They are the behemoths of the deep, the largest mammal on the face of the earth and, until now, a rare sighting off the coast of British Columbia. But the blue whales, which can be the length of two school buses and weigh 200 tonnes, are once again returning to where they were slaughtered to near extinction in the last century. Research that will soon be published in the journal Marine Mammal Science shows the whales are increasingly being spotted off the coasts of British Columbia, Alaska and Mexico's Baja. Experts say the massive mammals are following the krill, the tiny shrimp that are its main source of food, as they follow a cool water phenomena known as Pacific Decadal Oscillation. The El Nino-like pattern of Pacific Ocean climate changes is bringing the krill, and the whales along with them, both north to British Columbia and Alaska and south to Mexico, thousands of kilometres away from their usual home off the California coast. The report was co-authored by John Calambokidis, of the fisheries science centre Cascadia Research in Washington State, and John Ford, with Fisheries and Oceans Canada. "We think it's the fact that blue whales are highly-specialized feeders and that they feed only on krill and it looks like there are variations and conditions that kind of create better areas for feeding in different cycles," Calambokidis said. "And we show how some of these shifts seem to match the timing of shifts in this Pacific Decadal Oscillation. " Once thought to number about 300,000, its believed there are only about 10,000 blue whales left, more than 40 years after commercial whaling was stopped. One census of the blue whales spotted in shipping lanes says the population is dwindling, but Calambokidis believes the whales have just moved on and aren't being spotted by the ships. He said about 2,000 blue whales ply the waters between Alaska and Central America. Over more than two decades of study, Calambokidis has documented distinctive features of the whales off the California coast. Ford said "mug shots" confirmed the same whales being spotted up and down the coast are those same whales. Ford began offshore surveys off the B.C. coast in 2002 and said when they started looking the blue whales were there. "Whether they just weren't there in the past or whether they were there and nobody was out looking at them to document it is a little uncertain. We do know, from the many thousands of kilometres that we surveyed, that they're very scarce off our coast." They're labelled as endangered under the Canadian Species At Risk Act. Blue whales still haven't recovered from commercial whaling like the grey and humpback whales, and experts can't say why. Part of the problem is very little is known about the world's largest creature. Ford said another two-week survey will be conducted again this July off the West Coast and they hope to find more clues to help the whales thrive. "If there's such a thing as critical habitat that might be needed for their recovery so that those habitats can be protected from threats," Ford said. "It's finding out where and how many there are and how well they're doing, if they're increasing their numbers, what their habitat use patterns are like." Few people have been lucky enough to see a blue whale in person, which Ford said is "incredibly impressive." Calambokidis agreed that it was impossible to describe how large the whales really are until you're up close and personal. "When you get that close, you get a new appreciation."