A new study estimates routine energy use across nearly the full size range of living fish species and finds that warm-bodied “mesothermic” fishes consume roughly four times more energy than cold-blooded counterparts and that larger mesotherms face a compounding risk of overheating as ocean temperatures rise.
Most fish are ectotherms: their body temperature matches the surrounding water. A small minority including tunas, some sharks, and billfishes are mesotherms, meaning they retain metabolic heat and maintain body temperatures above ambient water. These species tend to be top predators with outsized ecological roles. Despite that importance, reliable estimates of how much energy large fish actually use in the wild have been scarce, partly because standard laboratory methods (placing animals in sealed chambers and measuring oxygen consumption) don’t work well for animals weighing hundreds of kilograms.
Researchers have long known that body temperature elevates metabolic rate. They also know that mesotherms are faster and more wide-ranging than ectotherms of similar size. It has also been broadly understood that large animals retain heat more easily than small ones, a phenomenon sometimes called gigantothermy. However, what remained unclear was how heat production and heat loss scale together as fish grow larger. It was also uncertain whether this relationship creates practical limits on where large, warm-bodied fish can live.
The research team, led by Nicholas Payne at Trinity College Dublin, developed a method to estimate metabolic rate from temperature data recorded by biologgers attached to free-swimming fish. They fitted a heat-exchange model to simultaneous measurements of body temperature and surrounding water temperature. Using this approach, they could back-calculate the rate of internal heat production. From that, they estimated oxygen consumption, which is a standard proxy for metabolic rate.
To better represent large fish, the team tagged seven free-swimming basking sharks. These individuals ranged from approximately 800 to 3,500 kilograms and were equipped with body and water temperature sensors. The researchers then combined these new estimates with published temperature-biologging data from 10 species. They also conducted a separate literature search that returned 443 metabolic rate measurements from 137 species using conventional respirometry. The combined dataset spans body masses from 1-milligram larvae to fish exceeding 3,000 kilograms. It therefore covers essentially the full size range of living fish.
After accounting statistically for body mass and body temperature, mesotherms had metabolic rates approximately 3.78-fold higher than ectotherms. That gap held consistently across the entire size range studied. The model explained 97% of variance in metabolic rate on average.
The team also compiled 103 measurements of whole-body heat transfer coefficients, which measure how easily heat moves between a fish and the surrounding water, from 19 species. They found that as fish grow larger, their ability to lose heat declines faster than simple geometry would predict. Specifically, the rate is proportional to mass raised to the power of approximately –0.63, which is steeper than the –0.33 expected from surface-area-to-volume reasoning alone.
Heat production scales with mass to the 0.83 power, while heat loss capacity declines more steeply with increasing size. As a result, larger fish generate metabolic heat faster than they can shed it. The researchers quantified this imbalance as a body temperature elevation above the surrounding water. This elevation increases with both body size and thermal strategy. A large mesotherm, therefore, is predicted to maintain a substantially higher body temperature than the surrounding water, even without specialized heat-retention anatomy.
This mismatch also implies upper water temperature thresholds. Beyond these limits, fish of a given size cannot maintain heat balance without behavioral or physiological adjustments. The model estimates that a 500-kilogram mesotherm reaches this threshold at roughly 20°C ambient water temperature. For a one-tonne mesotherm, the threshold drops to around 17°C. In contrast, a two-tonne ectotherm such as a whale shark has a higher estimated threshold of about 27°C. Above these thresholds, a fish would need to reduce swimming speed, increase heat loss, or move to cooler water.
Payne and colleagues argue that this scaling imbalance between heat production and heat loss helps explain long-observed ecological patterns. Mesothermic fish tend to inhabit higher latitudes or greater depths than similarly sized ectotherms. In addition, large fish are generally less common near the equator. The researchers also note that mesothermy in fish appears to have emerged during a period of ocean cooling, which supports their model.
Looking ahead, the team mapped current and projected future sea surface temperatures against their estimated thresholds. Under warming scenarios for 2080–2100, based on the SSP4-6.0 climate pathway and Bio-ORACLE sea surface temperature data, regions exceeding these thresholds expand. This expansion is particularly pronounced during summer months. As a result, large mesotherms are expected to shift toward higher latitudes or deeper waters.
The researchers also point out that some species can partially compensate for these challenges. For example, Atlantic bluefin tuna can significantly increase their thermal conductance when approaching their predicted threshold. However, available evidence suggests this adjustment is not sufficient, as individuals confined to warm surface waters experience high mortality. Basking sharks, despite having mesothermic traits, lack red muscle heat exchangers. They also appear to have unusually high thermal conductance for their size, which may help them cope with heat in warm surface waters.
The heat-balance thresholds derived from the model are described by the researchers as boundaries that “demand physiological or behavioral adjustment” rather than hard lethal limits. Species can and do occupy waters exceeding their modeled thresholds, at least temporarily. The authors explicitly note that predicting which species will shift ranges versus buffer overheating physiologically remains a challenging open question. The role of evolutionary adaptation in future oceans is also flagged as difficult to predict. The dataset, while broad, still has relatively few data points from very large fish a gap the basking shark tagging work was designed in part to address.
Citation:
Payne NL, Snelling EP, Peralta-Maraver I, Cade DE, Chapple TK, McInturf AG, Watanabe YY, Sims DW, Queiroz N, da Costa I, Sousa LL, Goldbogen JA, Dolton HR, Jackson AL (2026). Mesothermic fishes face high fuel demands and overheating risk in warming oceans. Science, 392(6795): 301–306.
DOI: 10.1126/science.adt2981
