Whales swimming behind fishing vessels catch an easy feast of escaped fish, but experts warn this 'free meal' is risky, potentially lethal behavior

• Sperm and bottlenose whales are known to pursue fishing boats to catch fish that escape the nets.
• It's an easy way to get a meal but makes them more dependent on fishing boats, experts say.

Humans catch about 95 million metric tons of fish every year, contributing to the trillion-dollar frenzy of the global fishing industry.

Interestingly, some whale species have also found ways to profit from the fishing business.

It's not the money they're after — but the fish that outwit even the fishermen.

Whales discovered how to catch a free meal

Sperm whales and northern bottlenose whales off the coast of Newfoundland were observed time and again trailing fishing trawlers to catch the fish that escaped fishermen's nets.

But the whales didn't catch just any old fish. They were apparently picky eaters and only followed a vessel when it was fishing for Greenland halibut.

None of the whales that scientists observed pursued a trawler while it was catching redfish, thorny skate, or any other fish other than Greenland halibut. Scientists published their results in the journal PLOS ONE in August.

"The whales have learned to identify the fishing vessels as a source of easier fish to catch, reducing their time for foraging," lead researcher Usua Oyarbide told Insider.

Oyarbide first observed this behavior in 2007 as part of her Master's thesis project. Then five to six years later, she said a fisherman sent her video of one of the same whales still chasing down fish.

This study isn't the first to report whale and fishing boat encounters. Scientists have found similar behavior among at least 17 other whale species in different parts of the world dating as far back as 1997, Oyarbide said.

"It is a behavior that has been reported more and more over the past 20 years or so, in a number of parts of the world." Hal Whitehead, a sperm whale expert and biology professor at Dalhousie University, told Insider. He wasn't involved in Oyarbide's study.

What's even more interesting is that it's not only whales that have learned to catch fish escaping the fishermen's nets.

"As time goes on we hear more and more reports of everything from sperm whales to dolphins doing this. Probably, almost every species of marine mammals that eats fish somehow take advantage every now and then of a free meal," said Andrew Trites, director of the Marine Mammal Research Institute at the University of British Columbia, who also wasn't involved in the study.

Oyarbide further suggests that this behavior may become more common among whales in the future as whales learn socially from each other and can, therefore, pass tips and tricks onto the next generation, and so on.

Is it good for whales to follow fishing boats? No.

Oyarbide said this change in whale behavior may seem beneficial for them at first as it increases their access to prey and reduces foraging time.

But it can also make them more dependent on fish vessels for food, influencing their survival and reproduction. The whales likely began trailing trawlers because the boats overlapped their foraging areas, Oyarbide said.

Overfishing in such areas indirectly threatens the whales by decreasing the availability or altering the size or diversity of their prey. "It is important to understand fisheries' impacts in marine ecosystems," Oyarbide said.

Another issue is the "habitat degradation caused by bottom trawlers can have an indirect, negative impact on marine mammals that depend on those habitats for food," she added.

Instead of searching for food, some whales find it easier to catch prey just by following the trawlers. And that often leads to more risky interactions with vessels or fishing gear, Oyarbide said.

"It is probably bad for the whales in the long run," Whitehead said. "They sometimes get rammed, or shot by angry fishers, and can get caught in the nets. We saw a sperm whale killed by a trawler net close to the area mentioned in the study. They may also lose skills at catching their real food."

Fishing gear entanglement alone causes the death of 300,000 whales, dolphins, and other cetacean mammals every year, according to the International Whaling Commission. And a study published in January suggested this estimate was low.

Moreover, a report from the National Oceanic and Atmospheric Administration suggests that females of the endangered North Atlantic right whale species are having fewer births because of stress and injuries from repeated fishing gear entanglements.

Old observations can still teach something new

Though Oyarbide is no longer researching whales and her observations in the study are many years old, Trites said her research is still helping the scientific community as it offers a glimpse at past whaling behavior in Canada that may be compared with data in other parts of the world like Africa, China, and Australia.

"I learned something, which I didn't know before," Trites, who has been studying marine mammals for over 30 years added. "I've known about sperm whales being engaged with different fisheries but I wasn't aware northern Bottlenose whales show similar behavior, so I've learned something new."

"The study tells us how whales can exploit new resources, also, occasionally with really good data, we may get a better understanding of how they learn the new behavior from one another," Whitehead told Insider.

Oyarbide added that her coauthor is using the study's data to better understand bottlenose whale depredation in Canada that may be going underreported.