Estimates of Adult Fall Chinook Salmon Spawner Abundance and Viable Salmonid Population Parameters in the Washington Portion of the Lower Columbia River Evolutionarily Significant Unit, 2013-2017

In 1999, Chinook salmon (Oncorhynchus tshawytcha) in the LCR Evolutionarily Significant Unit (ESU) were listed as threatened under the Endangered Species Act. As a result, WDFW began to strategically implement more intensive escapement monitoring for adult fall Chinook salmon on select populations over the next decade in an effort to improve estimates. In 2010, WDFW modified and expanded its existing fall Chinook salmon escapement monitoring program to ensure estimates of viable salmonid population parameters were being monitored for all populations within the LCR ESU and that these parameters met the National Oceanic and Atmospheric Administration Fisheries guidelines for accuracy and precision (Crawford and Rumsey 2011). This report covers methods and results from spawn years 2013-2017 and updates previously reported results from spawn years 2010-2012 as models have been improved. We report only adult fall Chinook salmon estimates (classified as 60 cm and larger) due to challenges in obtaining sufficient jacks to estimate accurately. Adult abundance was estimated using a variety of methods including weir counts, Lincoln-Petersen and Jolly-Seber mark-recapture models, area-under-the-curve based on live counts, redd expansion based on census redd surveys, and peak count expansion depending on resources and survey conditions. Carcasses recoveries were used to determine the marked and unmarked proportion based a combination of adipose and ventral fin clips and coded-wire tags (CWTs). The marked and unmarked was further adjusted by juvenile mass mark rates from hatchery releases to account for hatchery production that was released unclipped and untagged (~1-3%). Annual estimates of natural-origin Tule fall Chinook salmon spawner abundance for the Washington portion of the LCR ESU ranged from a low of 7,065 in 2012 to a high of 18,941 in 2015. The larger Cascade stratum populations, the Cowlitz and Lewis, had the largest annual estimates of natural-origin Tule fall Chinook salmon spawner abundance. The three populations which comprise the Coast stratum (Grays/Chinook, Elochoman/Skamokawa, and Mill/Aber/Germ) and the Lower Gorge population had smallest annual estimates of natural-origin Tule fall Chinook salmon spawner abundance. Weir operations were successful at reducing the proportion of hatchery-origin spawners for some populations. However, the proportion of hatchery-origin spawners exceeded Hatchery Scientific Review Group recommendations for most populations in most years. Most Tule fall Chinook salmon populations were comprised of predominately hatchery-origin spawners except the Coweeman. Age structure varied by population, but most Tule fall Chinook salmon were age-3 and age-4. A total of 2,106 snouts were collected from the field and CWTs decoded successfully. CWT recoveries were uploaded to the regional database (Regional Mark Information System) and unexpanded CWT recoveries indicate most hatchery-origin Tule fall Chinook salmon returned to the basin of release or an adjacent basin. Bright stock fall Chinook salmon not native to this ESU are successfully spawning in the Grays/Chinook, Lower Gorge, Upper Gorge, and White Salmon populations. Assumption testing indicated our abundance and proportion estimates were relatively unbiased.