Some sharks return to the same sites to breed for decades

Some species of sharks return to the same breeding grounds for decades and live longer than previously thought, according to scientists who study the animals off Florida.

New England Aquarium scientists have found that nurse sharks return to waters off Dry Tortugas, 113 kilometers from Key West, to mate for up to 28 years. They also found that the lifespan of sharks appears to extend at least into their 40s, rather than around 24 years as previously believed.

The researchers published their findings in October in the journal PLOS ONE as part of the world’s longest study of shark mating behavior. The research sheds new light on how sharks reproduce and the role their environment plays in their reproduction, said Nick Whitney, lead scientist at the aquarium and co-author of the study.

“This is the first example that showed the long-term use of a mating ground,” Whitney said. “Observing the natural behavior of sharks in the wild is incredibly rare to begin with, and observing mating behavior is truly unusual.”

Scientists have known that nurse sharks have used the waters off the Tortugas as a breeding ground since at least the late 19th century, but whether the sharks have returned to the area remains a question. Aquarium researchers tagged 118 sharks from 1993 to 2014 and found that more than two-thirds returned to breeding grounds in subsequent breeding seasons.

The scientists wrote that this evidence of long-term site fidelity “reveals the importance of identifying and protecting mating sites for this and other species”.

Research team members said they had not guessed that the same animals would continue to mate at the site for years. Of all the sharks returning to the site, almost 60% have been tracked for over 10 years and 13% have been tracked for over 20 years.

David Shiffman, a marine biologist at Arizona State University who was not involved in the study, said the work could open up new insights into sharks and how they migrate and use their habitat.

That could be a key to helping conserve them, he said.

“Some of the best-known shark species are highly migratory, regularly crossing oceans. So learning that other species are homebodies using the exact same habitat year after year is fascinating,” Shiffman said.

The Dry Tortugas are a group of remote islands in the Gulf of Mexico popular with divers and birdwatchers. The area’s status as a breeding ground for nurse sharks could be compromised by a direct hit from Hurricane Ian in September, Whitney said.

Scientists have not yet been able to tell whether their underwater monitoring stations weathered the storm, they said. They also don’t know if the sharks have returned. Female sharks come to the area’s shallow waters to prepare to give birth in September and October, said Ryan Knotek, associate scientist at the aquarium and co-author of the study.

The site’s vulnerability is a strong reason to protect both the sharks and their breeding grounds, Whitney said.

“These sharks clearly have a strong interest in returning in this state,” he said. “It was a shark that was once thought to be a sedentary couch potato of a shark. It turns out they’re a lot more active.

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