Sharks and the ocean’s twilight zone: Some female great white sharks can deep dive for hours

Much of the shark focus around the Cape is on great whites roaming close to the shoreline as they prowl for seals, but researchers are finding out that several sharks are actually diving deep into the twilight zone out in the middle of the ocean.

Scientists at Woods Hole Oceanographic Institution on Cape Cod are researching the sharks’ deep diving behavior and how sharks’ bodies have evolved to handle these deeper conditions.

They’re learning that deep diving is far more frequent and extensive across species than previously thought, said Simon Thorrold, a senior scientist in the biology department at the Woods Hole Oceanographic Institution.

Even some great whites will head offshore into very deep, middle of the ocean habitats.

“It’s super interesting,” Thorrold told the Herald on Monday. “Young white sharks tend to be very coastally attached. They make this very predictable annual migration from Florida up to us in the summer, and turn around in the winter.

“But what happens as the great whites get older, they also get better at retaining heat, and the mature female white sharks do something completely different that we’re only just beginning to understand,” he said.

The mature female great whites will deep dive up to three hours in the twilight zone, he said. Two of the large female great whites have even traveled out into the eastern Atlantic Ocean.

“We don’t know exactly why they do it, but for whatever reason they just want to get away,” Thorrold said. “They may have mated in the summer, and are getting away from the aggressive males during gestation.”

The ocean’s twilight zone is home to some of the largest fish biomass on Earth, making it a unique feeding ground for the ocean’s largest predators.

Many shark species venture from surface waters to the deep ocean between 656 to 3,280 feet, and experts believe the main purpose is to hunt for prey when food is scarce in surface waters.

Different shark species spend varying lengths of time in the twilight zone. Blue sharks spend up to an hour hunting for a meal; whale sharks can spend a few hours; and basking sharks take it to the extreme, spending up to four months feasting on twilight zone creatures before re-emerging to the sunlit surface waters.

“I hadn’t appreciated how much time that these predators were spending in the twilight zone with less light where it’s more difficult to hunt visually,” Thorrold said. “The more tags we put out, we’re seeing how important the twilight zone is to all of these species.”

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