Specialists work together to improve salmon sonar's accuracy

Posted: Sunday, June 20, 2004

In the murky world of underwater fish imaging, things aren't always as they seem.

What appears to a sonar operator to be one big fish moving upstream can actually be two smaller fish swimming side by side. Debris floating downstream can be confused with migrating salmon. And when it comes to determining what species of fish is actually swimming past a sonar site, it often can be more of a guess than science.

With the advent of new technology being tested on the Kenai Peninsula, however, much of the guess work may soon be taken out of how people view the unseen lives of migrating salmon.

Debby Burwen is an acoustic imaging specialist who works for the Alaska Department of Fish and Game. For the past couple of years, she's been working with new sonar techniques aimed at sorting out the differences between various species of fish. Since traditional sonar gives little more than a blip on a screen, Burwen's work is considered to be on the cutting edge of sonar technology.

"Species classification is the holy grail of acoustics," Burwen said last week while working with two other scientists at Fish and Game's Kenai River sonar site.

Along with physicist Tim Mulligan of the Canadian Department of Fisheries and Oceans and biologist Guy Fleischer with the U.S. National Marine Fisheries Service, Burwen spent a week on the Kenai doing research for a new technique that could revolutionize how fisheries managers and fishers themselves look at salmon and other schools of fish.

The technique being tested on the Kenai involves using four different frequencies of sonar aimed at the same section of river. The scientists hope that by using four sonar arrays, they'll be able to get a handle on what kind of fish are swimming past. Because the Kenai is home to a number of similar looking fish of different sizes chinook salmon average around 30 pounds, while sockeye come in at between 4 and 10 it's the perfect proving ground for the new technique.

Mulligan, who came to Alaska specifically to work on the Kenai sonar site, said the Kenai River also is a good place to try to weed out the chinook from the sockeye, especially during the height of the salmon run.

"The sockeye, when they come in, outnumber the kings a thousand to one," Mulligan said.

Figuring out which fish are which is important to fishers and biologists because the number of each type of salmon has a major impact on how the area's sport and commercial fisheries are managed. Since a couple hundred chinook one way or the other could make a big difference as to when fisheries are opened, it's important to be able to get a precise measurement of how many fish are actually entering the river.

On the Kenai, the team of scientists combined high-tech computers with low-tech fishing to collect data on the two types of salmon. While Mulligan and Fleischer staffed the sonar equipment, Burwen and an assistant were on the water, trying to get a 50-pound king salmon to cooperate.

Because the sonar technique requires that an individual fish be looked at very closely to gather as much data as possible, Burwen's job was to tether individual salmon to a rope and get the fish to swim within the sonar's view. After a separate crew netted the salmon, Burwen tied the fish to the rope, communicating with the two scientists on-shore to make sure the fish was in the right place.

The conversations between the scientists got a bit confused at times on the Kenai, as the salmon seemed unwilling to stay in the right spot for very long.

"OK, he's in ... maybe a little less than 10 meters (from shore)," Burwen radioed to the sonar tent.

"OK, we got him," Mulligan said into his radio, telling Burwen the salmon was in the right place to be viewed with the sonar.

"I wish he'd sit still," Mulligan continued. "... OK, I got him in the center of the grid."

While Mulligan homed in on the fish on his monitor, Fleischer guided him with a high-tech DIDSON sonar machine that provided relatively high-resolution pictures of the fish as it swam back and forth underwater.

"He's moving forward ... now back a little," Fleischer said.

"A little bit for you, but a lot for me," Mulligan replied, struggling to keep the fish centered on his more sensitive four-array sonar system. "He's keeping me busy."

Finally, the fish settled down long enough for Mulligan and Fleischer to gather enough data on its size and individual characteristics. Later, the scientists joked that their work would be easier if the fish would simply swim in one place.

"If only these fish would cooperate," Fleischer said.

Mulligan, pointing out the obvious, noted the fish can't be blamed for wanting to get free.

"You wouldn't cooperate either if you were tied to a line," he told his colleague.

Although trying to gather data on the sonar characteristics of individual salmon isn't easy, Mulligan said the work on the Kenai is a chance to view live fish in an almost-natural environment.

"It's a rare opportunity to be able to get fish of a known size and have them swimming alive in the beam," he said.

With the data gathered at the Kenai site, the scientists hope to figure out exactly what a chinook salmon looks like on the sonar array as opposed to a sockeye. If the four-array technique works, and scientists are able to tell the different species apart, it could eventually have applications for fisheries worldwide.

For example, Fleischer said, if fishers on the high seas were able to target only certain species of fish, it could cut down on the amount of unwanted bycatch that usually ends up getting tossed overboard.

"This has applications all over the world," he said.

Burwen, a recognized leader in the field of underwater acoustics, said the work done on the Kenai is among some of the most exciting she's done in her career. However, she said what will really be exciting is when she's able to make positive species identifications, rather than just educated guesses.

"You're doing cutting-edge science, which is really fun and exciting. But on the other hand it would be even better if it was a solved problem," she said.

The dream of identifying individual fish species could be one step closer to reality once Burwen and her team finish their research. However, that won't be accomplished for at least a few more months. That's because collecting the data on the Kenai was just the first step.

Now, Burwen, Fleischer and Mulligan must return to their respective offices and spend months analyzing the data to see if they really can tell the difference between chinook and sockeye salmon.

"This was the fun and quick part of it," Mulligan said. "When we get home, we'll be keeping the Internet busy for a while."

Because Mulligan works in Vancouver, B.C., Fleischer in Washington state and Burwen in Anchorage, the three scientists will have to compile their data over the course of the next year and finalize their research from different parts of the Pacific Northwest.

However, all three said the shared nature of their work is what made the whole thing come together in the first place.

"It's a real good collaboration," Mulligan said, pointing out that none of the three agencies involved likely could have picked up the roughly $750,000 price tag alone.

"It's how science is done most effectively," he said. "Getting a team together, that's how to get things done."



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