Upper Chehalis River Smolt Production, 2023

This report provides the third year of results from a juvenile salmonid monitoring study on the Upper Chehalis River main stem near Pe Ell, Washington in 2023. The primary objective of this study is to describe the freshwater production (e.g., smolt abundance) of Pacific salmon (Oncorhynchus spp.) and steelhead trout (O. mykiss) in the Upper Chehalis River. Specifically, we describe the abundance, timing, and diversity (body size, age structure) of juvenile outmigrants for wild Chinook (O. tshawytscha), coho salmon (O. kisutch), and steelhead trout. Based on the location and timing of our study, the results reflect juveniles that completed their freshwater rearing phase in habitats upstream of river kilometer (rkm) 151.7 (river mile 94.3) of the main stem Chehalis River.

To meet the study objectives, a 1.5-meter (5–foot) rotary screw trap was operated near rkm 151.7 (river mile 94.3) of the main stem Chehalis River from March 7 through July 16, 2023.

Scale age data indicated the presence of two age classes, one-, and two-year-olds. Coho outmigrants were predominately of the yearling (or “1+”) age class (98.5%). Scale age data indicated that there was a small 2+ year-old component of the coho out-migration (1.5%). The average fork length of known yearlings was 114.1 mm (± 10.7 mm SD), and two-year-old outmigrants was 114.2 mm (± 10.8 mm SD). Abundance of wild coho outmigrants in 2023 was estimated to be 26,028 (95% confidence intervals, CI = 18,177 – 39,266) with a coefficient of variation (CV) of 20.2%.

Steelhead outmigrants were Age-1 through Age-3, based on scales. Age-1 steelhead represented 33.7% of juveniles, Age-2 represented 65.3% of juveniles, and Age-3 represented 1.0% of the juveniles. A total of 20.4% of the scales sampled were unreadable and therefore age could not be determined from those fish. Fork length averaged 153.7 mm (± 20.6 mm SD) for Age-1, 160.2 mm (± 21.7 mm SD) for Age-2, and 201.5 mm (± 25.2 mm SD) for Age-3. Abundance of wild steelhead outmigrants was estimated to be 11,247 (95% CI = 8,050 – 16,206) with a CV of 18.2%.

The Chinook salmon outmigrants quantified were subyearlings, not fry. The majority of Chinook fry (≤ 45 mm fork length) out-migrate when flow conditions are not suitable for smolt trapping in the Chehalis River (e.g., January and February). Therefore, the goal was to estimate the subyearling (> 45 mm fork length) component of the Chinook outmigration that generally occurs from March through July. Fork length of Chinook subyearlings increased steadily throughout the trapping period with an average of 54.1 mm (± 3.3 mm SD) and 85.9 mm (± 5.5 mm SD) in the first and last sampled week of trapping, respectively. During this time roughly 98.5% of the total catch of wild Chinook outmigrants were > 45 mm. Abundance of wild Chinook subyearling outmigrants was estimated to be 79,900 (95% CI = 72,245 – 88,792) with a CV of 5.3%.