Chehalis River Smolt Production, 2022

This report provides the 2022 results from the juvenile salmonid smolt monitoring study on the Chehalis River main stem near Rochester, WA. The primary objective of this study is to describe the freshwater production (e.g., smolt abundance) of Pacific salmon (Oncorhynchus spp.) and steelhead (O. mykiss) in the Chehalis River. Specifically, we describe the timing and diversity (body size, age structure, genetics) and estimates of abundance of juvenile outmigrants for wild coho salmon (O. kisutch), steelhead, and Chinook salmon (O. tshawytscha). 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 84 (river mile 52) of the main stem Chehalis River.

To meet the study objectives, a 2.4 meter (8–foot) rotary screw trap was operated near river kilometer 84 (river mile 52) of the main stem Chehalis River from March 24 to July 25, 2022.

Coho outmigrants were predominately of the yearling (or “1+”) age class (81.6%) with rare occurrences of subyearlings (or “0+”) and 2-year-old outmigrants (18.1% and 0.3%, respectively). Fork length averaged 111.8 mm (± 7.2 standard deviation, SD) for subyearlings, 125.9 mm (± 12.2 SD) for yearlings, and 165.0 mm (one individual captured, no SD) for 2-year-olds. Abundance of wild coho outmigrants was estimated to be 163,354 (95% credible interval, CI = 118.767 – 246,552) with a coefficient of variation (CV) of 19.7% (Table 1).

Steelhead outmigrants ranged from Age-1 to Age-4 based on scales (47.8%, 42.8%, 8.2% and 1.3% for Age-1 through Age-4, respectively), indicating four different juvenile life histories. Fork length averaged 168.7 mm (± 20.6 SD) for Age-1, 165.4 mm (± 14.7 SD) for Age-2, 197.2 mm (± 22.0 SD) for Age-3, and 220.5 mm (± 6.4 SD) for Age-4. Abundance of wild steelhead outmigrants was estimated to be 36,236 (95% CI = 15,936 – 107,650) with a CV of 58.9% (Table 1).

Chinook outmigrants were subyearlings (Age-0). Fork length of Chinook transitional and smolt subyearlings increased steadily throughout the trapping period with an average of 49.1 mm (± 7.9 SD) in the first week of trapping and 106.6 mm (± 5.5 SD) in the last full week of trapping. Abundance of wild Chinook subyearling outmigrants was estimated to be 247,707 (95% CI = 215,914 – 289,634) with a CV of 7.9% (Table 1).

Table 1. Abundance of coho, steelhead, and Chinook outmigrants that completed their freshwater rearing phase upstream of river kilometer 84 (river mile 52) of the main stem Chehalis River.