Final Report: Spawner Abundance and Distribution of Salmon and Steelhead in the Upper Chehalis River, 2013-2017

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. Stock assessments 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 within and above 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 - index surveys were conducted at approximately 7 to 14 day intervals throughout the spawning period and supplemental surveys were conducted once or twice during peak spawning. Together, index and supplemental surveys covered the entirety of known spawning habitat for each species. Surveys started the first 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 late May or early June when no new redds were observed for two consecutive weeks at the end of the project spawning period.

Surveys were conducted either on foot or by pontoon-style boats. 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, scale collection for aging Chinook and Steelhead, tissue (genetic analysis) and otoliths (life history, Chinook only). Additional data taken for Chinook and Steelhead included scales (aging), tissue (genetic analysis) and otoliths (life history, Chinook only). Other data collected included water clarity, stream flow, riffle and pool visibility, direction being surveyed, and weather.

Results from four survey seasons document the importance of the reaches upstream of the proposed dam and 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 to absent.
• Chinook spawning activity was observed between September and November. The total number of spawners ranged between 3 and 65 spring Chinook and 297 and 424 fall Chinook. Both spring and fall Chinook spawned primarily in the main stem river, with the majority of redds (72% to 100%) found within the dam inundation footprint. Chinook spawning distribution was variable among years and responsive to river flows. For example, low flows in fall 2015 resulted in only 7% of Chinook (spring and fall) spawning outside of the main stem river whereas high flows in fall 2016 resulted in 45% of Chinook (spring and fall) spawning outside of the main stem river.
• Coho spawning activity was observed between October and February. The total number of Coho ranged between 174 and 1,590 spawners. An average of 41% of Coho spawning above the proposed dam site occurred within the inundation footprint over the four years of study.
• Steelhead spawning activity was observed between the months of December and June. The total number of steelhead ranged between 1,048 and 1,850 spawners with 7% to 16% of the redds observed prior to March 15th. An average of 35% of Steelhead spawning above the proposed dam site occurred within the dam inundation footprint over the four years of study.