Great white sharks and tuna face overheating risk as oceans warm

Some of the ocean’s most powerful predators, including great white sharks, basking sharks, mako sharks and tuna species, are far more energy-hungry than previously understood and might be pushed closer to their physiological limits as oceans warm.

This is according to a new study published in Science, led by researchers at Trinity College Dublin in collaboration with professor Edward Snelling, an experimental physiologist in the University of Pretoria’s Faculty of Veterinary Science, alongside scientists from institutions in Europe, the US and Japan.

The research shows that warm-bodied fish, known as mesotherms, burn nearly four times more energy than cold-blooded fish of similar size. This group includes species such as great white sharks, basking sharks, mako sharks and tuna. 

The findings suggest they will probably face increasing risk from rising ocean temperatures, including reduced suitable habitat and forced movement towards cooler, polar regions. 

This places them in what the researchers describe as “double jeopardy”: high baseline energy demands combined with a rapidly warming ocean and declining food availability.

Ocean temperatures are warming because greenhouse gas emissions trap heat in the atmosphere, with the oceans absorbing most of the excess heat. 

“This research shows that being a high-performance predator in the ocean comes at a greater cost than we previously appreciated,” Snelling said. “As the oceans warm, these species are being pushed closer to their physiological limits, which could have consequences for where they can live and how they survive.”

Mesothermic fish are rare, representing fewer than 0.1% of all fish species. They can retain metabolic heat, keeping parts of their bodies warmer than surrounding seawater. 

The evolutionary adaptation, which has arisen independently in some sharks and tuna, supports faster swimming, long-distance migrations and more efficient hunting.

The researchers note how body size and temperature set metabolic rates and the pace of life, “yet our understanding of the energetics of large fishes is uncertain, especially of warm-bodied mesotherms, which can heavily influence marine food webs”. 

To quantify the energetic cost of the lifestyle, they developed a new method to estimate metabolic rates in free-swimming fish. 

Using biologging devices that record body and water temperatures, they calculated heat production and loss in real time. The data, including measurements from large basking sharks weighing up to 3.5 tonnes, were combined with hundreds of laboratory measurements from smaller species.

The results were striking, noted lead author Dr Nicholas Payne of Trinity’s School of Natural Sciences. “After accounting for body size and temperature, we found that mesothermic fish use about 3.8 times more energy than similarly sized ectothermic fish. 

“A 10°C increase in body temperature more than doubles their routine metabolic rate, meaning these predators must consume far more food to sustain themselves.”

But energy demand was only part of the problem. As fish grew larger, their bodies generated heat faster than they could dissipate it.

“This creates a mismatch driven by basic physics — larger bodies retain heat more effectively, and in mesotherms, high metabolic rates amplify this effect,” Payne explained.

The scaling imbalance meant larger fish became increasingly warm-bodied, creating a risk of over-eating that had important ecological consequences.

The team used the data to model “heat-balance thresholds” — the water temperatures above which large fish could no longer shed heat efficiently enough to maintain stable body temperatures without changing behaviour or physiology.

“Our models show that a typical one-tonne mesotherm has a threshold of about 17°C,” said the study’s senior author, professor Andrew Jackson of Trinity College Dublin. “Above this, the animal must take countermeasures to avoid overheating.”

The responses might include moving into deeper, cooler waters, altering blood flow or avoiding warmer regions altogether. While overheating might not be immediately lethal, it forces fish to increase food intake significantly to cope with the metabolic cost.

The findings help explain long-observed patterns in the oceans, where large predatory fish were more commonly found in cooler waters, at higher latitudes or at depth, and often migrate seasonally in search of optimal temperatures.

Under future warming scenarios, the researchers predict that suitable habitat for large mesotherms will shrink, particularly during warmer months. Even highly mobile species such as Atlantic bluefin tuna, which can temporarily regulate heat loss or dive to cooler depths, might eventually reach their limits if surface temperatures continue to rise.

The implications were sobering, Payne said. “Many mesotherms are already heavily affected by over-fishing of both themselves and their prey, so their high energy demands make them especially vulnerable when food becomes scarce.”

Snelling added: “What’s particularly concerning is that these animals are already operating on a tight energy budget and climate change is narrowing their options even further. Understanding these constraints is essential if we want to predict how marine ecosystems will shift in the coming decades.”

The study provides a new framework for assessing vulnerability in marine predators, suggesting that some of the ocean’s fastest and most formidable hunters might also be among its most physiologically constrained. As climate change accelerates, understanding the hidden energy budgets of marine giants could prove critical to their conservation.