Kings count in the Kenai —  Sonar sites track fish numbers

By Patrice Kohl

Photo courtesy of Patrice Kohl. The Kenai king sonar site camp is a familiar sight among sport fishermen who troll the lower Kenai River for kings. Sonar site crew monitor the site’s sonar operations from the camp’s beige tent, staffing the site 24/7 throughout the Kenai king sonar site field season.

For the Redoubt Reporter

It may seem like only a passionate trout fisherman could get excited about waking up at the crack of dawn in early May to jump into a 42-degree river — but then there’s Jim Miller.

“It’s better than coffee,” Miller said as he shimmied into a pair of chest waders early Thursday morning for a dip in the lower Kenai River.

Miller is the project leader for the Alaska Department of Fish and Game’s Kenai River king salmon sonar site. On Thursday, Miller waded ankle, thigh and finally waist deep into the cold river water to deploy the sonar transducers now collecting Kenai king run data for this year’s fishing season. Miller, with the help of his crewmembers Trevor Davis and Mike Hopp, deployed a total of four sonar transducers mounted on two tripods — one for each bank — during low tide.

The king sonar site is 8.6 miles upstream from the river mouth’s meeting with Cook Inlet, and river width at the site can swell from a low of about 70 meters to 100 meters wide under the influence of Cook Inlet’s tides. On Thursday morning, the river’s exposed muddy banks yawned wide above Miller as he shuffled around in the water to position the tripods just the right distance from shore.

Once Miller had positioned both tripods, Hopp adjusted the tripod-mounted transducers’ sonar beams to align them with the river’s bottom. But unlike Miller, Hopp stayed high and dry, adjusting the transducers remotely from the sonar site’s camp, which from the perspective of a boat floating downstream, is situated just above the Kenai’s right bank.

Before the tide turned and water returned, the crew also needed to secure the sonar transducers’ cables. Cables from the right bank transducers run straight up the bank to the site’s camp, while cables from the left bank transducer run up the left bank and are suspended over the river to the camp. The crew secured the sections of cable submerged in the river by sinking them into the mud and pinning them down with sandbags. But like all rivers, the Kenai prefers nothing remain in one place for long.

“When we’re all done here with our aim, I’ll step on the cable and sew it into the mud,” Miller said. “It stays there until probably the first week of June, when the current washes all this mud out and the cable pops free.”

When the cable comes free and flags in the current, it creates a greater danger that fishermen will catch fishing gear or

Kenai River king sonar site project leader, Jim Miller, pulls a tripod equipped with two sonar transducers toward deeper water for deployment in the Kenai River while fisheries biologist Trevor Davis provides some slack in the transducers’ cables.

anchors on it. Most often fishermen will catch their fishing gear on the cable and the gear will just break off, but sometimes they can tug the tripod out of place. And when in July 2009 a lake dammed behind Skilak Glacier broke loose and flushed through the Kenai River, the current played with the right-bank tripod like a toy on a rope.

“It was pulling so hard on that cable it was just swinging the tripod in,” Miller said. “We’d get it out to 54 meters (from our marker onshore) where we have it now, and it would just get pulled in by the cable and current.”

Once the cable comes free, there’s not much the sonar site crew can do other than hope that nothing gets caught on it and re-deploy the tripod if it gets tugged out of position.

Dealing with cable mishaps is just one of the many challenges that keep the Kenai king sonar site’s crew on its toes throughout the spring and summer. The department has 15 sonar sites monitoring salmon throughout the state — among which the Kenai king site stands out as one of the most difficult.

At many of the department’s sonar sites, fish swim close to shore where sonar can most easily detect them. The department is also monitoring the most abundant salmon species present. Neither of these conditions is true at the Kenai king sonar site.

At the Kenai site the department is interested in monitoring kings and only kings. Yet during a great deal of the king run, the river is filled mostly with sockeye. Approximately a million sockeye and king salmon migrate up the Kenai River each year, but sockeye outnumber kings by about 17-to-1. And in the wide Kenai River kings have a tendency to swim in the middle of the stream, where fish can only be detected using very advanced sonar tools and methods.

In recognizing the challenges that distinguish the Kenai king site from most of its other sonar sites, the department has decided to change the way it reports Kenai king estimates. Instead of reporting a daily estimate, as was the practice in previous years, this year it will report estimates only on Tuesdays and Fridays.

Regional Fish and Game sonar biologist Debby Burwen says that, for many years, the king sonar site has tried to live up to

Kenai king sonar site project leader, Jim Miller, and fisheries biologist, Trevor Davis, prepare the left bank transducers for deployment into the Kenai River.

the standards set by less-complex sonar sites in the state by providing the public with a daily number. But there are big differences in the type of information that can be provided by other sonar sites and the Kenai king sonar site, she said.

Compare, for example, the Kenai king sonar site with the Kenai sockeye sonar site at River Mile 19. If estimating salmon runs were like reporting the weather, the sockeye estimate would read more like a report on the current weather and the king estimate would read more like a three-day forecast. The daily Kenai sockeye estimate is pretty much final. It changes little, if at all, after it is reported. But on a daily basis the Kenai king sonar site can only report a very preliminary estimate that changes as fisheries biologists receive additional data. When all of the data comes in, the preliminary estimate may change only a little, or it might change a lot.

The strength of Kenai king runs is gauged using many pieces of information, including data collected using sonar and nonsonar tools. In particular, fisheries managers take a hard look at four pieces of information — the rates at which sport fishermen and the sonar site’s test gillnet project are catching kings, and two estimates generated based on sonar and test gillnet data.

The two estimates include an estimate generated using sonar-detection techniques meant to account for large kings and a sonar estimate adjusted to account for large and small kings based on the proportion of large kings, small kings and sockeye caught by the site’s test gillnet project over a period of three days. None of these pieces of information can stand on its own, and an estimate accounting for all of them requires three days to mature.

Although the Kenai king site will likely always remain one of the department’s most challenging, improved sonar technology may help reduce the complexity of estimating king runs in the near future. The department hopes to replace the site’s current sonar technology, split-beam sonar, with a more advanced sonar technology known as DIDSON, which can capture high-resolution, ultrasoundlike video of fish.

DIDSON first came out in 2002 and is now being used to collect fisheries management data at 11 sonar sites. The Kenai king sonar site has deployed and field-tested DIDSON for several years, but due to the DIDSON’s short range and the tendency for Kenai kings to swim in the middle of the river, fisheries biologists have not yet been able to use it to collect management data at the Kenai king sonar site. But that might change with a recently developed long-range DIDSON equipped with a special, high-resolution lens now being tested at the site.

DIDSON has several advantages over other sonar technologies and could improve and simplify Kenai king data collection, including greater fish-sizing accuracy and ability to identify fish that mill or hold in the current. And while neither split-beam nor DIDSON can identify fish by species, fisheries biologists speculate that they might eventually be able to use the DIDSON to identify fish by species through tail-beat observations.

Burwen says it’s important that the department’s sonar and fisheries biologists continually seek out new ways to improve estimates.

“You always want to move the field forward as fast as possible, because our fish stocks are at stake,” Burwen said. “It’s too late for the East Coast, for so many of those fisheries, but not too late for us over here.”

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Filed under fishing, Kenai River, salmon

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