In the scientific journal "Marine Mammal Science," researchers from Cascadia Research Collective in Washington state, NOAA's Southwest Fisheries Science Center in California, and Canada's Department of Fisheries and Oceans identified 15 separate cases where blue whales were seen off British Columbia and the Gulf of Alaska.
Four of the whales were identified as animals previously observed off the coast of California, suggesting a re-establishment of their historical migration pattern.
Researchers made this identification by comparing photographs of blue whales taken in the North Pacific Ocean since 1997 with a library of nearly 2,000 photographs of blue whales off the West Coast.
Researchers identified individual blue whales by the shapes of their small dorsal fins. (Photo by John Calambokidis courtesy Cascadia Research Collective)
A positive match was determined based on pigmentation patterns in skin color and shape of the dorsal fin.
Reaching lengths of nearly 100 feet, the blue whale is the largest animal on Earth today and the largest known to have ever existed.
They were nearly hunted to extinction throughout the world and are currently listed as endangered under the U.S. Endangered Species Act, the Canadian Species at Risk Act, and on the authoritative IUCN Red List.
During the early 1900s in the North Pacific and along the West Coast as far south as Baja California, blue whales were nearly wiped out during commercial whaling activities. Because they were the largest whales, blue whales were a prime target for whalers.
Formerly large populations of blue whales in the North Pacific never rebounded after commercial whaling ended, while those animals off southern California have apparently fared much better, researchers say.
Large concentrations of blue whales have been documented off California and Baja California and in the eastern tropical Pacific since the 1970s, but it was not known if these animals were part of the same population that previously ranged into Alaskan waters.
John Calambokidis of the Cascadia Research Collective, who has a long history of blue whale research, said of this study, "We document 15 blue whale sightings off British Columbia and in the Gulf of Alaska made since 1997, and use identification photographs to show that whales in these areas are currently part of the California feeding population."
A blue whale spouts off Moresby Island, British Columbia. (Photo by John Calambokidis courtesy Cascadia Research Collective)
"We speculate that this may represent a return to a migration pattern that has existed for earlier periods for eastern North Pacific blue whale population," he said.
"The recent sightings and photographic matches that we report here have changed our view of blue whale population structure in the eastern North Pacific," the authors state in the study.
"We had previously considered the population of blue whales that feed off California to be separate from the population of whales that had historically inhabited the waters off British Columbia and in the Gulf of Alaska," they explain.
"Although the sample size is small, the number of matches does indicate that blue whales seen off British Columbia and at least in the eastern Gulf of Alaska are likely part of the same population that is seen off California," the authors say.
The scientists are still not certain why blue whales are now beginning to migrate from southern California to the North Pacific Ocean, although changing ocean conditions may have shifted their primary food source of krill further north.
"One possible explanation for a shift in blue whale use is changes in prey driven by changes in oceanographic conditions, including the Pacific Decadal Oscillation, which coincides with some of the observed shifts in blue whale occurrence," the authors say.
Living between 70 and 80 years, blue whales reproduce every two or three years. There are an estimated 5,000 to 12,000 animals surviving today, with the largest population of approximately 2,000 off the U.S. West Coast.
Click here to view the research paper, "Insights into the population structure of blue whales in the Eastern North Pacific from recent sightings and photographic identification."
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