Resident killer whales are one the most studied mammals on the West Coast, but after more than 40 years of observations, researchers are unable to definitively say where they go in winter.

From November until April, the estimated 300 orcas that make up the northern and southern resident populations disappear from B.C. waters, leaving a hole in the scientific understanding of an iconic species that in some areas has become endangered. Knowing where they go, and what they feed on year-round, is important to protecting whales in the long term, scientists say. And while it remains a mystery for the moment, there are increasing signs that the mystery could soon be solved.

"Certainly if you are out in a boat, they do [vanish] because you just can't find them," says Graeme Ellis, a technician with the department of Fisheries and Oceans' cetacean research program.

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There are two distinct populations of killer whales: transients, which constantly roam along the coast, feeding on seals and other large mammals; and residents. What scientists want to know is where the residents, which feed on salmon and are more localized in their range, go for six months over the winter. Why has such an important question stumped researchers for so long?

"The biggest difficulty is winter weather. It definitely restricts when, where and how we can get out to find these animals," said Mr. Ellis, referring to the season's short hours of daylight and frequent storms.

But decades of research are slowly beginning to pay off for Mr. Ellis and his colleagues at the Pacific Biological Station.

For several years they have operated a string of listening stations along the coast, which record the calls of any whales passing by. Because researchers are able to identify specific pods by the sounds they make, the team can filter out the calls of passing transient whales, and slowly piece together a pattern of movement for the residents.

Most of the signs are pointing north, to Dixon Entrance, which lies due west of Prince Rupert, between Haida Gwaii and the southern tip of the Alaska Panhandle.

"Through the seasons we see these animals more frequently on the north coast early on in the year, and I think it's because they are basically following the Chinook [salmon]," Mr. Ellis said.

His working hypothesis is that the resident populations go where the salmon go. "Early spring, March, April, May, there are killer whales out there almost every day in Dixon entrance," Mr. Ellis said. "So we've gone out there and made a concerted effort in ships to try to find the whales.… We're mainly interested in what they are feeding on in the winter. If we can determine where the hot spots are in different months of the year, then it just helps us focus our boat-based research efforts."

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While most of the underwater listening devices are repositioned every two years, one has been operating at Langara Island, on the north coast of Haida Gwaii, for seven years. And that has produced more proof that resident killer whales are spending at least some of their winter months in Dixon Entrance.

"You are listening through these recorders and, oh my gosh, there are these guys and it's February and they are out there off Langara making calls and echolocation like they are hunting fish, just like summer time," Mr. Ellis said. "So we get a glimpse into their movements at a time of year when it is really tough to work out there … it certainly answers a lot of questions we had about where they are."

Mr. Ellis said one pod of B.C. resident killer whales was also sighted in the Gulf of Alaska in winter, suggesting they range farther north than Haida Gwaii.

"It`s very neat that we can get them in Dixon Entrance day after day after day," Mr. Ellis said. "That was enlightening. If we had listening stations up in southeast Alaska, we maybe could find other good feeding areas in the winter months."

Once the winter range is known, he said, the next challenge will be to get out in boats and study them up close, to figure out why they are there.