Evaluation of Adult Fish Weirs Used to Control the Proportion of Hatchery-Origin Fall Chinook Salmon in Six Washington Lower Columbia River Tributaries, 2013-2017

Chinook salmon (Oncorhynchus tshawytcha) in the Lower Columbia River (LCR) Evolutionarily Significant Unit (ESU) were listed under the Endangered Species Act in 1999. The Hatchery Scientific Review Group, which completed a review of hatcheries throughout the Columbia Basin, found that interactions between natural- and hatchery-origin fish on the spawning grounds was one of the key factors limiting recovery of naturally spawning populations of LCR Chinook salmon (HSRG 2009b). In 2009, the Washington Fish and Wildlife Commission adopted the Fishery and Hatchery Reform Policy, which mandated that Washington Department of Fish and Wildlife reform its hatchery operations to comply with HSRG recommendations. As a result, Washington Department of Fish and Wildlife began a series of hatchery reform and monitoring actions oriented around LCR fall Chinook salmon, including the installation and operation of river-spanning weirs. The first weir was installed on the Grays River in 2008, followed by four weirs that were operational by the fall of 2011, and an additional weir that was installed in 2015. These projects have three objectives: (1) to complement existing adult salmonid monitoring efforts, (2) to promote recovery of fall Chinook salmon by controlling the proportion of hatchery-origin spawners (pHOS) on the spawning grounds, and (3) to assist collection of hatchery broodstock. This report focuses on the effectiveness at meeting objective two for weirs operated on the Grays River for the fall of 2008-2016, the Elochoman River for the fall of 2009-2017, the Green River for the fall of 2010-2017, the Coweeman and Washougal rivers in the fall of 2011-2017, and the lower Kalama for 2015-2017. In order to measure effectiveness, we quantified weir efficiency (the proportion of the upstream population captured at a weir) and pHOS, and began to document effects of weirs on natural spawning populations. Weir efficiencies were highly variable depending on the site and year. The three sites with permanent infrastructure (Green, Kalama, and Elochoman) had weir efficiency in excess of 90% for adult fall Chinook salmon in 16 out of 20 year by location combinations. For the three sites without permanent infrastructure (Grays, Coweeman, and Washougal), weir efficiencies were in excess of 90% for only 3 of 23 year by location combinations. In terms of managing pHOS, the Kalama River Weir had the greatest success, reducing pHOS by 35-48% during its three-year evaluation period. All other locations had significantly less impact on pHOS, with Grays River Weir showing the least impact at 1-12% reduction in pHOS over its nine-year period of evaluation. Overall, pHOS levels have been trending down for many of the populations since weir operations began. However, for most populations, pHOS targets are still not being met for several reasons, including spawning below the weir sites, contributions of hatchery-origin spawners to pHOS from subpopulations without weirs, and the inability to remove unclipped hatchery-origin spawners at weirs. Additionally, we have documented several unintended consequences of weir operations, including a downstream shift in the spatial distribution of fall Chinook salmon spawners, lower apparent residence time of spawners above weirs, and clustering of spawning in areas below weirs, all of which likely result from weirs impeding migration. We have not yet quantified the effects of these impacts on the dynamics of naturally spawning populations, but these results may be harmful, potentially offsetting benefits resulting from reduced pHOS. As a result of their high efficiency, we recommend continuing status quo weir operations on Kalama, Green, and Elochoman rivers. In order to have a greater impact at reducing pHOS, we recommend investing in permanent infrastructure and acquisition of land in order to improve weir effectiveness for the Grays, Coweeman, and Washougal weirs. Finally we believe it is imperative to better measure and understand the population dynamic effects of weir-induced migration delays in order to determine whether weirs are able to act as a net benefit to naturally spawning populations through pHOS reduction, or instead act as a net harm to wild populations through reduced population productivity due to migration delay and redistribution of spawners