The Alaska Department of Fish and Game changed how it counts the fish that pass through the sonar site on the Nushagak River this year.
The sonar, about 25 miles upriver of the commercial fishing district, allows technicians to get population estimates of Chinook, chum and sockeye salmon as they swim upstream toward their spawning grounds.
But in a memo quietly published in mid-June, state biologists said a recent study showed the existing method of counting may have underestimated Chinook and chum runs. And they said they hope the new methodology provides more accurate counts of the salmon species going forward — which could be increasingly important as managers grapple with low king salmon counts on the Nushagak and around Alaska.
Bristol Bay area research biologist Jordan Head said the sonar gives Fish and Game technicians a visual of the fish swimming past so they can get a count of the run.
“It almost looks like an ultrasound, where you can see fish swimming past the sonar,” he said.
The sonar records 10-minute intervals, every hour, at two different sections on the Nushagak River — one near shore and one off-shore. Head said that sockeye usually swim near shore while kings and chum are usually further offshore. But the sonar doesn’t differentiate between species.
“We can see the fish swimming upriver just like on a [counting] tower, but we can't see what they are,” he said. “So we then run a drift gillnet program. We drift several different gill net mesh sizes through each of those two strata on each bank.”
Technicians analyze the sonar and the catch in those two places to estimate how many fish in each species is running upriver.
Until this summer, the department has also factored in the number of fish swimming downriver — called downstream fish. Technicians would subtract that downstream count from the upstream count for their final estimate of upstream migrators for that time period.
“That works really well,” Head said. “But there's a lot of assumptions that we're making with it.”
He said until now, the sonar project has operated under two major assumptions. One is that the downstream fish count breaks down into the same species makeup as the upstream fish. For example, if they apportion out that the run upriver at 90% sockeye, 7% chum and 3% kings, they apply that ratio to the downstream count, too. The other assumption, Head said, is that the fish swim both downstream and upstream in the same section of the river — either in-shore or offshore.
“We always want to try and make the least amount of assumptions that we can, especially when we can't test the assumptions in our project design,” he said.
Head said the department’s study last year indicated that subtraction may have undercounted both Chinook and chum runs.
When technicians didn’t subtract the downstream count, the Chinook run numbers increased by approximately 9%. The chum run went up 3% and the sockeye run increased by 1%.
“What we think is going on up there, and why this change was made, is because you have sockeye escapements that are in the millions of fish, and you have king escapements that are in the, you know, 50-ish thousand fish range,” Head said. “And so if on average, about 1% of the fish do circle back and come downstream, if we're misapplying those downstream sockeye as kings, that makes a big deal in the king count.”
The change comes as area managers, fishermen and residents are watching escapement counts closely after several years of low Chinook returns up the Nushagak. This year, just over 44,000 Chinook have escaped, far under the minimum goal of 55,000.
The Nushagak District’s Chinook harvest is 4,605 fish so far. That’s the largest catch of all districts in the bay. The bay-wide harvest of 7,558 Chinook is far lower than the 20-year average of around 40,000 kings.
Head said they are still counting the downstream fish this season, and that a report on the change will be available before the Alaska Board of Fisheries meeting this winter.
He added that the difference in counts between the two methodologies varies each year. And so far, this year it hasn’t meant much of a difference.
“It might not be a better estimate. But scientifically speaking, we're making less assumptions. And we have smaller errors – potential error – associated with making less assumptions,” he said.
Head is planning to re-analyze counts dating back to 2006 to see how the new way of counting may affect the department’s escapement goals for the Chinook run.
“Hopefully within the next two weeks, I'm going to be able to get out and do the postseason Chinook aerial surveys in the tributaries and kind of be able to tell us if it was better than last year, or if it wasn't better than last year,” he said.
Preliminary estimates for Chinook escapements in the Nushagak District are expected to 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 said the sonar site on the Kenai River is similar to the Nushagak’s in that it captures 10-minute intervals every hour. But the Kenai sonar is dedicated to just counting Chinook. They monitor runs by the size of the fish, counting salmon 34 inches and larger.
“We know with some pretty good certainty that that is going to be a king as the other species present would not be of that size. And due to the large proportion of large fish present in the Kenai run, we are able to then use that as our assessment metric,” he said.
Lipka said they started that program in 2017 to more accurately monitor Chinook escapement there. Prior to that they monitored all species, but they made the change to focus on kings. He said it works for the Kenai because of the bigger population of larger fish for that run.
Kenai is also facing a decrease in Chinook runs, which have declined since 2010. Just this week, sport and set-net fishing was closed due to low returns.
This year, the run is estimated at 12,700, that’s still below the escapement goal of 15,000 to 30,000. But Lipka said the season goes through August 20, so they’re hopeful the run will increase.
