Killer Whales Target Favorite Fish With Sonar?

Ker Than
for National Geographic News
November 17, 2008
Killer whales can use their natural sonar to find their favorite fish from a distance, a new study suggests.

Previous research had revealed that some killer whales off the coasts of British Columbia and Washington State (see map) have an uncanny ability for finding chinook salmon, even in months when chinook are vastly outnumbered by other salmon species such as coho and sockeye.

"Chinook salmon have a higher concentration of [fat] than any salmon species, and apparently killer whales like that," said study co-author Whitlow Au, a bio-acoustician at the Hawaii Institute of Marine Biology.

(Related: "Wolves Prefer Salmon to Deer?" [September 3, 2008].)

Echo Patterns

Like other porpoises and whales, killer whales, or orcas, emit high-frequency clicks that are reflected back when the sound waves strike an object.

The animals use sonar information to navigate, hunt, and communicate in murky waters.

But Au speculated that killer whales also use natural sonar to select specific types of prey.

To test this idea, Au and his team used simulated echolocation clicks resembling those of wild killer whales to measure the echoes produced when the sound waves bounced off the bodies of three kinds of salmon.

In research to be detailed this week at the annual meeting of the Acoustical Society of America in Miami, Florida, the team found that each salmon species has a unique echo pattern based on the different sizes and shapes of their swim bladders.

The air-filled sacs show up clearly in the echo images because they have a different density than the surrounding flesh and water.

The swim bladder "is responsible for at least 90 percent of the [sound] energy that is reflected from the fish," said study team member John Horne of the University of Washington. "Think of it as a hard wall."

Although Chinooks on average are larger than the other two salmon species, individual sizes overlap between the three groups, so the team doesn't think killer whales are selecting prey based on body size.

Horne added that the team plans to continue testing their theory using captive killer whales.

"Cultural" Diets

The new finding is further evidence for the multiple uses of sonar that some marine mammals have developed, said John Ford, head of the Cetacean Research Program at the Pacific Biological Station in Canada.

Ford, who was not involved in the new study, noted that there is another population of killer whales in the same North Pacific region that seems to favor marine mammals over fish.

"It seems that the dietary specialization of these two forms of killer whale is cultural," Ford told National Geographic News.

"It is likely that young whales are born with a blank slate and learn what constitutes food and how to catch it from their mothers and others in the kinship group."

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