A study by the ICM-CSIC and the UdG warns that young individuals of this key species have lost nearly 60% of their energy reserves over the last two decades.
The increase in water temperature is transforming marine ecosystems and is already directly hitting one of the most common species in our cuisine: the blue whiting (Micromesistius poutassou). A new study by the Institute of Marine Sciences (ICM-CSIC) and the University of Girona (UdG) shows that juvenile blue whiting have lost nearly 60% of their energy reserves over the last two decades.
The findings, published in the scientific journal Marine Environmental Research, warn that the warming of the Mediterranean is compromising the recovery capacity of this species, which is important both for the marine food web and for fisheries along the Catalan coast.
A key species for Catalan fish auctions
Blue whiting is an affordable and common fish in Catalonia's fishmongers which, historically, has been caught by the trawling sector (as it inhabits depths between 100 and 550 meters). From an ecological perspective, it is a key species in the ecosystem, as it transfers energy from plankton to higher trophic-level predators, such as fish, squid, and marine mammals. Despite its relevance, catches in Catalonia have plummeted in recent decades, causing great concern within the fishing sector.
To assess the health of the population, the scientific team combined two complementary indicators: fatty acid profiles in the muscle, which reveal what they eat, and the hepatosomatic index, which measures the fat and nutrients stored in the liver (their energy "battery"). Comparing data from the early 2000s with the most recent period (2017-2022) revealed that the fat of young blue whiting has drastically decreased by 58%, while that of adults has remained stable. This weakness in the early stages is associated with persistent water warming in the northwestern Mediterranean and translates into lower productivity for the species.
Juveniles, the most vulnerable stage
The main reason for this drop in energy reserves lies in a decrease in the productivity of the Mediterranean, which affects plankton—the primary food source for juvenile blue whiting.
"Juvenile and adult individuals live in different areas and have different diets. The young rely entirely on small planktonic organisms that are extremely sensitive to rising temperatures. Water warming in the northwestern Mediterranean is causing a drop in both the quality and quantity of this plankton," explains Mar Vila, a predoctoral researcher at the UdG and lead author of the study.
In contrast, adult blue whiting feed on larger, fat-rich prey, making them more resilient. "The problem is that if the young do not have enough food to grow healthily, the population replenishment is at risk," Vila adds.
"These results align with the sharp decline in catches in Catalonia that we had already documented in previous research by the ICM-CSIC," points out Joan Mir (ICM-CSIC), co-author of the study.
Global warming acts in synergy with fishing pressure, making the recovery of the species even more difficult. For his part, Josep Lloret, also a co-author on behalf of the ICM-CSIC, highlights that this work demonstrates the importance of using biochemical analyses and health indicators to "early detect the impact of climate change and design a more flexible fisheries management adapted to the current challenges of overfishing and climate change."
Ultimately, the results of the study show that protecting fish stocks can no longer rely solely on catch control; it is also essential to understand how climate alters the biology of species. Only by integrating these health indicators into conservation policies will it be possible to design truly effective management strategies that guarantee, in the long term, both the ecological richness of the Mediterranean and the viability of fishing.