The Marine heatwaves are changing the western Mediterranean at a speed that seemed unthinkable a few years ago. sustained increase in water temperature It is no longer limited to isolated summer episodes: it has become a constant pressure that reorganizes the ecosystem and directly affects sectors such as fishing.
According to a broad international investigation led by the Institute of Marine Sciences (ICM-CSIC) A study by the French laboratory LOCEAN-IPSL, published in the journal Scientific Reports, found that over 60% of the western Mediterranean's surface area has been affected by marine heatwaves in the last decade. Scientists warn that these extreme events are no longer the exception but have become part of the new climate normal for the sea.
An extreme phenomenon that is becoming routine in the western Mediterranean
Marine heatwaves are defined as prolonged periods in which the sea temperature clearly exceeds normal values for a certain area and time of year. In the western Mediterranean, this situation is occurring more and more frequently, for longer periods of time, and over a greater spatial extent, both at the surface and in deeper layers.
The study confirms that, in recent years, these warm spells have not only spread horizontally, covering large areas, but have also gained depth and thermal intensityThis increases the volume of water affected and amplifies the impacts on organisms living from the surface to the seabed.
According to data collected by the international team, more than 60% of the western Mediterranean It has been subjected to heat waves in the last decade. This percentage reflects a structural change in the behavior of the sea, which no longer responds to the known pattern of natural variability, but rather adjusts to a context of accelerated global warming.
The researchers also emphasize that the heat waves act in parallel with other historical pressures in the region: overfishing, pollution and intensive use of marine spaceThe combination of all these factors weakens the ability of ecosystems to recover, reducing their resilience to new disturbances.
Thermal inequality between the north and south of the western Mediterranean
The warming is not distributed evenly. Models and observations used by the ICM-CSIC and LOCEAN-IPSL show clear differences between the northern and southern areas of the western Mediterranean, both in temperature anomalies and in the maximum temperatures recorded.
In the northern strip, closer to the European Mediterranean coast, the following are detected: greater temperature increases compared to historical average valuesIn other words, the water there is being diverted more than would be expected based on records from previous decades, indicating an intensification of relative warming.
In contrast, in the southern western Mediterranean —with key areas such as the Alboran Sea and the Algerian Sea—, heat waves reach the highest absolute temperatures. In these areas, the sea reaches especially high levels, generating very severe heat stress for many marine species, including those of commercial fishing interest.
This uneven distribution greatly complicates environmental management. Biological responses and impacts on biodiversity are not the same in all regions, so Conservation and fishing measures must be adapted to each area and they cannot be considered as a single, homogeneous solution for the entire basin.
Scientists add that this spatial variability necessitates improvements to monitoring systems and strengthened international cooperation in the Mediterranean, where ocean currents and species migrations They make it so that what happens on one coast has repercussions, sooner or later, on the others.
An ecosystem that responds at different speeds
One of the most striking aspects of the work is that it shows how The marine ecosystem does not react uniformly in the face of accelerated warming. Each group of organisms responds at different rates and timescales, which complicates the assessment of the real impact of heat waves.
Species at low trophic levels, such as phytoplankton and zooplanktonThey are usually small and have short life cycles. This means their populations react quickly to temperature changes, with increases or decreases occurring in a matter of weeks or months. These early alterations can trigger cascading effects on the upper links of the food chain.
Conversely, organisms at higher trophic levels—larger fish, seabirds, and other predators—exhibit slower growth and longer lifespans. In these cases, The effects of heat waves can take years or even decades to become visible.which gives the false impression that the system holds up better than it can actually withstand.
The researchers describe the process as a “reconfiguration” of the ecosystem, resulting from direct effects from heat stress and indirect impacts resulting from changes in interactions between speciesIf plankton is altered, for example, the availability of food for small fish changes, and this ends up affecting top predators.
This dynamic causes much of the damage to accumulate silently. The marine system appears stable for a time, but in reality It is losing resilience year after year until certain thresholds are exceeded and abrupt changes occur in abundance, species distribution, or fishery productivity.
Direct impact on commercial species and fishing
Beyond the ecological effects, the research pays special attention to the consequences for commercially valuable species And for the fishing economy of Spain and the rest of the western Mediterranean. Warming waters are not an abstract issue: they translate into smaller catches and greater uncertainty for those who work at sea.
Among the most closely monitored species are the hake and sardineThese species have historically been mainstays of fishing in many Mediterranean fleets. The study indicates that their populations could decline more sharply than previously thought, due to a combination of heat stress, changes in food availability, and other pressures such as overfishing.
The increase in temperature alters the survival, growth, and reproductive success of these fish, and can modify their distribution patterns. Some populations move into deeper or colder waters, forcing fleets to change fishing grounds or assume higher costs to maintain catches.
This scenario poses a direct threat to the local and regional fishing economyThe sector was already facing challenges such as quota regulations, rising fuel costs, and competition from other uses of marine space. If available biomass continues to decline, it will be more difficult to sustain current catch levels and the income of many coastal communities.
Experts also warn that heat stress and disruptions to the food chain are not limited to just a few emblematic species. The entire fishing system becomes more fragile and vulnerableThis increases the risk of local collapses, temporary closures of fishing grounds, or drastic changes in the fleet's target species.
Combined pressures that accelerate the transformation of the sea
The work led by ICM-CSIC and LOCEAN-IPSL insists that marine heat waves They cannot be analyzed in isolationIts impact is multiplied when combined with other pressure factors that have been deteriorating the western Mediterranean for years.
Among these pressures, the following stand out: historical overfishingThis has reduced the populations of numerous species; pollution from industrial, agricultural, and urban activities; and the increasing use of marine space for transport, tourism, and coastal infrastructure. Each of these forces pushes the ecosystem in the same direction: less capacity for recovery.
Heat waves act as a accelerator of deterioration processes already underwayWhere already there were diminished populations or degraded habitats, an extreme episode of high temperatures can cause mass mortalities, displacement of species or the expansion of opportunistic organisms, such as some invasive species and potentially harmful microorganisms.
Scientists point out that this cocktail of pressures is transforming the western Mediterranean at a rate that exceeds the natural capacity of many organisms to adapt. Corals, sponges, mollusks, and other invertebrates Heat-sensitive species are especially vulnerable, and their decline profoundly alters food chains and the structure of marine habitats.
In this context, marine heatwaves cease to be mere extreme weather events and become a structural component of the new Mediterranean climate, with direct implications for biodiversity, fishing and the well-being of human communities that depend on the sea.
Management challenges and measures to strengthen resilience
Given this scenario, the study raises the need to rethinking fisheries and environmental management in the western Mediterranean, explicitly incorporating marine heatwaves into planning for the coming decades.
The researchers propose a series of measures aimed at to increase the resilience of the ecosystem and human activities linked to the sea. Among them, strengthening the protection of key marine areas, where sensitive habitats or essential breeding grounds for many species are concentrated.
They also consider it a priority reduce overfishing Through more cautious resource management, adapting quotas, seasons, and fishing gear to the new warming context, the aim is to prevent the most vulnerable species from bearing the brunt of the impact.
In parallel, an additional effort is being called for reduce pollution which reaches the sea from land and from maritime activities themselves. Less chemical and physical pressure on the ecosystem can translate into a greater capacity to recover from heat waves.
The scientific community insists that integrating these extreme events into decision-making will allow anticipate risks and reduce vulnerabilitiesInstead of reacting only when the damage is already evident. In a sea as densely populated and exploited as the Mediterranean, anticipation can make the difference between a system that adapts and one that enters a spiral of continuous degradation.
Everything points to the The future of the western Mediterranean will be marked by more frequent and severe marine heatwaves.These events are closely linked to the advance of climate change. Science has already made it clear that these events affect more than half of the ecosystem, reorganize food chains, put pressure on key species such as hake and sardines, and test the fishing economy; what happens in the coming years will depend, to a large extent, on the collective capacity to reduce emissions, alleviate local pressures on the sea, and implement management policies that give the ecosystem a real opportunity to adapt.
