Population Monitoring of Chinook Salmon in the Washington Portion of the Lower Columbia River Evolutionarily Significant Unit, 2018-2019

The Washington Department of Fish and Wildlife (WDFW) is one of several stakeholders that manages Pacific salmonids within the Columbia River basin. Following the listing of several species of Columbia River salmon and steelhead under the Endangered Species Act, the state mandated mass marking and WDFW adopted an adaptive management-based approach to limit the escapement of hatchery-origin fish in the Lower Columbia River by installing several resistance-board weirs. In 2010, WDFW restructured the agency’s annual monitoring program for naturally spawning Spring and Fall Chinook salmon populations around the viable salmonid population (VSP) concept, and more recently, expanded efforts to characterize the effects of the weirs on natural populations including quantifying the proportion of hatchery-origin (pHOS) spawners above and below the weirs. In survey years 2018 and 2019, WDFW conducted weekly spawning ground surveys from August to December and operated seven resistance-board weirs to quantify adult spawner abundance, diversity, productivity, and spatial structure for Fall and Spring Chinook salmon present in Southwest Washington tributaries. Approximately 2100 Spring Chinook, 26600 early fall/tule (EFT) Chinook, and 60600 late fall/bright (LFB) Chinook total adult spawners were estimated for the twenty Spring and Fall Chinook populations in Southwest Washington when summing the two survey years. For designated populations within the Lower Columbia River Chinook Evolutionarily Significant Unit (ESU), the Lower Cowlitz had the highest adult spawner abundances of Spring Chinook and EFT Chinook in both survey years. The North Fork Lewis population is the only designated LFB Chinook population in the ESU, and had the highest overall spawner abundance of LFB Chinook in 2019 for all LFB Chinook populations monitored by WDFW. In 2018, the Lower Gorge population had the highest spawner abundance of LFB Chinook (specifically upriver bright Chinook) though these fish are not a designated population within the ESU. Most of the LFB Chinook that are not recognized as belonging to any established recovery population with the Lower Columbia River Chinook ESU were observed in the populations of the Gorge strata. Based on the minimum natural-origin spawner recovery goals set by the National Marine Fisheries Service, four of the thirteen designated Lower Columbia River Chinook populations are above their goals based on 5-year geometric means of the smoothed spawner abundances. Five additional Chinook populations are within 25-75% of their recovery goals. All the Lower Columbia River Spring Chinook populations are far from their minimum natural-origin spawner recovery goals with three populations believed to be at critically low abundance or extirpated. Based on limits outlined in the Mitchell Act Biological Opinion, estimated mean pHOS for one of the ten Chinook populations discussed in the BiOp is under the limit; however, full BiOp implementation has not yet occurred. Future work is needed to discern EFT from LFB Chinook in several tributaries, and assess the possible negative impacts that spawning LFB Chinook adults are having on recovery of natural-origin populations of EFT Chinook in the Lower Columbia River Chinook ESU.