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To fish or not to fish. That has been the big question this summer on the Kenai Peninsula. And the Alaska Department of Fish and Game’s new DIDSON sonar system has played a deciding role in that.
DIDSON stands for “Dual-frequency IDentification SONar.” It was developed for military uses such as harbor surveillance and mine identification, but Alaska is the first to use it to count fish.
The Department has two sonar stations on the Kenai river, one for counting kings and another for sockeyes.
The king counting sonar has been the most controversial this summer, due to one of the lowest runs on record.
The sonar site is about eight miles upstream from the mouth of the Kenai River. If you’re boating along the shore in search of a scientific outpost, you might miss it. The beige plastic-covered tent looks like something more suited to a camping trip. But inside you’ll find an array of computers that serve as a set of sonic eyes that see what’s happening under the water.
Several species of salmon travel together in the river, and differentiating between them has never been an exact science. But researchers like Jim Miller, Fish and Game’s project manager for the sonar stations, believes new technology is helping to make counting fish more reliable.
He points to one of two laptops that are sitting side by side in the tent.
“This is collecting data that’s coming across the wireless,” said Miller.
Wireless Internet has removed the need for stringing cables across the river, which were used when the DIDSON sonar was first introduced in the summer of 2010. Since then, the system has seen steady improvement.
Just recently, Miller waded into the river to install a new component called ARIS, Adpative Resolution Imaging Sonar. It doesn’t look all that sophisticated, just a metal box on a pole, but Miller says it will improve the range of the sonar and its resolution. He compared it to the transition from standard to high definition television.
“We are using the most accurate sonar tool we’ve ever had access to,” says Debby Burwen, one of Fish and Game’s sonar experts. “These actually look like fish,” she says, pointing to the screen. “It’s one of those, seeing is believing.”
Burwen says the salmon move through a fan of 48 sonar beams, which captures their swimming behavior. The data is recorded on a computer hard drive and viewed like a video.
“It fills in some of the unknowns,” said Burwen. “It fills in the uncertainty about whether it was a downstream fish or a piece of debris.”
Technicians can pause the video and measure individual fish by dragging a mouse from tail to snout. Anything above 75 centimeters, or about 29-and-a-half inches is counted as a king, or chinook salmon. Eventually, researchers hope to develop software to automate the measuring process.
Fishermen have questioned the reliability of the data, because Fish and Game discontinued use of an older split-beam sonar system this summer.
For the past two seasons, both sonars were running side-by-side. But fishery managers say, after using the DIDSON, and comparing the split-beam counts to other sets of data, it became clear that system was unreliable. They also began to believe that the split-beam technology wasn’t appropriate for use on a river, but more suited to larger bodies of water.
Biologists say the transition between the two systems has put them in a situation where they’re comparing apples to oranges.
To insure that enough kings reach their spawning grounds, fishery managers set a minimum goal for the number of kings to pass the sonar station. The target was 17,800 kings, but it was based on numbers from the old sonar system.
But this summer, researchers were relying exclusively on the DIDSON and weren’t sure what target they should set to make sure enough kings escaped to spawn.
In a two year, side-by-side comparison, it appears the DIDSON was counting fewer kings than the split-beam system, so scientists haven’t been quite sure what their goal for DIDSON sonar passage should be. Instead, they erred on the side of caution. (Next page)
