Sonar device may improve salmon counting
Source: OnlineMariner
Publication date: 2002-02-04
Anchorage Daily News
ANCHORAGE (Feb. 4) The same sonar technology that gives parents a close-up glimpse of their unborn children in the womb may be used to focus on Alaska salmon runs. Engineers at the
University of Washington's Applied Physics Laboratory who developed a long-range ultrasound
video system about a year ago will test it in mid-July on the Kenai River.
State biologists long have used sonar to count fish swimming up the Kenai, but it has been, at times, impossible to differentiate between salmon species -- namely reds and kings -- or to calculate whether enough kings make it up river to spawn. The new device, called a surrogate camera, uses lenses and delivers images crisp enough to resemble video of salmon moving upstream. It shoots 96 sonar beams at high frequency and can discern objects as far as 30 meters 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 undoubtedly begin buying the $80,000 units, said Debby Burwen, a state fisheries biologist who specializes in sonar counting. Sample images viewed on the lab's Web site show a grainy, blue-hued "movie" of salmon, their tails swishing side to side.
"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. And though commercial setnetters target the more plentiful red salmon, they too can be restricted because a fair number of kings are still caught. The state has used sonar to count fish in the Kenai River since 1985. The existing split-beam sonar, installed in 1995, has been considered state of the art.
Each summer, a wooden shack capped with a blue tarp is erected on the river's grassy marshlands at mile 8.6 of the river. A couple of computers inside the shack display a mosaic of colored blips, each signifying a fish. Biologists typically hand count wormlike images of the sonar readings printed out on paper. With only squiggles to look at, they 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.
Tests with the new sonar camera will be monitored closely by Belcher and Alaska fisheries biologists, who should encounter these types of conditions. But Burwen said the camera's improved definition should be able to discern 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. You see the swim bladder. You see the bone structure."
The small unit, a little bigger than a coffee can, is called a dual-frequency identification sonar and was originally developed for the military by engineers at the university's lab. The military plans to use the cameras to identify enemy divers or floating mines in sensitive harbors, but fisheries biologists also quickly seized on the potential, Belcher said. Eight units, each made by hand, have been produced for military and fisheries research buyers, Belcher said. The lab has one demonstration model, which will be shipped for testing on the Kenai. It will be stationed alongside the existing state sonar counter.
If it works, a moving image rendered on a shore-side computer screen will show a view of the river bottom from above, as if a camera were aimed down from a tower. The top-down view is an optical illusion peculiar to sonar, caused by sound waves first bouncing off the near side of a fish and then the far side, Belcher said. He explained that objects farther away appear higher on a computer display screen because it takes longer for the sound waves to bounce back.
"We call it an out-of-body experience because it's confusing," he said. "You feel like you're looking down at the object when in fact you're looking at it from the side."
Publication date: 2002-02-04
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