Great white sharks are overheating

This will disrupt ecosystems as mesotherms are typically apex predators that exert disproportionate control on 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 survive,” said Snelling in a press release. “These animals are already operating on a tight energy budget, and climate change is narrowing their options even further.”

Using tiny sensors on a range of fish, including basking sharks weighing over three tons, researchers calculated how much heat fish produce and lose in real time. From this, they calculated that a one-ton warm-bodied shark may struggle to remain in waters above 62.6° Fahrenheit  (17° Celsius) without taking countermeasures. Discovering these “hidden heat budgets” could prove critical to any hope of conserving them or mapping protection areas, researchers said.

In South Africa, the stakes are both ecological 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 long sensationalized as feared predators, they’ve increasingly become 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 keystone species essential to maintaining ocean health,” Nicolaides said.

Declines of great white sightings in False Bay, Mossel Bay, and Gansbaai, however, are multifaceted. Though thermal relocation may be a contributor, their population decline is also linked to a history of overfishing, shark netting, and habitat destruction.

Indeed, though warming waters heighten mesotherms’ vulnerability worldwide, other manmade harms exert the most danger. “If we had to say what is the one thing that we need to urgently address for these animals, it’s the fishing problem,” said Payne. “The most acute, urgent crisis these animals face is from overfishing, and particularly now from 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 they suffered disproportionately during past ocean temperature increases as they likely struggled to secure food to fuel their large, warm bodies.

“Today’s oceans are changing at unprecedented speeds,” Payne said. “The alarm bells are ringing loudly at this point.”

This article originally appeared on Inside Climate News, a nonprofit, non-partisan news organization that covers climate, energy, and the environment. Sign up for their newsletter here.