Research to use ultrasound to count Kenai River salmon

Posted: Monday, February 04, 2002

ANCHORAGE (AP) -- The same sonar technology that allows parents to view an unborn child in the womb may be used to to estimate the number of salmon that make it upriver to spawn.

Engineers at the University of Washington's Applied Physics Laboratory developed a long-range ultrasound video system about a year ago and will test it on the Kenai River in July.

State biologists long have used sonar to count fish swimming up the Kenai, but it has been, at times, impossible to differentiate between red salmon and king salmon or to calculate whether enough kings make it up river to spawn.

The new ultrasound device delivers images crisp enough to resemble video of salmon moving upstream. It shoots 96 sonar beams at high frequency and can discern objects up to 100 feet away, designers say. Software accompanying the sonar automatically counts and measures the length of passing fish.

''It represents a quantum leap in what you can do with acoustics,'' said Brett Huber, director of the Kenai River Sportfishing Association, which is paying $10,000 for the testing. ''You've probably seen sonograms of babies. That's what this does. I mean, you can see the fins on the fish.''

If the camera can successfully peer through the Kenai's cold, silt-choked water and dense fish this summer -- conditions known to trip up less sophisticated sonar systems -- the state will begin buying the $80,000 units, said Debby Burwen, a state fisheries biologist who specializes in sonar counting.

''The idea is you can actually see the shape of the fish, the length of the fish, and you can see the fish swimming,'' said Ed Belcher, the camera system's principal engineer. ''Whereas in previous sonars for fish assessment, you get kind of a return, a pulse.''

With the current system, biologists rely on those pulses to open, close and restrict fishing, affecting the Kenai's multimillion-dollar sport and commercial fisheries.

Kings may be giants among fish, but their numbers are relatively small. They are counted in the thousands, compared with reds, which reach the Kenai by the hundreds of thousands.

If the king run looks weak, state biologists move swiftly to limit or close sportfishing.

The state has used sonar to count fish in the Kenai River since 1985. The existing split-beam sonar, installed in 1995, had been considered state of the art. That system produces wormlike images of the sonar readings printed out on paper.

With only squiggles to look at, biologists rely on the fact that fish generally follow predictable behavior patterns. Kings, which weigh up to 90 pounds, typically hunker down in the middle of the river, while smaller reds stay closer to shore, where the current is gentler.

But when incoming tides turn the river slack, schools of red salmon can spread into the middle. And sometimes the large number of reds smother the sonar signal and affect the count.

Burwen said the new ultrasound camera's improved definition should be able to help biologists see fish, even at the peak of the red run.

Burwen said she and other biologists traveled to Seattle in November to see a demonstration of the system. What they saw was astounding, she said.

''We were all just pretty shocked. You don't usually see such big leaps,'' she said. ''It actually looks a little like an X-ray as (the salmon) swims through the beam.''

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