Sonar system provides data for 'in-season' fish regulations

Posted: Tuesday, June 24, 2003

Many peninsula residents are familiar with the sonar estimates for king salmon on the front page of the Peninsula Clarion. They eagerly watch the numbers like a Wall Street broker watching the Dow, waiting for that exciting moment when they can spread the word, "The salmon are in!"

However, few people know just when, where and how these numbers are collected, or how important the numbers are in making major fishery management decisions.

Sports bag limits, catch-and-release, commercial openings, emergency closures and long-range run forecasts all are based on the sonar estimates of in-river returns.

"We do our best to ensure healthy returns of escapements while still allowing people to fish," said Debbie Burwen, a sonar coordinator and fisheries biologist with the Department of Fish and Game.

"The benefits of the data are 'in-season' fishery management," said Burwen.

"We use the sonar data in season, as opposed to setting quotas before, to maximize fishing opportunities for commercial and sport fishing, while subsequently ensuring enough protected and healthy fish make the return up the river."

The state has a sonar counter for kings at Mile 8.5 of the Kenai River.

"We've got a crew of four that works in eight-hour shifts to track fish, monitor equipment and maintain a 24-hour presence at this site," said fishery biologist Jim Miller.

He explained they burn the midnight oil at the site during the brief, but active salmon season.

There rarely is a moment when numbers aren't flashing on a computer screen or when the printer isn't humming and spitting out sonar images of life beneath the surface of the river.

The crew tracks chinooks throughout the day. At midnight they transfer the data to disk. The next morning during crew change, the data comes to the Fish and Game office, where Miller and others review the data and synthesize the chinook passage estimate for the previous day.

It sounds like a lot of work, but no one at the site was complaining. Most said they enjoy what they do. Many of the fisheries technicians request the assignment year after year.

"The hardest part is contending with the tidal fluctuations," said Miller. "The tides have a tendency to knock over the underwater equipment."

The equipment Miller was referring to is two tripods which contain transducers the sonar imaging devices.

According to Burwen, initially dual-beam sonar technology was used, but it was replaced in 1995 by a split-beam sonar system.

Dual-beam technology revealed the size of the fish, but split-beam revealed the direction of the fish as well as providing a size estimate.

"That's how we separate kings from sockeyes," said Burwen. "That, combined with location in the river, since kings stick to the middle and sockeye run close to the bank. We also support the sonar data with data from a netting program that alerts us to times when the sonar estimates may be inflated by sockeye."

However, new sonar equipment, developed from military technology for spotting enemy divers, is in the process of being tested and may replace the split-beam.

"It's very exciting," said Bur-wen. "The new technology, called DIDSON imaging sonar, produces a much nicer image, compared to the cryptic image of the current system."

This has the potential to make the current system even better, particularly in regard to discriminating between salmon species that migrate concurrently.

There is a drawback, though.

"Unfortunately, the new system is range limited, which makes using it on the Kenai difficult," she said.

The DIDSON only has a range of about 12-meters as opposed to the 30- to 50-meter range of the currently utilized sonar system, but there still is hope for its use in the Kenai.

"We're experimenting with the equipment to see if we can anchor them from a boat in the middle of the river," said Burwen.

She said tests already have occurred in the Anchor River, Copper River and the Nushagak, and so far the results have been promising. The Kenai test will be later this summer.



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