The marine behemoths, which can weigh more than 10,000 pounds, are returning in numbers not seen in decades

Giant basking sharks have reappeared off the coast of southern California in numbers not seen in decades.

“The sight just takes your breath away – it’s magic,” said Lotti Keenan, who saw nearly a dozen basking sharks while on a whale-watching cruise in April. Three of them circled the boat. “We knew this was such a rare thing. And people on the boat were screaming with excitement – it was like you were at a soccer game.”

The sharks, the second largest fish in the world, were grazing along the coast, gliding slowly through the water, their cavernous mouths hanging wide open so they could catch krill and plankton.

A century ago, basking sharks along the west coast numbered in the hundreds or even in the thousands. The marine behemoths can grow more than 30ft long and weigh more than 10,000 pounds, said Heidi Dewar, a biologist with the National Oceanic and Atmospheric Administration based in La Jolla, California.

Facebook Twitter Pinterest A basking shark in the waters off southern California. Photograph: Courtesy of Lotti Keenan

But by the 1960s, they were a rare sight. Many were fished for their liver oil, for human consumption and for use in animal feed. In the 1950s and 60s, Canada’s department of fisheries and oceans oversaw an eradication program because the sharks were interfering with salmon fishing operations. The method was unflinching: boats with large blades attached rammed into the creatures.

“It’s too early to say whether the increased numbers we’re seeing this year are reflective of a population in recovery,” said Dewar. “But it’s certainly hopeful.”

Seeing as so few of them have been spotted in past decades, “we researchers don’t really know much about them at all – we don’t know how their populations are doing, we don’t know much about their biology,” she said.

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Since 2010, Dewar and her colleagues at Noaa have tagged a handful of sharks with satellite trackers in order to follow their movements through the ocean. Over the past few years, Noaa, which has listed the sharks as a “species of concern”, has also asked for the public’s help in spotting them. Basking sharks are now protected in Canada, and internationally, the IUCN Red List assessment has categorized them as vulnerable.

“Thirty years ago when I was a grad student in southern California, I’d see them all the time between of between Los Angeles and Catalina Island,” said Chris Lowe, the director of the shark lab at California State University, Long Beach. “And I literally have not seen one since.”

Though he is “cautiously optimistic” about their resurgence this year, Lowe said they still face many threats, including ocean warming due to climate change and plastic. “There’s so much plastic in the ocean – and since these sharks are filter feeders, they’re undoubtedly ingesting a lot of it.”

Lowe said he is hoping to see some this week for the first time in decades. “I always describe them as buses with fins,” he says. “They’re just so impressive.”