Basking sharks can be over 30 feet long and are characterized by their huge gill spines and three-foot high dorsal fins. But these mysterious, massive, filter-feeding cousins of the great white shark aren’t just a scientific curiosity — they also appear to be disappearing from California and the rest of the northeast Pacific.
The National Oceanic and Atmospheric Administration has conducted aerial surveys since 1962-2004 monitor populations of commercially valuable fish such as sardines. Aerial surveys have proven to be a valuable historical record for sightings of basking sharks, which are easily large enough to spot from the air.
Pilots reported their sightings to NOAA every year, and researchers found that sightings dropped significantly over the four decades of the survey.
“The size of the group has decreased, as well as the decrease in sightings,” said Alexandra McInturfpostdoctoral researcher at Oregon State University and lead author on a new basking shark paper in the review Marine Science Frontiers. McInturf authored the paper as a doctoral candidate at UC Davis. “Before the 2000s, you could see up to 500 individuals in a group…then after the 2000s, we only saw 10.”
Basking sharks have been listed as globally endangered by the International Union for Conservation of Nature in 2019, but the decline took place largely out of public view.
“They’re big, charismatic animals and they look so weird,” McInturf said. “They’re not going to hurt you, you know, they’re basically whales, but the shark version. I’m surprised people didn’t get hooked on them sooner.
“Basking sharks are usually filter feeders, and their teeth really aren’t much bigger than your little pink fingernail,” said David Ebert, the program director for the Pacific Shark Research Center at Moss Landing Marine Laboratories and one of the paper’s co-authors. “They basically filter water – they swim with their mouths open… Their gill spines are basically set up to be like big sieves.”
Although spotted on the surface, basking sharks can spend up to 90% of their time underwater, Ebert said. They often surface to feed on copepods, a type of zooplankton that NOAA fisheries biologist Heidi Dewar, also one of the paper’s co-authors, describes as “sea cows” for their role as grazers of the ocean’s primary photosynthesis producers.
Basking sharks also feed on other small prey such as fish eggs. Despite their small prey, they become huge, making them the second largest fish in the ocean behind the whale shark. They are known to be long-distance migrants that are usually found at depths of 300 to 500 meters, although they are able to dive much deeper.
Basking sharks often feed in areas of the ocean called fronts, Dewar explained, which are subject to “physical forcing mechanisms that act to concentrate prey.” These food-filled fronts can occur both above and below the water surface.
For a separate study published in 2018, Dewar et al. tagged three basking sharks off the California coast, which they then tracked across the Pacific, including waters near Hawaii and Baja California. While sharks tend to be seen off the California coast from spring through fall, they remain largely a mystery in terms of habits, distribution, and population size.
“Very little is known about them,” McInturf said. “They do these kind of highs and lows, where there’s a lot of variability in when they show up and how many come at a certain time.”
“We don’t even know where they have babies,” Dewar said. “There are so many things that are not known.”
Even estimating the age of the few basking sharks observed is an impossible task. Age is often estimated by counting the stripes on a shark’s vertebrae and “for some species this tends to work, where the stripes tend to be annual, like you count the stripes on a tree,” said Ebert. “Basking sharks have nice stripes to count, but it turns out that the stripes are not age-equated. We have absolutely no idea how old the sharks are. Anything you see doesn’t is just a pie in the sky because we just don’t know.
Knowing more about this species is extremely difficult due to the rarity of its occurrence.
Researchers predict that a mix of factors – among them the residual effects of historical basking shark overfishing – are behind the decline in sightings in recent years off the California coast.
During the 20th century, basking sharks were targeted by fisheries in the Pacific and Atlantic oceans for their highly profitable foie gras and large fins.
“In Canada, it was more this eradication program they had because the sharks were swimming into the salmon nets and destroying them,” Dewar said. “So they designed these ships that basically had a big knife in the front and they would just ram into basking sharks to protect their fishing nets.”
This culling practice and basking shark fisheries in the eastern North Pacific ended in the mid to late 1900s, and today basking sharks are protected within 200 miles of US coasts. United States and Canada, and by the United Nations Convention on International Trade in Endangered Species beyond that.
“It’s a long-lived species,” Dewar said. “It takes them a long time to recover from overexploitation, and they probably don’t have a lot of young.”
Fishing and killing of basking sharks began decades before aerial surveys indicated a potential decline in basking shark populations, and “this is likely due to a lag due to the long lifespan and maturity of these animals,” McInturf wrote in an email. “It’s really hard to say why this effect wasn’t immediately apparent.”
Now researchers are hoping for greater awareness and monitoring of this rarely seen fish. McInturf spent three summers studying basking sharks in Ireland, and during that time she only saw them once. “It’s common,” McInturf said. “And that’s really a shame. I call them shark droughts. And they happen a lot. It’s really hard to predict where they’re going to be and when they’re going to show up.
For researchers to search for sharks, they also need to be in contact with people who are out on the water regularly. “You have to get a tip like, ‘Hey, we see them off Monterey, Santa Cruz or somewhere,’ and you have to go out and see if you can find it and put a tag on it,” Ebert said.
“I grew up in the Monterey area and have been on the water since I was very young,” he added. “And you would see them, but it wasn’t something you would think, ‘Oh, who am I going to call?'”
Since 2010, the Pacific Shark Research Center, NOAA, and other collaborators in Canada, the United States, and Mexico have helped generate the Spot a basking shark citizen science project that allows members of the public to submit sightings of basking sharks via a gate.
“People on the water deserve a lot of respect for what they know, and we really rely on them a lot,” McInturf said.
Dewar also mentioned that the public can call in and report sightings by calling the Southwest Fisheries Science Center at (858) 546-7000, and she urged people to take photos and give space to basking sharks when they are spotted, as they are susceptible to collisions with boats.
For a species so rarely seen, staying in touch with fishermen, whale watchers and the public at large is a key part of the monitoring process for this species.
“People lost sight of the fact that basking sharks were happening in huge numbers off California,” Dewar said. “The sightings would have been sporadic, but they were there – they were part of the flora and fauna of California. And people have lost track of it. I think this concept is really important as we deal with the impacts of climate change. Our normality is not necessarily without impact.
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