September 30, 2008
"We don't know if they are an open-ocean population or a continental shelf population," said Doug DeMaster, Director of the Alaska Fisheries Science Center. "But we do know that offshore killer whales can move great distances over a relatively short period of time. "
The offshore killer whales are genetically different from their kin, the marine mammal-eating transient killer whales and fish-eating resident killer whales.
"Offshore killer whales differ in size, shape and behavior from other two killer whales eco-types," said Marilyn Dahlheim, a researcher from the Alaska Fisheries Science Center. Offshore killer whales are shyer, moving evasively and unpredictably when approach by boats, she explained. They are smaller and they tend to live in larger groups-up to 75 or 100 individuals.
Although the ranges of the three eco-types occasionally overlap, offshore killer whales have never been seen to intermix with resident or transient killer whales.
Offshore killer whales most likely subsist on fish. They have, for instance, been seen with salmon in their mouths. Scientists have observed many other foraging behaviors which also support the idea that they are fish-eaters. Scientists have watched offshore killer whales in the company of sea lions, gray whales, fin whales and dolphins. In no case did the offshore killer whales target these animals as prey, nor did the other marine mammals act as if the offshore killer whales were a predatory threat.
In recent years, researchers, using photographs of individual whales, have found 57 matched sightings of offshore killer whales between Alaska and Washington, six matches between Alaska and Oregon and 81 matches between Alaska and California. Of these, 46 individual whales were seen in all three places: Alaska, Washington and California.
One offshore killer whale traveled 4,435 kilometers (2,756 miles) from Dutch Harbor, Alaska to Dana Point, California. Another long-distance trip was 4,345 kilometers (2, 700 miles) 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 kilometers (26 miles) per day.
Dahlheim said that it is very difficult to determine the total number of killer whales that comprise this "offshore" assemblage. They are not seen that often and they acquire nicks quickly. The overall shape and nicks on the trailing edge of the dorsal fin are used to help identify individuals. But if the nicks change rapidly over time and the whales are not seen for a few years, it's easy to count the same whale as two different individuals. Taking this into consideration and being conservative in their estimates, Dahlheim said she and her co-authors think that there may be as many as 300 or 400 individual offshore killer whales.
Dahlheim and California co-authors Alisa Schulman-Janiger, Nancy Black, Richard Ternullo, plus Washington co-authors Dave Ellifrit, and Kenneth C. Balcomb III have recently summarized the results from 20 years of observations on offshore killer whales in a scientific paper; "Eastern temperate North Pacific offshore killer whales (Orcinus orca):Occurrence, movements, and insights into feeding ecology"
For a time, researchers believed that offshore killer whales had appeared recently. However, a valuable store of genetic materials from stranded killer whales has allowed modern scientists to perform DNA testing which indicates that offshore killer whales were coastal visitors off California nearly 50 years ago and possibly as long as 100 years ago.
Dahlheim and other NOAA researchers plan to continue documenting offshore killer whales during field studies. Cooperation with other marine mammal researchers working in areas from Alaska to California will remain important given the extensive range of this offshore eco-type.
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