Spawner Abundance and Distribution of Salmon and Steelhead in the Upper Chehalis River, 2017-2018

Flood control alternatives being analyzed as part of the Chehalis Basin Strategy include a dam that would be located in the main stem Chehalis River at river mile 108.2 upstream of the town of Pe Ell. Monitoring efforts in the Chehalis River basin have not historically focused on delineating population trends above or below the location of the proposed dam. Information on spawner abundance and distribution data in this area of the river was identified as a data gap by the Aquatic Species Enhancement Plan Technical Committee of the Chehalis Basin Strategy (Aquatic Species Enhancement Plan Technical Committee, 2014). This study was undertaken to understand the numbers and species of salmonids that would be affected above, within and below the footprint of the proposed dam and its associated reservoir. This work also informs fish passage needs should a dam be chosen as a structural solution to flooding within the Chehalis Basin.

Two types of surveys methods were used to evaluate salmonid spawners above the proposed dam site - index surveys were conducted at approximately seven day intervals throughout the spawning period and supplemental surveys were conducted once during peak spawning. Together, index and supplemental surveys covered the entirety of known spawning habitat for each species. Surveys started the third week of September based on prior knowledge of when fish (spring Chinook) begin spawning and continued on a weekly basis through the spawning seasons for spring and fall Chinook salmon, Coho salmon, and winter Steelhead. Surveys concluded in mid-June when no new redds were observed for two consecutive weeks at the end of the project spawning period. An additional survey of the main stem Chehalis River from river mile 108.2 (proposed dam site) downstream to the Newaukum River confluence (RM 75.4)was conducted four times, once during the peak spawn timing for each species and run type. The purpose of these additional surveys was to document the spatial distribution of spawning in the main stem river.

Surveys were conducted either on foot, pontoon-style boats or helicopter. Crews identified and recorded all spawning activity by species per reach segment. Individual redd locations were georeferenced. Live and dead fish counts included the species and sex. Dead fish or carcass sampling included fin mark sampling, fish length, coded wire tag (CWT) status and scale collection for aging Chinook and Steelhead. Additional tissue samples were taken for Coho genetic analysis.

Results from 2017-2018 coincide with the previous year's results (2013-2017) and document the importance of the reaches upstream, within, and below the proposed dam footprint and associated reservoir as a spawning area for spring and fall Chinook, Coho, and winter Steelhead:

• The majority of spawners observed for all species in this area of the watershed were wild; hatchery fish were rare.
• Abundance of spring and fall Chinook salmon within the study area was estimated to be 8(spring) and 239 (fall) adult spawners. Chinook spawning activity was observed between September and December. Both spring and fall Chinook spawned primarily in the main stem river, 100% and 97% respectively. The percentage of fall Chinook redds found within the dam inundation footprint was 95%. Spring Chinook had only one of the three (33%) redds observed in the inundation footprint.
• Abundance of Coho salmon within the study area was estimated to be 1240 adult spawners. Coho spawning activity was observed between mid-November and February with 33% spawning in the inundation footprint.
• Abundance of winter Steelhead within the study area was estimated to be 942 adult spawners. Steelhead spawning activity was observed between the months of February and June. Steelhead redds observed prior to March 15th composed 9% of the total observed redds. The percentage of estimated redds of pre-March 15th and post March 15th in the inundation footprint were 39% and 23%, respectively.
• On the main stem river from the proposed dam site to Newaukum River, we observed very little Coho and Steelhead spawning but substantial spring and fall Chinook spawning activity. The highest density of fall Chinook occurred between the proposed dam site (RM 108.2) and Elk Creek (RM 100.2). In contrast, spring Chinook spawning distribution was more evenly spread between the proposed dam site and the Newaukum River.