How scientists use sonar to figure escapement counts

Jun 15, 2018

Ever wonder how biologists and fishery managers estimate how many of each salmon species are escaping upstream? Here's your answer!

Matt Nemeth and Chris Sewright get a sonar pod ready for deployment in the Kvichak River on May 29, 2015.
Credit Molly Dischner / KDLG

Commercial fishermen in Bristol Bay's Nushagak district stepped closer to opening their season Friday as officials released the year’s first escapement counts divvied up by species after eight days of counting fish near Portage Creek. 

On Thursday, 2,759 king salmon swam past the Nushagak River sonar along with 1,977 sockeye and a big push of 7,516 chums. That brings the yearly total to 3,406 kings, 5,107 sockeyes and 10,250 chums.

Tim Sands, westside area management biologist for the Alaska Department of Fish and Game, said fair weather and lack of wind have put us slightly behind on king salmon escapement for this time of year.

"That’s not a surprise really this year because we knew we were behind, and we can catch up really fast. It’s not like it’s the middle of the season where being behind means it’s hard to catch up," Sands said.

As for when commercial fishermen will be able to put their nets in the water, Sands can only say sometime next week. Howver, getting apportioned escapement counts is one hopeful sign it’s coming over the horizon.

So, how exactly does the apportionment process work?

Sands explained it’s a combination of high-tech sonar and an old-school, handheld tally counter. Sonar records 10-minute snapshots every hour all day long from a site about 25 miles upstream from the commercial fishery – just downstream from Portage Creek. Technicians then total up the number of fish swimming by to get a raw count.

"So that's what the image looks like," Sands said, pulling up an ultrasound-like video on his computer. "You can't tell species from this. All you can tell is they're fish. To break them down into species, you have to catch some of them. The proportion caught of each species, then you can apply that proportion to the number you counted in each zone, and there's your numbers." 

Greg Buck is a biologist with the Alaska Department of Fish and Game who's spent decades studying Bristol Bay. He said they used to catch and release 100 fish across the whole 1,000-foot-wide Nushagak River to apportion escapement counts. That ended up taking too long to be useful for fishery management, so they took a different tack approximately around the year 2005.

"Now, we've started fishing more and broke the river into different strata, and that’s allowed us to drop the sample size. So now we apportion on a daily basis rather than a pool across days across the whole river," Buck said. 

There’s an in-shore and off-shore zone on both sides of the river, and Sands said they have to catch five fish in each zone before they can be confident in their apportionment data.

He refuses to release any count before then because he’s been burned in the past. That’s why it took eight days of counting this year to get apportionment data. Sands recalled using historical data one year to apportion escapement counts that turned out to be nowhere near the real count.

"The public got really bent out of shape because the number they were concerned about went down," Sands said. "People were like, 'We'd be fired if we kept books like that!' It's easier to say, 'Sorry we don't have any numbers. We'll tell you when we have numbers, but right now we don't have any numbers.' Because we don't. And to pretend that we do can be worse than saying that we don't."

Now that we’re getting this year’s numbers, it’s a waiting game. Sands has to be confident that Nushagak River will at least hit its lower escapement goal of 55,000 king salmon before opening the district up for commercial fishermen hunting sockeye.

"We've been fishing since 1884 and they're still here, so that's an indicator that it's somewhat sustainable," Sands said, knocking three times on his wooden desk.

For more details on the Nushagak River sonar, visit the website here.

Contact the author at austin@kdlg.org or 907-842-2200.