Sonar helps whales with menu choices

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 have an uncanny ability to find 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.

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 presented at the annual meeting of the Acoustical Society of America in Miami, Fla., the team reported that each salmon species has a unique echo pattern based on the sizes and shapes of the species' 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 in the study, individual sizes overlap among 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 its theory using captive killer whales.

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 said. “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.”


