Scientists have discovered that warm-bodied fish like great white sharks and tuna burn nearly four times more energy than cold-blooded species, facing heightened risks as oceans warm. The research highlights a 'double jeopardy' from increased metabolic demands and shrinking food supplies. Large specimens may struggle to maintain stable body temperatures above certain water thresholds.
Researchers from Trinity College Dublin and the University of Pretoria's Faculty of Veterinary Science analyzed mesothermic fish, a rare group comprising less than 0.1% of species, including tunas, great white sharks, and basking sharks up to 3.5 tonnes. These fish retain body heat for advantages like faster swimming and better hunting, but the study, published in Science, shows they use about 3.8 times more energy than similar-sized ectothermic fish after accounting for size and temperature differences. A 10°C rise in body temperature more than doubles their routine metabolic rate, demanding far more food to sustain their lifestyle. Dr. Nicholas Payne, the lead author from Trinity's School of Natural Sciences, stated: '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 fishes.' The team developed a new method using biologging sensors on wild fish to measure heat production and loss in real time, combining this with lab data from smaller species. Larger fish face amplified risks because their bodies generate heat faster than they can dissipate it due to geometry and physics, leading to higher body temperatures over time. Professor Andrew Jackson, senior author, defined 'heat-balance thresholds,' noting a 1-tonne warm-bodied shark may struggle above 17°C waters, forcing behavioral changes like slowing down or diving deeper, which could hinder foraging. Dr. Edward P. Snelling from the University of Pretoria added: 'As the oceans warm, these species are being pushed closer to their physiological limits.' The findings suggest shrinking habitable ranges toward cooler polar regions or depths, compounded by overfishing and prey scarcity amid climate change.