Chehalis River Smolt Production, 2019

This report provides the 2019 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) of juvenile outmigrants for wild coho salmon (O. kisutch), steelhead, and Chinook salmon (O. tshawytscha). In addition, we generated estimates of abundance for wild coho and steelhead in 2019, but not for Chinook. 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 27 to June 16, 2019. Coho outmigrants were predominately yearlings, however scale age data indicated a small subyearling component of the coho outmigration starting sometime near the middle of May. Abundance of wild coho outmigrants was estimated to be 363,214 ± 31,169 standard deviation SD with a coefficient of variation CV of 8.5% (Table 1).

Steelhead outmigrants were one, two, and three years of age, indicating three different juvenile life histories. Fork length averaged 157.4 mm (± 11.9 mm SD) for one-year olds, 184.8 mm (± 26.9 mm SD) for two-year olds, and 245.4 mm (± 61.6 mm SD) for three-year olds. Abundance of wild steelhead outmigrants was estimated to be 29,024 ± 5,343 SD with a CV of 18.4% (Table 1).

Chinook outmigrants were subyearlings. High stream temperatures and associated issues precluded trapping over the entirety of the Chinook outmigration. Therefore, data collection was not adequate to produce an estimate of subyearling Chinook abundance. Fork length of Chinook transitional and smolt subyearlings increased steadily throughout the trapping period with an average of 50.5 mm (± 2.6 SD) and 74.2 mm ± 8.2 SD in the first and last full week of trapping, respectively.