Chehalis River Smolt Production 2020

This report provides the 2020 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 (O. mykiss), 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 18 to July 21, 2020.

Coho outmigrants were predominately of the yearling (or “1+”) age class (98.8%) with rare occurrences of subyearlings (or “0+”) and 2-year-old outmigrants (0.59% and 0.59%, respectively). Abundance of wild coho outmigrants was estimated to be 463,566 ± 42,944 standard deviation (SD) with a coefficient of variation (CV) of 9.2% (Table 1).

Steelhead outmigrants were one, two, and three years of age (45.4%, 50.3% and 4.3%, respectively), indicating three different juvenile life histories. Fork length averaged 162.4 mm (± 15.9 mm SD) for fish that were 1-year-old, 172.3 mm (± 17.2 mm SD) for 2-year old, and 187.3 mm (± 33.3 mm SD) for 3-year old. Abundance of wild steelhead outmigrants was estimated to be 38,647 ± 10,746 SD with a CV of 26.5% (Table 1).

Chinook outmigrants were subyearlings. Fork length of Chinook transitional and smolt subyearlings increased steadily throughout the trapping period with an average of 55.3 mm (± 5.9 SD) and 99.6 mm (± 7.0 SD) in the first and last full week of trapping, respectively. Abundance of wild Chinook subyearling outmigrants was estimated to be 320,358 ± 34,319 SD with a CV of 10.5% (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.