A new study, published in the journal Aquatic Biology has found that British Columbia's (B.C.) offshore killer whales also prey on large Pacific sleeper sharks. Until now their diet has remained quite a mystery due to their wide-ranging and distant movements.

Scientists have long known that resident killer whales depend on a diet of fish, especially salmon, and that transient killer whales eat marine mammals such as seals and sea lions. Offshore killer whales were first identified on the B.C. coast in 1989. This latest study proves through DNA evidence that offshore killer whales prey on large Pacific sleeper sharks, whose skin is so abrasive, it is believed to be wearing the whales' teeth flat, even among younger offshore killer whales, something not seen even in older transient and resident killer whales.

Other potential prey of offshore killer whales includes salmon shark, blue shark and spiny dogfish, as well as related elasmobranch species such as skates and rays. They may also consume fish such as halibut, meaning their diet could overlap with resident killer whales. The study documents how offshore killer whales preyed on at least 16 Pacific sleeper sharks on two occasions: at B.C.'s Learmonth Bank, in western Dixon Entrance, in May 2008; and Montague Strait in Alaska's Prince William Sound, in June 2009.

This news represents the first confirmed prey species of offshore killer whales based on field observations of foraging and the first record of Pacific sleeper sharks as the prey of killer whales anywhere.

Researchers speculate that the whales might be targeting the large, rich livers of the sharks, since in one of the attacks it would have been unlikely for the whales to consume all their prey. The liver of a four-metre Pacific sleeper shark can weigh about 180 kilograms, or one-fifth of its body mass. It's remarkable that three distinct groups of killer whales exist off the B.C. coast eating different prey. Scientists who started studying killer whales decades ago never would have predicted that there could be different populations that are specialised and living separate lives in the same waters around BC.

Offshore killer whales differ in size, shape and behaviour from the other two killer whales, they are generally shyer, moving evasively and unpredictably when approach by boats. They are smaller and tend to live in larger groups of up to 75 or 100 individuals. These particular whales can move great distances over a relatively short period of time. One offshore killer whale travelled 4,435 kilometres from Dutch Harbor, Alaska to Dana Point, California. Another long-distance trip was 4,345 kilometres between Dutch Harbor, Alaska and Los Angeles, California. For the speed record, two whales moved from Kodiak, Alaska to Monterey, California in 77 days. That's an average of 42 kilometres per day and quite impressive!