Today, 22 April is Earth Day.

New research by scientists at Trinity College Dublin in collaboration with the University of Pretoria’s (UP) Faculty of Veterinary Science  focuses on “mesothermic” fishes.

Warm-bodied fish such as tunas and some sharks, including the legendary Great White and Ireland’s iconic basking shark, burn nearly four times more energy than their cold-blooded counterparts. This means they are likely to face an increasing risk of overheating as oceans warm, which may result in a reduction of suitable habitat and an enforced relocation towards the poles.

Mesothermic fishes are a rare group comprising fewer than 0.1% of all fish species, which can retain metabolic heat and keep parts of their bodies warmer than the surrounding seawater. This ability has evolved independently several times in some sharks and tunas, enabling higher swimming speeds, long-distance migrations, and enhanced predatory performance.

The researchers analysed biologging data—from tiny sensors that record body and water temperature in the fish. The team calculated how much heat fish produce and lose in real time. They combined these new measurements, including data from huge basking sharks weighing up to 3.5 tonnes, with hundreds of lab measurements from smaller species.

Dr Nicholas Payne, from Trinity’s School of Natural Sciences explained:

“The results were really quite striking – after accounting for body size and temperature, we found that mesothermic fishes use about 3.8 times more energy than similarly sized ‘ectothermic’, or ‘cold-blooded’ fishes. In addition, a 10°C increase in body temperature more than doubles a fish’s routine metabolic rate which, in practical terms, means warm-bodied predators must consume far more food to fuel their lifestyle.”

“But that heighted energy demand is only part of the story because as fish grow larger their bodies generate heat faster than they can lose it.

“This creates a mismatch driven by basic geometry and physics because bigger bodies retain heat more effectively, and in mesotherms, high metabolic rates amplify this effect.” 

The team found that larger fish become increasingly “warm-bodied” simply because of this imbalance, and it is this scaling mismatch that creates an overheating dilemma with significant implications for these species. 

Professor Andrew Jackson from Trinity’s School of Natural Sciences  added:

“Based on the data we were able to create theoretical ‘heat-balance thresholds’, which are the water temperatures above which large fish cannot shed heat quickly enough to maintain stable body temperatures without changing their behaviour or physiology. For example, a 1-tonne warm-bodied shark may struggle to remain in heat balance in waters above about 17°C.” 

“Above such thresholds, fish must slow down, alter blood flow, or dive into cooler depths to avoid dangerous warming but that comes at a cost too; it might be harder to find food, or catch it, for example – especially if your main weapon is speed and power.” 

These findings seemingly help to explain long-observed patterns in the ocean, where large fishes tend to occur in cooler waters, at higher latitudes, or at greater depths. They also migrate seasonally, tracking favourable temperatures. 

Unsurprisingly, the scientists predict that under future warming scenarios suitable habitat for large mesotherms will shrink, and particularly so during summer months. And while some species, such as Atlantic bluefin tuna, can temporarily increase heat loss or dive to cooler waters, even they may be pushed to their limits if surface waters continue to warm.

Dr Snelling, University of Pretoria commented:

“This research shows that being a high-performance predator in the ocean comes at a greater cost than we previously appreciated. 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.”  

“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 implications are really sobering as this new finding essentially places these animals in ‘double jeopardy,” added Dr Payne.

“Many mesothermic fishes are already heavily impacted by overfishing of themselves and also their  prey species, so their elevated energy needs make them especially vulnerable when their food becomes scarce.” 

“Fossil evidence suggests that warm-bodied marine giants, like the infamous extinct Megalodon shark, suffered disproportionately during past climate shifts when seas changed and today’s oceans are changing at unprecedented speeds, so the alarm bells are ringing loudly at this point.”

What are the potential implications of this research?

Ultimately this crucial research provides a new framework for predicting which species are most at risk in a warming world and shows that many of the ocean’s fastest and most formidable predators may also be among its most physiologically constrained. As climate change accelerates, understanding the hidden heat budgets of marine giants could prove critical to conserving them.

Click on this link to access, Mesothermic fishes face high fuel demands and overheating risk in warming oceans, published in Science.