How Orcas Determine Which Fish to Eat

Scientists pinpoint whales' dining plan

Swim bladder key to locating chinook in a sea of sockeye

Judith Lavoie

Victoria Times Colonist; Canwest News Service

Saturday, November 15, 2008

VICTORIA - Killer whales off the coast of Vancouver Island know the precise sound of their favourite dinner, according to a new study by a team of U.S and Canadian researchers.

Orcas like their fish dinners fat and greasy, which means their prey of choice is chinook salmon, and scientists have discovered the whales can identify a juicy chinook from up to 100 metres.

John Ford, marine mammal scientist at the Department of Fisheries and Oceans' Pacific Biological Station, said it's well-known that killer whales have a sophisticated echo-location system for finding prey, but scientists were confused about how whales can identify a chinook that's surrounded by sockeye.

"We know they like the larger chinook, but they also take the two- and three-year-olds which are the same size as the sockeye, so they can discriminate," he said.

The secret is in the swim bladder, an internal organ that helps fish maintain buoyancy, says marine ecologist John Horne of the School of Aquatic and Fisheries Science at the University of Washington.

Horne, an author of the study presented at a meeting of the Acoustical Society of America in Florida this week, said the team of scientists bounced recorded replicas of the whales' echo-location clicks off tethered sockeye, coho and chinook salmon to see what patterns emerged.

"We were looking at how killer whales perceive the fish," he said.

The swim bladder on a chinook salmon is half the size of other species, and is responsible for most of the reflected sound energy, the study found.

Scientists now know how the whales transmit the clicks, and the shape of the echo coming back from the target fish -- but little is known about how the whales then interpret the information, Horne said.

"We really need to find a co-operative killer whale to explain how they process the data," he said.

In addition to their built-in sonar systems, there are probably also learned skills, Horne said.

"It is the same as someone interested in picking blackberries. You get a sense of when the berries are ripe, what areas they are in and how they grow," he said.

The research may help to develop a system for more accurately calculating fish populations, Horne said.

Chinook are in short supply this year and, with some whales showing signs of malnourishment, there is a push to find out why the whales are not switching their menu.

© The Edmonton Journal 2008