Take a number. The “patients” wait briefly in the trap box before being seen and released upstream to continue their migration. (Photo by Ryan Hagerty/USFWS)

Take a number. The “patients” wait briefly in the trap box before being seen and released upstream to continue their migration. (Photo by Ryan Hagerty/USFWS)

Refuge Notebook: Anatomy of a fish weir — Keeping a finger on the pulse of Alaska’s salmon runs

During a routine physical exam, your doctor checks your vitals: Weight, heart rate, blood pressure and so on. Annual visits establish a baseline of personal health against which unhealthy trends can be detected before they become risk factors.

We need baselines for the health of salmon runs too. Weirs help us to establish those baselines and then detect changes in populations over time. They also help fisheries managers evaluate and adjust their management actions, reconstruct past salmon abundances and forecast future salmon returns.

So, get your scrubs on. We’re going to dissect how weirs work and what they tell us about the health of much-valued wild Alaska salmon.

The weirs we run in Alaska are temporary, permeable “fences” across rivers with an opening in the middle. We construct our weirs with panels of evenly spaced PVC pickets aligned parallel to the direction of flow.

The upstream end of each panel is anchored to the river bottom and the downstream end is lifted above the surface by a resistance board that planes upward in flowing water.

The pickets don’t go all the way across the river. They guide migrating fish into a trap box that is occasionally closed so fisheries biologists can briefly take a few measurements and samples from a subset of fish: length, sex, scales for aging and a small fin clip for genetic analyses.

This information can tell us many things about the health of salmon populations. For example, we discover if fewer bigger, older females are returning to a particular tributary or how different populations are contributing to “mixed-stock” harvests downstream.

Finally, fish enter a narrow chute and pass in front of a video box on their way out. Standard video components include a sealed box filled with clear water that houses a video camera, pond lights and a digital video recorder equipped with motion detection software.

Power includes green energy sources like solar panels. With this setup we can record passing fish 24 hours a day even when the water is cloudy. The analog video signal is routed to the digital video recorder and converted to digital format instantaneously.

The digital video recorder removes blank footage and only reports motion events. The video footage lets us see the entire run of fish through time. The remote video cameras only record motion events, which saves staff many hours during review.

Much like a team of doctors and nurses and other specialists keep tabs on the human health, teams of fisheries professionals keep tabs on the health of Alaska’s salmon runs. For well over two decades, fisheries biologists have been pioneering innovative underwater video technology at weirs.

This has led to more accurate and efficient assessments of the health of salmon populations returning to the tributaries of the Kenai River in Kenai National Wildlife Refuge.

In Alaska, Fish and Aquatic Conservation staff are currently operating weirs in Yukon Delta, Yukon Flats, Koyukuk and Kenai national wildlife refuges. Weirs are one way we collect information to make management decisions that ensure that the fisheries are here for future generations.

In next week’s Refuge Notebook article, we will be talking more specifically about Kenai River chinook salmon.

Katrina Liebich is co-host of the award-winning “Fish of the Week!” podcast and part of the Alaska External Affairs team with the U.S. Fish & Wildlife Service. To find more Refuge Notebook Articles go to https://www.fws.gov/kenai-refuge-notebook

Ken Gates (left) and Jim Boersma at the Funny River weir in Kenai National Wildlife Refuge, Alaska. (Photo by Katrina Liebich/USFWS)

Ken Gates (left) and Jim Boersma at the Funny River weir in Kenai National Wildlife Refuge, Alaska. (Photo by Katrina Liebich/USFWS)

More in Sports

Kenai Central’s Kylee Verkuilen races Nikiski for control of the ball during a soccer game at Ed Hollier Field in Kenai, Alaska, on Friday, April 12, 2024. (Jake Dye/Peninsula Clarion)
Kenai sweeps, shuts out Nikiski in Friday soccer games

Kenai girls and boys teams opened with early goals

Head coach Taylor Shaw (center) talks to the Kenai River Brown Bears during a timeout at the Soldotna Regional Sports Complex in Soldotna, Alaska, on Saturday, March 23, 2024. (Photo by Jeff Helminiak/Peninsula Clarion)
Season review: Brown Bears hope to build on solid finish to season

It’s not easy to finish a season in last place in a division, yet have plenty of reasons for optimism for the next season

tease
Homer snow shuffles sports schedule

The Mariners were supposed to host Kenai Central in Thursday

A pair of Trumpeter Swans break through the thin ice in search of emergent vegetation at the Kenai River Flats with Mt. Redoubt in the background. (Photo courtesy T. Eskelin/USFWS)
Refuge Notebook: Has spring sprung?

I have always found the arrival of spring to be championed by the first sightings of geese at the Kenai and Kasilof Flats

tease
Peninsula Piranhas win South Central Area Championships

The Peninsula Piranhas won the South Central Area Championships in Homer on… Continue reading

Lily Langham of Kenai Central High School and Brooklyn Larsen, Rylie Thompson and Ava Fabian of Soldotna High School, stand for a photo with their medals after competing in the USA Hockey National Championships as part of the Alaska All Stars 19U Division 2A hockey team. (photo provided)
Local skaters help Alaska All Stars to semis of national tourney

Four central Kenai Peninsula skaters were on the Alaska All Stars 19U… Continue reading

tease
SoHi girls win distance medley relay at Big C Relays

A number of Kenai Peninsula athletes nabbed top-eight finishes at the Russ… Continue reading

tease
Saturday: Brown Bears finish season with comeback victory in Fairbanks

The Kenai River Brown Bears finished their season with a 4-3 victory… Continue reading

Most Read