The Alaska Department of Fish and Game changed the way it counts fish passing through the sonar site on the Nushagak River this year.
The sonar, located about 25 miles upstream from the commercial fishing district, allows technicians to get population estimates of Chinook, chum and sockeye salmon as they swim back to their spawning grounds.
But in a memo released quietly in mid-June, state biologists said a recent study showed the existing counting method may have underestimated Chinook and chum races. And they said they hope the new methodology will provide more accurate counts of salmon species in the future – which could be increasingly important as managers grapple with low numbers of king salmon on the Nushagak and around Alaska.
Bristol Bay area research biologist Jordan Head said sonar gives Fish and Game technicians a visual of fish swimming past them so they can count the run.
“It almost looks like an ultrasound, where you can see fish swimming past the sonar,” he said.
Sonar records 10-minute intervals, every hour, at two different sections of the Nushagak River – one near shore and one offshore. Head says sockeye usually swim close to shore while kings and pals are usually further offshore. But sonar does not differentiate between species.
“We see the fish go up the river as if on a [counting] round, but we can’t see what they are,” he said. “So we started a driftnet program. We derive several different gillnet meshes through each of these two strata on each bank.
Technicians analyze sonar and catches at these two locations to estimate how many of each species are coming up the river.
Until this summer, the department also took into account the number of fish swimming downstream – called downstream fish. Technicians would subtract this downstream count from the upstream count for their final estimate of upstream migrants for that time period.
“It works really well,” Head said. “But there are a lot of assumptions we make with it.”
He said that so far the sonar project has operated under two main assumptions. The first is that the number of fish downstream breaks down into the same species composition as the fish upstream. For example, if they split the upstream count to 90% sockeye, 7% chum, and 3% kings, they apply that ratio to the downstream count as well. The other hypothesis, Head said, is that fish swim both upstream and downstream in the same section of the river – either onshore or offshore.
“We always want to try to make as few assumptions as possible, especially when we can’t test the assumptions in our project design,” he said.
Head said the department’s study last year indicated subtraction may have underestimated Chinook and chum runs.
When the technicians did not subtract the downstream count, the number of Chinook runs increased by about 9%. The chum return increased by 3% and the sockeye return increased by 1%.
“What we think is happening up there, and why this change was made, is because you have sockeye escapements that have millions of fish, and you have king escapements that are are in, you know, a 50,000 fish range,” Head said. “And so if on average about 1% of the fish go back and go downstream, if we misapply those sockeye downstream as kings, that’s a big deal in the king count.”
The change comes as area managers, anglers and residents closely monitor escapement numbers after several years of low chinook runs in the Nushagak. This year just over 44,000 Chinooks have escaped, well below the minimum target of 55,000.
The Nushagak District Chinook harvest is 4,605 fish so far. It is the biggest catch of all the districts in the bay. The bay-wide harvest of 7,558 Chinooks is well below the 20-year average of around 40,000 kings.
Head said they are still counting fish downstream this season and a report on the change will be available before the Alaska Fisheries Council meeting this winter.
He added that the count difference between the two methodologies varies each year. And so far this year, that hasn’t meant much of a difference.
“That may not be a best estimate. But scientifically speaking, we make fewer assumptions. And we have smaller errors – potential errors – associated with making fewer assumptions,” he said.
Head plans to reanalyze counts dating back to 2006 to see how the new counting method may affect the department’s escapement goals for the Chinook run.
“Hopefully in the next two weeks I can go out and do the Chinook post-season aerial surveys in the tributaries and kind of be able to tell us if it was better than last year, or if it was no better than last year,” he said.
Preliminary estimates of chinook escapements in Nushagak District should be available this fall.
There are a number of sonar sites across the state, some of which count salmon differently.
Sportfish area management biologist Colton Lipka says the sonar site on the Kenai River is similar to the Nushagak in that it captures 10-minute intervals every hour. But the Kenai sonar is dedicated to Chinook counting. They monitor the courses based on the size of the fish, counting salmon 34 inches and over.
“We know with fairly good certainty that it will be a king because the other species present would not be of this size. And because of the large proportion of big fish in Kenai Pass, we can then use that as an assessment metric,” he said.
Lipka said he started this program in 2017 to more accurately monitor the Chinook’s exhaust there. Before that, they monitored all species, but they changed to focus on kings. He says it works for the Kenai because of the larger population of big fish for that race.
Kenai is also facing a decline in Chinook returns, which have been declining since 2010. Again this week, sport and set net fishing was closed due to low returns.
This year, the comeback is estimated at 12,700, which is still below the breakaway target of 15,000 to 30,000. But Lipka said the season continues until August 20, so they hope that the race will increase.