% in the most exposed zones, affecting fish, invertebrates and demersal species. These effects add to the impacts of gradual warming and existing fishing pressure.
What does the north-south dipole in the Mediterranean mean?
It is the geographic pattern that emerged from the research: northern waters, such as the Ligurian Sea and the Gulf of Lion, show neutral or slightly positive responses to marine heatwaves, while the south, especially the Alboran Sea and the Algerian coast, experiences significant biomass losses.
