This would disrupt the ecosystem because mesotherms are typically apex predators who have disproportionate control over the species below them in the food chain, said co-author Edward Snelling, a physiologist at the University of Pretoria.
“These species are being pushed closer to their physiological limits, which may have consequences for where they can live and how they can survive,” Snelling said in a press release. “These animals are already operating on limited energy budgets, and climate change is reducing their options even further.”
Using tiny sensors on a variety of fish, including basking sharks weighing more than three tons, the researchers calculated how much heat the fish produced and lost in real time. From this, they calculated that a warm-bodied shark weighing one ton might struggle to survive in water above 62.6° Fahrenheit (17° Celsius) without any means. The researchers said discovering these “hidden heat budgets” could prove vital to any hope of their conservation or mapping out protection zones.
There are both environmental and cultural threats in South Africa. Here, great whites have emerged as a “sentinel species”: when their patterns change, it signals a deep change in the marine ecosystem.
Although long sensationalized as ferocious predators, they have increasingly become symbols of marine conservation and eco-tourism, said Stephanie Nicolaides, a marine conservation researcher at the University of the Western Cape. Nicolaides said, “Many local and international conservation narratives now present the great white not as a villain, but as a key species essential to maintaining the health of the ocean.”
However, the decline of great white views in False Bay, Mossel Bay and Gansbaai is multifaceted. Although thermal migration may be a contributor, their population decline is also linked to a history of overfishing, shark nets, and habitat destruction.
Indeed, although warming increases the vulnerability of mesotherms worldwide, other man-made losses pose the greatest threat. “If we had to say what’s the one thing that needs our immediate attention for these animals, it’s the problem of fishing,” Payne said. “The most serious, urgent threat facing these animals is from overfishing, and especially now from bycatch.”
Bycatch refers to fish and other marine animals caught inadvertently by fishermen using huge nets or long lines strung with thousands of hooks.
However, history itself sets a grim precedent for physical vulnerability. Fossils of extinct warm-bodied species – such as the infamous Megalodon shark, which was nearly 60 feet long – show that they suffered disproportionately during past ocean temperature increases as they likely struggled to secure food to fuel their large, hot bodies.
“Today’s oceans are changing at an unprecedented pace,” Payne said. “The alarm bells are ringing loudly right now.”
This article originally appeared on Inside Climate News, a nonprofit, nonpartisan news organization covering climate, energy and the environment. Sign up for their newsletter here.
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