Sharks and tuna are overheating and running out of options

The research, led by scientists at Trinity College Dublin in partnership with the University of Pretoria's (UP) Faculty of Veterinary Science, shows that certain warm-bodied fish such as tunas and some sharks, including the Great White and the basking shark, use nearly four times more energy than cold-blooded fish of similar size. As ocean temperatures climb, these species are more likely to overheat, which could shrink their habitable range and push them toward cooler regions closer to the poles.

What Makes "Mesothermic" Fish Unique

The study, published in the journal Science, focuses on "mesothermic" fishes. These are a rare group, making up less than 0.1% of all fish species, that can retain body heat and keep parts of themselves warmer than the surrounding water. This adaptation has evolved independently in several shark and tuna species, giving them advantages such as faster swimming, long-distance travel, and improved hunting ability.

To better understand the cost of this high-performance lifestyle, the researchers developed a new method to estimate metabolic rates in fish swimming freely in the wild. They used biologging data collected from small sensors that record body and water temperatures. This allowed them to calculate how much heat the fish produce and lose in real time. The team combined these findings, including data from massive basking sharks weighing up to 3.5 tonnes, with hundreds of laboratory measurements from smaller species.

Dr. Nicholas Payne from Trinity's School of Natural Sciences, the study's first author, said: "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."

Why Bigger Fish Face Greater Heat Risks

Dr. Payne explained that higher energy use is only part of the problem. "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," he said. "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."

Because of this imbalance, larger fish naturally become more "warm-bodied" over time. This scaling effect creates a growing risk of overheating, which has important implications for how these species survive and where they can live.

Heat Limits Could Restrict Habitat

Professor Andrew Jackson, senior author of the study, said the team used the data to define "heat-balance thresholds." These thresholds represent the water temperatures at which large fish can no longer release heat quickly enough to maintain stable body temperatures.

"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," he explained.

"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."

Shifting Habitats and Migration Patterns

These findings help explain why large fish are often found in cooler waters, at higher latitudes, or deeper in the ocean. Many species also migrate seasonally to stay within favorable temperature ranges.

Looking ahead, the researchers expect that suitable habitats for large mesothermic fish will shrink as global temperatures continue to rise, especially during warmer months. Some species, such as Atlantic bluefin tuna, can temporarily cope by diving deeper or increasing heat loss, but even these adaptations may not be enough if surface waters keep warming.

Climate Change Is Tightening the Pressure

Dr. Snelling from UP emphasized the growing challenge: "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."

He 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."

A Double Threat for Ocean Giants

"The implications are really sobering as this new finding essentially places these animals in 'double jeopardy," said 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."

He also pointed to lessons from the past. "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."

Why This Research Matters

This study offers a new way to predict which marine species are most at risk in a warming world. It highlights that many of the ocean's fastest and most formidable predators may also be among the most physically constrained. As climate change accelerates, understanding how these animals manage heat could be key to protecting them and preserving marine ecosystems.