Killer whales are discriminating diners

Sophisticated predators scan wide ocean regions listening for favorite fish

A killer whale's favorite meal is king salmon, according to a new study that found these sophisticated predators scan wide ocean regions listening for their favorite fish.

Echolocation, which involves creating a sound in order to produce an echo, allows the whales to zone in on king salmon, also known as Chinook salmon, at distances up to half a mile.

But why do killer whales go to so much trouble to hunt down king salmon, picking them out like sushi chefs even when they represent just 5 to 10 percent of the available salmon population?

"Salmon are not necessarily equally nutritious," Whitlow Au, who led the study, told Discovery News. "Chinook salmon has the highest concentration of lipids, or fats, that orcas seem to prefer."

Au, a marine mammal researcher at the University of Hawaii's Hawaii Institute for Marine Biology, and his colleagues mechanically recreated killer whale echolocation pulses at Lake Union in Seattle, Wash. The researchers tied Chinook, Coho and Sockeye salmon to a rotating net set out at different water depths.

Although these fish look similar to human eyes, the study showed the echo structure created by each type of salmon was unique and could be used by killer whales to discriminate among the various species.

"Fish gas" appears key to the process, as the study revealed echolocation tuned especially well into information released by each salmon's swim bladder.
Swim bladders are gas bags within a fish that help the fish to be buoyant at any specific depth," Au explained. "Gas bags are probably the best reflector of acoustic energy underwater."

Au likened killer whale echolocation to a person wearing a miner's cap with a blinking light on it. Each time the light blinks on, the individual receives information about what's around.

The study's findings will be presented at next week's Acoustical Society of America meeting in Miami, Fla.

Among whales, only toothed species use echolocation. Au therefore suspects other toothed whales, such as sperm whales, possess the killer whale's choosy, long-distance mealtime behavior.

Bottlenose dolphins appear to fall in the discriminating eater group too.

Marine biologist Ronald Schusterman of the University of California's Long Marine Laboratory told Discovery News that the new study results "are consistent with work done on bottlenose dolphins in captivity showing that they can recognize objects rather easily by means of echolocation."