Deep beneath the surface of the Gulf of Alaska an extraordinary marine crime has been taking place.

Sperm whales, the ocean’s largest predators, have been targeting the boats of black cod fishermen and swiping their catch off their lines.

This giant animal’s deft trick was filmed for the first time by a group of scientists based in Alaska, US, and is shown in the BBC Two programme Alaska: Earth’s Frozen Kingdom.

It shows a sperm whale using its long jaw to create tension on the line, which in turn snaps fish off the hooks. This feeding behaviour is called depredation and experts think it is learned by the whales.

“I don’t know how to quantify their intelligence but their effectiveness is almost perfect,” says Stephen Rhoads, a boat skipper who has been fishing in the area for 20 years.

“That they’re getting better at this every single year and it’s less work for them to hang out with us and take our fish than it is to dive down and get them off the bottom.

“There’s no doubt that these creatures are very smart.”

Long-line fishermen fish on the sea bed using hook-and-line equipment that can be up to five miles long.

Black cod and other deep water fish are natural prey for sperm whales in the region.

Crews fishing for black cod, or sablefish (Anoplopoma fimbria) first reported sperm whales (Physeter macrocephalus) removing fish from their lines in the Gulf of Alaska in the 1970s.

Since then the fishing season has been extended from around two weeks to eight months and commercial whaling has also been banned, both of which are thought to have added to the problem by increasing the numbers of whales and the length of time they have to perfect their technique.

Reported incidents of depredation began to rise in the mid 1990s and the damage done by the whales’ attacks is now estimated to cost hundreds of thousands of dollars each year and led the industry to approach scientists for help.

The Southeast Alaska Sperm Whale Avoidance Project (SEASWAP) was launched in 2003 to try to understand the mysterious mammals and uncover ways to stop them from following fishing vessels for their dinner.

They’re premeditated

Scientific research has been combined with the fishermen’s observations and has produced a number of revelations about sperm whale depredation.

It has also provided several new findings about the Gulf of Alaska sperm whale population as a whole, including determining their genetics and their movements.

In the SEASWAP study area 120 sperm whales have been observed, all of which are male, scientists estimate there may be 235 in total. Up to 10 whales have been seen around fishing boats, which is unusual as adult males usually hunt on their own and could also point towards depredation being socially transmitted between whales.

They also discovered that the whales, who hunt using echolocation, are alerted to the fact fishing is taking place by the sound of boat engines shifting gear as the crew haul in the catch, this can be detected from several miles away.

“They’re premeditated,” says Mr Rhoads, “but they are using clues of how we’re fishing to know that there’s definitely gear on the bottom, that we have fish on hooks a half mile under the surface and the whales have figured out the sounds of our boats in action meaning that the fish are coming up.”

Sperm whales have also been shown to feed off the line at different depths, some diving fairly shallow and others much deeper, similar to natural foraging dives.

Of the solutions the SEASWAP team have tried to keep sperm whales away from the boats, monitoring has proved the most successful. This involves putting satellite tags onto the whales so they can be tracked, fishermen can then use a website or contact the SEASWAP team directly, to find out where the whales are.

The scientists hope to eventually tag 10 individuals – known as the 'bad boys' – who are seen around boats most often. One of the most regular visitors has even earned the name Jack the Stripper after being seen nine out of the last 10 years in the same part of the Gulf.

“We think that they’re a little bit better at what they do or a bit more interested in depredation,” says Lauren Wild from SEASWAP.

“Maybe the other whales drive by and take fish if they’re by a boat but they’re not seen as frequently.”

Four of the 10 serial depredators have so far been tagged, giving the fishermen the option to avoid fishing near them.

“I don’t know if there’s one answer to the depredation issue but I think being informed and aware of how these animals are behaving are all important to really get a grasp on what they’re doing and how they’re doing it in order to prevent it,” says Ms Wild.

“As we learn more, we hone down better ideas of what to do.”

SEASWAP is now collaborating with scientists studying depredation by whales in other parts of the world and continue to work with the fishermen to devise and test potential answers to the problem.

“It was only 160 years ago that the classic novel Moby Dick was written, capturing the dramas at sea of whalers,” says Alaska: Earth’s Frozen Kingdom producer Jane Atkins.

“Now the tables have turned, whaling is banned, and sperm whales are returning and learning to take on fishermen in bold and surprising ways – and so far there is very little the fishermen can do about it.

“The conservation questions are far more complex, and it will certainly be an interesting story to follow in years to come.”

For more information about the SEASWAP project click here.

Watch the first episode of Alaska: Earth’s Frozen Kingdom at 20:00 GMT Wednesday 4th February, BBC Two.

You can follow BBC Earth on Facebook, Twitter and Instagram.