What's Killing the West Coast's Young Great White Sharks?

Sharks are one of the ocean's fiercest predators, but a new study shows how often they're taken down by fishing nets.

Published 9 May 2018, 22:25 BST
A great white shark swims in clear water off Guadalupe Island.
A great white shark swims in clear water off Guadalupe Island.
Photograph by Mauricio Handler, National Geographic Creaitve

Two of the ocean's biggest predators — sharks and humans — are at odds with each other in southern California.

A new study in the Journal of Applied Ecology found that fishing was the greatest cause of death for juvenile great white sharks off the western coasts of Southern California and Mexico. From 2002 to 2016, researchers tagged 37 sharks with satellite tags that remotely sent information about each shark, such as its location and temperature. Of the sharks they observed during this time period, only two died 'natural' deaths: one was preyed upon and the other's death was inconclusive.

John Benson, an ecologist now at the University of Nebraska-Lincoln, conducted research with the Monterey Bay Aquarium to quantify exactly how young sharks were dying in the northeast Pacific. He suspected fishing had been a major cause of death for this species, but he was aware of no exact estimates that showed just how often juveniles were becoming bycatch.

Bycatch refers to the animals that fishermen catch incidentally while fishing for a different species. For example, if a fishing boat catches shark while fishing for swordfish, the shark is considered to be bycatch.

Most of the sharks that died from accidental fishing were hooked by drift gillnets, a controversial type of fishing method that often indiscriminately catches whatever swims in its way. Drift gillnets can be spread a mile long and hang as much as 30 metres deep. Sharks die when they get entangled and can't escape. Both California and Mexico protect sharks from intentional capture, but gillnets are still allowed for catching non-protected species like swordfish.

“It's certainly fair to say that human fishing is a major threat, probably the biggest threat causing [shark population] declines around the world,” says Benson.

Sharks in the Water

In recent years, great white shark sightings have increased off the coast of Southern California. After suffering population declines for centuries, stricter protections have turned the decline around, and sharks are showing some signs of rebounding.

But whether or not sharks will increasingly become bycatch as their numbers rise is unclear, says Benson. He notes that fishing presents one of the greatest threats to sharks on a global scale, especially in waters that are less highly policed than the U.S. or Mexico.

“The population trends have been a very elusive thing for us to understand,” he says, but adds that this new study will be an important building block for helping quantify how many sharks are heading for Southern California. “Hopefully this is a step toward making that less of mystery.”

What Can Be Done?

“In terms of reducing white shark mortality, avoiding setting nets close to shore and checking them frequently appear to be the best practices,” Chris Lowe, a coauthor of the paper, said in an emailed statement. (Read about the mysterious fungus killing sharks.)

“Shark bycatch is a solvable problem,” says Oceana’s marine scientist Mariah Pfleger. She recommended three ways to solve the issue: “count everything that is caught in a fishery (including animals caught accidentally as bycatch), cap the amount of wasted catch in each fishery using scientifically based limits, and control and avoid shark bycatch by making improvements such as using cleaner fishing gear and enhanced monitoring.”

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