Local orcas' favorite meal: B.C. chinook

While here at home in their summer range, Puget Sound's endangered orca whales dine almost exclusively on salmon from Canada, scientists have learned, underscoring the connected nature of the Salish Sea ecosystem.

During the summers of 2004-08, scientists tracked the J, K and L pods of orcas (also known as killer whales) in the western end of the Strait of Juan de Fuca and San Juan Islands, to learn what they were eating and analyze where their food came from. No easy task, the work involved following the orcas in small boats and gathering killer-whale excrement and regurgitation, fish scales and other tissue with a fine mesh net after the whales ate.

Examination of the material, including DNA testing, revealed that the orcas select chinook salmon nearly exclusively for food, despite far more abundant numbers of pink and sockeye in the area at the same time.

"They would literally knock pink salmon out of the way to take a chinook," said Brad Hanson, biologist with the National Marine Fisheries Service Northwest Fisheries Science Center in Seattle, and lead author of the paper, published last month in the online journal Endangered Species Research.

Scientists took their examination a step further, to learn by DNA analysis where the fish came from. Of the chinook salmon sampled, 80 to 90 percent were from British Columbia's Fraser River, and only 6 to 14 percent from Puget Sound-area rivers.

"This means basically we are going to have to look very carefully at the link between chinook abundance and how well the whales do in terms of their survival," said John Ford, another author of the paper, and head of Fisheries and Oceans Canada's Cetacean Research Program at the Pacific Biological Station in Nanaimo, B.C.

The selection of Fraser River chinook might be because there simply are a lot more of them, Ford said. Other experts agreed.

"Why chinook? They are big. Why from the Fraser? They are abundant. Sometimes the simplest answer is where you start," said Patrick Pattillo, special assistant to the director of the Washington Department of Fish and Wildlife.

He noted the strongest Puget Sound chinook run might be 25,000 fish returning to the Skagit River, while strong summertime Fraser River runs number four times that, and more.

It's no surprise to Pattillo that orcas are found in the strait and the west side of the San Juans when those waters are alive with millions of salmon finding their way home to rivers in British Columbia and Washington. Biologists were surprised when even Columbia River chinook turned up in the west end of the strait last summer.

"People were really scratching their heads over that," Pattillo said. "Perhaps they were following food."

The most widely distributed cetacean, orcas exist in all of the world's oceans and most of its seas — though never in large numbers — and are always picky eaters.

Some, such as the orcas in the North Atlantic, eat mostly herring, slapping their tails on the water to corral the fish together for harvest, Hanson said.

Others, such as the transient killer whales of the eastern North Pacific, eat porpoises and seals, but shun fish.

Young orcas learn what is food for them from their families, with mothers and siblings in family groups, called pods, cooperating in the hunt and sharing their catch.

Smaller pink salmon may sometimes be used as "training" fish for young orcas in J, K and L pods. But when it's time to eat, chinook is what's for dinner.

Partly, scientists believe that might be because chinook deliver more calories for the effort, because they are the largest salmon, with the most oily flesh.

Chinook may also be easier to catch. They swim alone, rather than in schools, and go deep, where the water is dark.

Pink and sockeye travel in schools at the upper water column, where sunlight penetrates, which would enable them to see the orcas coming. Chinook might be easier to surprise in the dark — but these factors are still a matter of speculation for scientists, Hanson said.

The orcas apparently discern which fish to select by the shape of the salmon's swim bladder, which they detect with echolocation clicks, Hanson said.

The orcas' fancy for Fraser River chinook in the summer is just one piece of the puzzle in seeing to their survival needs.

The orcas' winter travels are poorly understood, but they have been tracked as far south as Monterey Bay and far north as the Queen Charlottes, and are known to take salmon all along the outer coast, as those fish return to the Columbia River and Sacramento River systems and beyond.

"It certainly has raised the question of providing suitable numbers of chinook available to sustain their current needs, both in the U.S. and Canada," Ford said.

Lynda V. Mapes: 206-464-2736 or lmapes@seattletimes.com