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.
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).
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:
Case Study: monitoring Kuskokwim River salmon runs with weirs
This noteworthy river—over 700 miles long—flows through the remote and wild landscape of southwest Alaska. It’s particularly significant in terms of supplying food for Alaska Natives: Alaska’s largest subsistence harvest of Chinook Salmon is taken from its waters each year.
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.
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.
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:
Katrina Liebich is based in Anchorage and has served in her current position as the Alaska Fisheries Outreach Coordinator for the U.S. Fish & Wildlife Service since 2010. Contact her at: Katrina_Liebich@fws.gov/(907) 786–3637
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.