Anatomy of a Weir

Keeping a finger on the pulse of Alaska’s salmon runs

During a routine physical exam, your doctor checks your vitals, right? Weight, heart rate, blood pressure etcetera. 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 our 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 our much-valued wild Alaskan salmon.

A weir is basically a temporary permeable fence across a river with an opening in the middle. This weir sits in Alaska’s Kenai National Wildlife Refuge on a Kenai River tributary. We’ve been using it as a platform to track the health of the Funny River early-run Chinook Salmon population since 2006. Video: USFWS/Katrina Liebich

If salmon are the life blood of Alaska and its rivers are the veins, then weirs are a health screening tool for river-specific populations

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, if fewer bigger, older females are returning to a particular tributary or how different populations are contributing to “mixed-stock” harvests downstream).

Basic anatomy of a typical video weir in Alaska. Photo: USFWS/Katrina Liebich

Take a number. The “patients” wait briefly in the trap box on Alaska’s East Fork Andreafsky River before being seen and released upstream to continue their journey. Photo: USFWS/Ryan Hagerty

This Chum Salmon swam up the Yukon River and passed through our East Fork Andreafsky River weir on the way to its spawning grounds in Alaska’s Yukon Delta National Wildlife Refuge. Photo: USFWS/Ryan Hagerty

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 (DVR) equipped with motion detection software. Power includes green energy sources like solar panels. With this set-up we can record passing fish 24 hours a day even when the water is cloudy. The analog video signal is routed to the DVR and converted to digital format instantaneously. The DVR removes blank footage and only reports motion events. The video footage lets us see the entire run of fish through time and only recording motion events saves staff many hours during review. Here’s what we see as fish pass through a weir:

Meet the practitioners

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. We’d like to introduce you to a few:

For well over a decade, fisheries biologists (like Ken and Jim, pictured here) from our Kenai Fish & Wildlife Conservation Office have been pioneering innovative underwater video technology at weirs. This has led to more accurate assessments of the health of salmon runs with less manpower. Photo: USFWS/Katrina Liebich

Andrew and Jason from the village of Kwethluk help count salmon, take samples, and keep our remote Kwethluk River weir camp operational. Photos: USFWS/Lisa Hupp

Chelsea, a crew leader, shows a scale she plucked from a Yukon River salmon at the Andreafsky weir. Like the rings of a tree, scales are used to determine the age of fish. Photos: USFWS/Ryan Hagerty

Aaron, also a fisheries biologist with our Kenai Office, displays the escapement counts from the Kwethluk and Tulusak River weirs.

Case Study: monitoring Kuskokwim River salmon runs with weirs

This noteworthy river—over 700 miles long—flows through southwest Alaska. Alaska’s largest subsistence harvest of Chinook Salmon is taken from its waters each year.

Tour the Kwethluk River weir with our staff and learn more about how weirs help us manage salmon for future generations. Video: USFWS/Lisa Hupp

Recent years have seen the lowest numbers of Chinook reach their spawning grounds in documented history. In fact, the total escapement (number of spawners that escape harvest and reach their spawning grounds) in several tributaries has been at or below the level thought necessary to sustain Kuskokwim Chinook into the future. Low returns have prompted fishing closures and gear restrictions for the Kuskokwim and its tributaries.

The Kwethluk River flows through Alaska’s massive Yukon Delta National Wildlife Refuge. This photo was taken from Three Step Mountain in Western Alaska by one of our past interns.

Fisheries biologists from our Kenai Fish and Wildlife Conservation Office use weirs as their primary platform to monitor the salmon returning to two of those tributaries—the Kwethluk and Tuluksak rivers. Daily escapement numbers are relayed to fisheries managers charged with in-season (realtime) management of the mixed stock fishery in the lower river. Each stock is important to the overall health of the Kuskokwim run.

Salmon are incredibly important both culturally and nutritionally to the people of Alaska and they want to know how the runs are doing. Here, Chris Tulik, a Refuge Information Technician who serves as a liaison between the Kuskokwim River communities and fisheries managers and scientists, presents in Yupik at the village of Kwethluk. Photo: USFWS/Lisa Hupp

Parting shots: weir camp life

Due to their remoteness, some of our weirs are only accessible by boat or helicopter. In these cases, we have seasonal camps where fisheries technicians stay and keep things operational:

Killey River weir camp. Deep within the heart of Kenai National Wildlife Refuge, the Killey River is only accessible by helicopter. Two fisheries technicians stay in this camp while they keep the weir functioning and monitor the Kenai’s largest run of early-run Kenai Chinook. Photos: USFWS/Katrina Liebich

East Fork Andreafsky River weir camp. Photos: USFWS/Ryan Hagerty

The tidal Chickaloon River weir camp includes a weather port for cooking, a couple one-man tent platforms, outhouse, and gear shed. Photos: USFWS/Katrina Liebich

Kwethluk River weir camp. Photo: USFWS/Lisa Hupp

Katrina Liebich works for the U.S. Fish and Wildlife Service in Alaska in External Affairs.

In Alaska we are shared stewards of world renowned natural resources and our nation’s last true wild places. Our hope is that each generation has the opportunity to live with, live from, discover and enjoy the wildness of this awe-inspiring land and the people who love and depend on it.

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