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See Inside January 2005

Sound Findings

Sonar offers a way to count salmon accurately

The debate is as fierce and perennial as the surf pounding the Alaska coastline. On one side, commercial and sports fishermen complain that calculated fishing quotas do not match the number of fish actually in the water. On the other, conservation authorities worry that overfishing will deplete the sockeye and chinook salmon stocks plying the Pacific Northwest waters. New techniques using existing acoustic sonar equipment may help both sides by determining how many salmon are in the water as well as distinguishing one species from the other. And that could make counting and catching them a lot easier.

In sonar setups such as fish finders, pulses of sound bounce off the water's bottom--and off any creatures that happen to swim by. Typically devices record the strength of the return echo, thereby indicating the rough numbers of fish and their sizes.

Echo pulses have other characteristics, too, such as width, shape and kurtosis (the size of the pulse's top half relative to that of the bottom half). Scientists have largely ignored these features, because they believed that background noises obscure whatever information they may contain.

Some investigators decided to challenge that belief. This past summer researchers at Fisheries and Oceans Canada and the U.S. National Marine Fisheries Service in Alaska mounted four acoustic transducers on a metal plate beside Alaska's Kenai River. The transducers then fired simultaneous sound waves, each at different frequencies, at approximately 40 chinook and sockeye salmon tethered to the river bottom with fishing line.

"We've learned," says physicist Tim Mulligan with Fisheries and Oceans Canada, "that, indeed, the fish's shape" and its positioning measurably alter the return echo, a finding "that wasn't really documented until now." For instance, the degree to which a salmon's head and tail point toward the transducer correlates strongly with the width of the echo pulse; other changes in the returning echo, including the kurtosis, also indicate the orientation of the fish. The next step will be to correlate echo components with other fish features, such as tail strength and swim speed, and to determine how those sonar characteristics vary according to species. Nailing down the differences between sockeye and chinook salmon could take years, however.

Mulligan insists the significance of the correlation between fish swimming behavior and long-ignored echo characteristics cannot be overstated. It "opens many more opportunities for fish-type discrimination based on behavior," he says. Alaska's Kenai River Sportfishing Association, which helped to finance the project, evidently agrees. Its hope: that conservation officials will have more accurate assessments of the yearly salmon runs up the Kenai and other rivers and be able to identify fish by species before netting or reeling them in. That could settle disputes between officials and fishers over just how much salmon can be harvested.�

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