Great white sharks are overheating

The evolutionary edge that fueled the great white’s dominance for millions of years could soon become its greatest undoing.

The ocean’s most famous predators maintain a warmer body temperature than the surrounding seawater, and they are paying an ever-increasing price for it. As it is oceans warm due to climate change, they now face the risk of potentially fatal overheating, according to a new report in the journal Science.

Several large species of tuna and shark, known as “mesothermic” species because of the way their bodies heat up, require more fuel to maintain their temperature and therefore face the “double jeopardy” of warming oceans and declining food, mainly due to overfishing. As water temperatures rise, these species will be forced to move to cooler waters.

“If you’re a shark, you can’t just go to the supermarket and buy more food,” said Nick Payne, lead author and associate professor at Trinity College Dublin, Ireland. “We see animals moving with climate change in every biome on land and in the sea; this is just another example of that mechanism.”

From South Africa’s mighty great whites to Ireland’s filter-feeding sharks, these mesotherms expend nearly four times as much energy as their cold-blooded counterparts, whose body temperatures match the surrounding water. As the oceans warm, these species must slow down, change their blood flow, or dive to cooler temperatures, all in search of dwindling food supplies.

A rare group comprising less than 0.1 percent of all marine animals, mesothermic fish—also including thresher sharks and sharks—trap metabolic heat to keep their bodies warmer than the surrounding seawater. This was evolutionarily key to allowing faster swimming speeds, improved predation and long-distance migration.

However, as fish grow, their bodies generate heat faster than they can shed it. This mismatch – caused by surface physics and heat retention – drives the overheating dilemma in warmer waters.

While some species such as Atlantic bluefin tuna may temporarily increase heat loss or dive into colder waters, suitable habitats for mesothermic species will shrink as larger parts of the ocean become inhospitably hot. This will be especially the case during the summer months when sharks will experience increased competition for prey.

This will disrupt ecosystems because mesotherms are typically apex predators that exert disproportionate control over species below them in the food chain, said Edward Snelling, co-author and physiologist at the University of Pretoria.

“These species are being pushed closer to their physiological limits, which could have consequences for where they can live and how they can survive,” Snelling said in a news release. “These animals are already operating on a small energy budget, and climate change is further restricting their options.”

Using tiny sensors on a range of fish, including sharks weighing over three tonnes, the researchers calculated how much heat the fish produce and lose in real time. From this, they calculated that a one-tonne warm-bodied shark could struggle to stay in waters above 62.6 degrees Fahrenheit (17 degrees Celsius) without taking countermeasures. Uncovering these “hidden heat calculations” could prove crucial to any hope of conserving them or mapping protected areas, the researchers said.

In South Africa, the stakes are both environmental and cultural. Here, great whites have emerged as a “sentinel species”: when their patterns change, it signals a deeper shift in the marine ecosystem.

While they have long been sensationalized as feared predators, they are increasingly becoming icons of marine conservation and eco-tourism, said Stephanie Nicolaides, a marine conservation researcher at the University of the Western Cape. “Many local and international conservation narratives now position the great white not as a villain, but as a key species essential to maintaining ocean health,” Nicolaides said.

The decline in great white sightings in False Bay, Mossel Bay and Gansbaai, however, is multifold. Although thermal displacement may be a contribution, their population decline is also associated with a history of overfishing, shark netting and habitat destruction.

Indeed, although warming waters increase the vulnerability of mesotherms worldwide, other man-made damages pose the greatest danger. “If we had to say what’s the one thing we need to urgently address for these animals, it’s the fishing problem,” Payne said. “The most acute, immediate crisis these animals are facing is overfishing, and especially now bycatch.”

Bycatch refers to fish and other marine animals caught unintentionally by fishermen using huge nets or long lines baited with thousands of hooks.

History, however, offers a grim precedent for physiological vulnerability itself. Fossils of extinct warm-bodied species – like the infamous megalodon shark, which reached almost 60 feet long— suggest that they suffered disproportionately during past increases in ocean temperatures as they likely struggled to provide food to fuel their large, warm bodies.

“Today’s oceans are changing at an unprecedented rate,” Payne said. “The alarm bells are ringing loudly at the moment.”