The giant whales have worked out that the sound of the boats means the approach of a ready-caught meal, and they have learnt to wait for the telltale sounds that the engines make when the catch is being raised from the seabed.

Researchers believe that about 90 male sperm whales are taking fish from lines in the eastern part of the gulf, an area where fishermen rarely saw whales 20 years ago.

As the fishing boats approach, the whales dive shallower than usual and wait until they hear the change in engine speed as the fishermen haul in their lines. “That’s the whales’ cue,” Jan Straley, an assistant professor at the University of Alaska Southeast, said. Since 2002 he has helped to lead a study of their behaviour by using sound receivers attached to fishing gear. The sensors record the loud clicks of the chattering whales as the boats approach.

As the longlines used to catch bottom-dwelling sablefish are brought to the surface, the whales move in, biting chunks of the fish from the lines. Some of the lines are up to three miles long. Lips and partly chewed bodies are often left dangling from hooks.

Scientists found that the sperm whales tended to feed on longlines from late spring until summer, during the height of the sablefish season.

The researchers have devised low-cost ways of trying to hoodwink the whales, but most fisherman believe that they are too smart. Suggestions have included fishing earlier or later in the season, hauling in the line without changing engine speed and making decoy runs to divert whales.

“We try to get creative, but there’s only so much you can do,” Steve Fish, who has been catching sablefish in the gulf for 27 years, said.

The sweet, flaky flesh of the sablefish, long prized in Japan and Hawaii, is gaining popularity on the American mainland, where it is sold as black cod or butterfish.

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