Basking Sharks Have Almost Vanished From California and Scientists Don't Know Why

Thousands of basking sharks have vanished from waters off the coast of California and scientists do not know why.

Scientists tracking basking shark numbers off California have found that between the 1960s and 1990s, around 4,000 sharks appeared there every year. But then all of a sudden, sightings stopped. Since the 1990s, there have been less than 60 sightings per year.

Why these sharks left California is a mystery.

Basking sharks are the second-largest shark species on the planet, growing to over 26 feet long and weighing over 11,000 pounds. The species is easily recognized for its enormous mouth, in which it collects the zooplankton it exclusively feeds on.
It is an endangered species, according to the International Union for Conservation of Nature, with around 20,000 estimated to be left in the wild.

The International Union for the Conservation of Nature estimates its population has fallen by between 50 and 79 percent in the last century.

In a study published in the journal Frontiers in Marine Science, researchers examined the disappearance of basking sharks off the coast of California. It looked at historical data between 1962 and 2018 to show the extreme extent to which the species has been lost from the region.

Their findings also showed the number of basking sharks in each school was also down. Average school size fell from 57.2 to 24 between the 1960s and the 1980s. From the 1990s onwards, there were no schools of basking sharks larger than 10 reported.

The fall in basking shark schools could impact their numbers—fewer schools could impact mating success.

Study co-author Alexandra G. McInturf, a postdoctoral fellow at Oregon State University, told Newsweek that fewer sightings and smaller schools likely meant fewer basking sharks in total.

"It is challenging to determine the major factor driving declines in sightings for certainty," McInturf said.

Basking sharks are faced with several threats, including changes to their habitats, shipping and being hunted for their fins and other body parts.

"We want to know why the declines are happening," McInturf said in a statement "Is it climate change? Human-induced pressures? What environmental cues do they respond to and how might that change in the future?"

She told Newsweek the disappearance of basking sharks from California could relate to historical conflicts with humans. "Until the 1970s, basking sharks were considered so numerous in Western Canada as to be a pest species for entanglement in fishing nets, and the government enacted an eradication program that certainly may have led to some of the trends we observe now," she said.

"Through the mid-1900s, basking sharks could be captured as bycatch, but they were also targeted by fisheries in Monterey and Morro Bays."

Today the animals are protected and no deliberate catches of them have been recorded since 2004. Researchers hope their findings can be used to help conserve and protect the species, and potentially lead to the population to recover.

"Currently, fishing for basking sharks is prohibited in the U.S., Mexico and Canada," study co-author Heidi Dewar, from Southwest Fisheries Science Center, said in a statement. "The hope is that with these protections in place, the population will recover. Additional work to understand the sources of mortality in international waters is also needed."

Stock image of a basking shark
Stock image of a basking shark. The animals are a Red List Endangered Species according to the International Union for Conservation of Nature. Rebecca-Belleni-Photography/Getty Images