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Sperm whales have turned burglar. They have learned to follow commercial fishing boats off the coast of Alaska, and then pick huge volumes of fish from the lines. It now seems they can take about 5% of the fishermen’s annual quotas.

Sperm whales are the largest toothed predators on Earth. . Lone whales typically harass boats in the eastern Gulf of Alaska. When they attack a boat they work feverishly to pick fish from the hooks, sometimes without damaging the gear or even being seen.

at Sierra Nevada College in Nevada and her colleagues tracked the whales’ impact on the Alaskan fishery over 27 years. These fishermen use longlines stretching kilometres along the ocean floor, with thousands of hooks baited with octopus or squid. These are tempting targets.

The team found that sperm whales can take a quarter of a ship’s catches in a single attack. They may attack many times an hour.

This translates to a $5-8 million annual loss for Alaska’s $100-million sablefish industry.

More whale attacks

, according to a 2017 study by Peterson and at the NOAA Alaska Fisheries Science Center. They work in pods of as many as 40, and can take up to half of a catch each time they attack.

This behaviour was first documented in the 1950s, when Japanese whalers in the Bering Sea reported orcas targeting their catches, says Hanselman. “Sperm whales are more of a new issue.”

Hanselman and Peterson say two factors are behind the upswing.

One is the 1986 whale hunting moratorium, as sperm whale numbers have risen 4% a year since. The other is Alaska’s 1995 move from seasonal 10-day fishing bonanzas to individual quotas. The marathon seasons have given both species plenty of opportunity to learn about fishing practices.

Stop the whales

Mariners cannot fight back, as both species are covered by the . Sperm whales are a species – whereas to assess how threatened they are.

“There’s very little that can stop the whales once they’re there,” says Peterson. So fishermen have tried to keep them away.

“They’ve tried decoy buoys, boats fishing close in tandem, they’ve tried making noises,” says Peterson. An attempt to confuse the whales’ echolocation with submerged noise-making devices actually worked as a dinner bell, she says.

The  identifies whale-free fishing grounds by tracking their movements.

Meanwhile, in Patagonia, where sperm whales and orcas also thieve, and less gear. Shorter lines mean fewer fish to steal.

A better – albeit pricier – fix may be to swap out fish hooks for pots with narrow openings. “So far, the whales haven’t figured out how to blast pots open,” says Hanselman. “But it’s hard to put it past these large-brained mammals.”