A study published in Science found that large, partially thermophilic fish, including white sharks and tuna, consume almost four times as much energy as similarly sized warm-blooded fish. As sea temperatures rise, their evolutionary advantage could become a physiological limitation.
Some of the ocean’s most impressive predators, including great white sharks, basking sharks and tuna, face a previously underestimated problem: their bodies heat up faster than they can easily dissipate heat to the environment. New research led by researchers from Trinity College Dublin and in collaboration with the University of Pretoria shows that large, partially warm-blooded fish, known as mesothermic fish, pay a heavy energetic price for their fast, powerful lifestyles. The study was published in the journal Science on 16 April 2026 (.)science.org
Mesothermic fish are a very rare group, less than 0.1% of all fish species. They are not warm-blooded like mammals, but they are able to keep parts of their bodies warmer than the surrounding seawater. This trait has evolved several times independently in sharks and tuna, and it allows them to swim faster, migrate longer distances, and hunt more efficiently. But now it turns out that the same trait that gives them a competitive advantage also creates a growing limitation in the era of the Climate crisis. (tcd.ie)
Researchers have developed a method for estimating the metabolic rate of fish swimming freely in the ocean. They used data from tiny sensors that recorded body temperature and water temperature, and combined it with laboratory data from hundreds of fish species. This allowed them to calculate in real time how much heat the fish were producing and how much heat they were losing to the environment. The data also included giant sharks weighing up to 3.5 tons.
Partially warm-blooded fish consume 4 times more energy
The key finding was stark: Mesothermic fish consume almost four times as much energy as similarly sized warm-blooded fish. In addition, a ten-degree Celsius increase in body temperature more than doubles a fish’s normal metabolic rate. This means that relatively large, warm-blooded predators need much more food to maintain their performance. When food is reduced, due in part to overfishing and damage to marine food webs, they enter what the researchers call a “double whammy”: more energy needs, and fewer opportunities to obtain it.
The problem gets worse the bigger the fish. According to the researchers, large bodies generate heat at a faster rate than they can dissipate it. Therefore, large fish actually become “hotter” as they grow. The study points to a theoretical heat threshold: A mesothermal shark weighing one ton may have difficulty maintaining heat balance in water above about 17 degrees Celsius unless it changes its behavior or physiology. Such a change could include slowing down its swimming, moving to cooler depths, or changing its blood flow. But any such solution comes at a cost: at depth or in colder areas, there may be less food, and the ability to hunt quickly and powerfully will be impaired.
Diving deep to cool down
The finding explains why such large species are often found in cooler waters, at higher latitudes or at greater depths. It may also explain some of their seasonal migration patterns. As oceans warm, their suitable habitats may shrink, especially in summer. Some species, such as Atlantic bluefin tuna, can temporarily dive to cooler depths or increase heat loss, but even such strategies are not a long-term solution if warming continues.
The researchers emphasize that this is not about all sharks or all fish, but rather a specific group of marine predators with partial body heat. However, this group specifically includes species that are important from an ecological and economic perspective. They are at the top of the food web, affect populations of other species, and play a central role in maintaining the balance of the marine system. Therefore, harming them may also affect broader ecosystems.
The study adds an important layer to our understanding of the vulnerability of marine apex predators. It was already clear that overfishing, prey damage, and changes in distribution threaten sharks and tuna. Now it turns out that even the simple physics of body heat, body size, and water temperature may set a limit for them. For animals that evolved to be fast, strong, and efficient, that same biological engine could become a burden in a warmer ocean.
for the scientific article DOI: 10.1126/science.adt2981
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