Unprecedented ocean heat is changing the way sharks eat, breathe and behave

By Rachel Ramirez, CNN

(CNN) — Sharks have been made villains in most stories, whether it’s fact or fiction. But as the planet’s climate and oceans rapidly change, these boneless, aquatic, apex predators are also misunderstood victims — under severe environmental pressure yet historically capable of incredible adaptation.

Sharks are among the most endangered marine animals on the planet, with 37% of the world’s shark and ray species threatened with extinction, primarily due to overfishing, coupled with habitat loss and the climate crisis, according to the International Union for Conservation of Nature Red List.

And as ocean temperatures climb, researchers say many sharks are beginning to change their behaviors — shifting where they live, what they eat and how they reproduce — which could cause cascading effects for the rest of the marine ecosystem.

“Sharks and rays are fascinating species that have been misunderstood and underappreciated for far too long,” Heike Zidowitz, shark and ray expert at the World Wildlife Fund-Germany, told CNN, noting that they are essential for the health of the oceans.

“If these beautiful animals were to be wiped out from our oceans, it would not only be a heartbreaking loss, it would trigger ocean imbalances with ecosystem consequences that we cannot yet imagine.”

The oceans are heating to record levels this year — a shocking temperature increase that shows no sign of ceasing. Rising ocean surface temperatures began to alarm scientists in March. Temperatures then skyrocketed to record levels in April, leaving scientists scrambling to analyze the heat’s potentially dire ripple effects.

As with most creatures, sharks need certain conditions to thrive. With the climate crisis impacting the temperatures and acidity of the oceans, these agile ocean creatures are sheering off their normal paths and traveling to unknown, often taxing, territories.

Valentina Di Santo, an ecophysiologist and biomechanist who studies swimming performance in fish, said temperature changes play a dominant role in the ways they breathe, digest food, grow and reproduce.

For sharks in particular, these physiological processes speed up as ocean temperatures get warmer, doubling in speed every 10 degrees, according to Di Santo’s research.

“An increase in metabolic rates means that sharks are using more energy to just be alive and swim,” Di Santo told CNN. “Every activity needs extra energy. An increase in digestion rates often mean that they absorb fewer nutrients as digestion becomes less efficient and they possibly need to eat more frequently.”

Sharks always seem to be on the hunt, maneuvering their way through the water in search of new fish or other sharks to eat. But research has shown that warming oceans have pushed many fish populations northward to cooler waters, which has disrupted the ocean’s availability of food. Some fish species are not able to find new, suitable habitats, which causes a decline in their population. Overfishing also intensifies the issue by pushing fish stocks to drop.

Di Santo said understanding the interplay between predator and prey behavior is critical when considering how sharks respond to the climate crisis.

“It is important to consider that sharks are very much tuned in the behavior of their prey,” Di Santo said. “Therefore, it is not surprising that they may track the geographic shifts of their preferred food sources.”

Di Santo also said that sharks respond to ocean warming in two ways: shifting their latitudinal range or choosing deeper, cooler waters to enhance their physiological processes.

A climate vulnerability assessment from the National Oceanic and Atmospheric Administration found that sharks off the Northeast coast have a high likelihood of shifting their distributions or expanding into new habitats to follow preferable ocean conditions.

“These small-scale movements can be just as crucial for their survival as poleward relocations,” Di Santo said. But “the shift in depth has been found to be more pronounced than the latitudinal shift,” and some temperate species are already exhibiting seasonal shifts toward deeper waters.

As the climate crisis escalates, sharks’ paths will only become further strained, Zidowitz said, which could ultimately close off vast swaths of the ocean to sharks.

But sharks also have “a remarkable history of survival,” Di Santo said, having withstood all five major mass extinction events in the last 400 million years. It’s the never-before-seen compounding consequences of overfishing, climate change, prey scarcity and habitat destruction that has shark experts worried about whether they can adapt and survive these huge planetary changes.

Zidowitz said progress on conservation to protect shark species is “too slow to keep pace” with the numerous threats they face, yet she remains hopeful.

“If we can find the last remaining refuges around the world where the most threatened sharks and rays live, and work together with local communities, we can bend the curve towards their recovery,” Zidowitz said.

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