A great white shark chasing a seal generates so much internal heat that its core muscles can run 14°C warmer than the surrounding ocean. That thermal engine, shared by shortfin makos and bluefin tuna, belongs to fewer than 0.1% of all fish species on Earth. It is the reason these predators can sprint, dive deep, and hunt across vast stretches of open water. And according to a study published in Science in April 2026, it is becoming a liability.
The paper, led by researchers at the University of Oxford, finds that these warm-bodied fish, known as mesotherms, burn energy at roughly four times the rate of comparably sized cold-blooded species. In cooler oceans, that metabolic premium bought dominance. In warming ones, it is pushing the animals toward a physiological wall: they produce heat faster than they can dump it into the water around them, and the bigger the animal, the worse the problem gets.
The geometry of overheating
The core issue is one of scaling. As any animal grows, its volume (and therefore its heat output) increases faster than its surface area (and therefore its ability to shed that heat through gills and skin). For most fish, this barely matters because they do not generate much internal warmth to begin with. Mesotherms are different. Their specialized countercurrent heat exchangers trap metabolic warmth in red muscle, viscera, and, in some sharks, cranial tissue. That retained heat is what lets a bluefin tuna sustain cruising speeds above 40 mph or a mako shark hunt in frigid water hundreds of meters below the surface.
But the system depends on a temperature gap between the animal’s body and the ocean. When ambient water warms, that gap narrows. The fish still produces the same heat, but it has less thermal headroom to work with. The Oxford team’s models show that aerobic performance and sustained swimming capacity erode as the gap closes, and that the largest individuals in any mesothermic species hit the danger zone first. A juvenile mako with a relatively high surface-area-to-volume ratio can still cool itself effectively in moderately warmed water. A fully grown adult cannot shed heat as efficiently, and a prolonged chase or a long migration could tip its internal temperature into damaging territory.
Species already under pressure
The timing is difficult. Several mesothermic species are already contending with intense human pressure that has nothing to do with temperature. The shortfin mako is classified as Endangered on the IUCN Red List, driven largely by overfishing and bycatch in longline fisheries. Atlantic bluefin tuna were recently downlisted to Least Concern after decades of strict quota management, but that recovery is fragile and depends on continued enforcement. Great white sharks remain listed as Vulnerable. For all three groups, thermal stress from warming oceans is not replacing existing threats. It is compounding them.
Oceanographic data reinforce the concern. Multi-decade records maintained by NOAA through products like the World Ocean Atlas show sustained warming in both surface and subsurface waters across the ranges these predators use. The warming is not uniform. Certain regions, particularly parts of the North Atlantic and the Mediterranean, have seen sharper increases that overlap with critical bluefin tuna spawning and feeding grounds. While no published analysis has yet mapped the Oxford team’s heat-budget models directly onto these spatial temperature records, the direction is clear: the thermal envelope that supports large mesothermic predators is shrinking.
What scientists still do not know
The Science paper establishes the biophysical mechanism convincingly, but several questions remain open. The most pressing is whether the largest great whites, makos, or bluefin tuna are already showing measurable declines in swimming performance, body condition, or foraging success tied to rising temperatures. That kind of evidence would require fisheries-independent tagging data linked to specific thermal thresholds, and no such synthesis has been published as of June 2026.
Behavioral flexibility is another unknown. Mesothermic predators might respond to warming by shifting poleward, diving to cooler depths, or adjusting migration timing. Some tagging studies have already documented range shifts in white sharks along the U.S. East Coast and in bluefin tuna across the North Atlantic, but attributing those movements specifically to overheating risk, rather than prey availability or other factors, remains difficult. Such adjustments could buy time, but they carry costs: unfamiliar waters may offer less prey, and new migration corridors may increase encounters with fishing gear.
Evolutionary adaptation is theoretically possible but unlikely to keep pace. Selection could favor individuals with lower metabolic rates, smaller maximum body sizes, or more efficient heat-exchange structures. The problem is that mesothermic species tend to be long-lived and slow to reproduce. A female white shark may not reach sexual maturity until her early 30s. A bluefin tuna can live past 40. That generational pace leaves little room for rapid genetic change in a climate shifting on decadal timescales.
Separate peer-reviewed work on mako thermoregulation and tuna muscle physiology, cataloged through the National Library of Medicine, confirms the underlying biology: these animals depend on elevated tissue temperatures for peak performance. But those studies were not designed to project population-level consequences under specific warming scenarios, so they support the Oxford model’s logic without independently validating its predictions.
Why the biggest animals disappear first
If the Oxford team’s framework holds, the practical consequence for fisheries managers and conservation biologists is that size-based monitoring may detect trouble before traditional abundance surveys do. The largest individuals in a population face the steepest version of the overheating problem, so they should be the first to lose performance, reduce reproduction, or vanish from catch records. A decline in maximum observed body size, a shift toward younger age classes in commercial catches, or changes in the timing of seasonal migrations could all serve as early warnings that a mesothermic population is approaching its thermal ceiling.
That matters for management. Bluefin tuna quotas, for example, are set based on stock assessments that track total biomass and recruitment. If warming selectively removes the largest, most fecund adults from the population, reproductive output could drop even while overall numbers appear stable. For mako sharks, already depleted by fishing, the loss of large breeding females to thermal stress could accelerate a decline that current bycatch reduction measures are struggling to reverse.
A metabolic strategy meeting its limits
Mesothermy evolved over tens of millions of years in oceans that were, on average, cooler and more thermally stable than what these animals face today. The trait is a genuine marvel of vertebrate biology: a way for a fish to function almost like a warm-blooded mammal, sustaining power and speed in frigid open water where cold-blooded competitors cannot follow. The Oxford study does not predict the imminent collapse of every warm-bodied shark and tuna. What it does is identify a fundamental constraint that will tighten as oceans continue to warm, one rooted not in ecology or fisheries policy but in the geometry of heat and flesh.
For the great whites patrolling seal colonies off South Africa, the makos slicing through the Gulf Stream, and the bluefin tuna crossing the Atlantic to spawn in the Mediterranean, the engine that made them apex predators is starting to run too hot. How fast that matters, and whether anything can be done about it, depends on decisions that have not yet been made, both about carbon emissions and about how we manage the last strongholds of these extraordinary animals.
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*This article was researched with the help of AI, with human editors creating the final content.
