Upper Chehalis River Smolt Production, 2021

This report provides the first year of results from a juvenile salmonid monitoring study on the Upper Chehalis River main stem near Pe Ell, Washington in 2021. 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 24 to July 2, 2021.

The Chinook salmon outmigrants quantified were subyearlings, not fry. The majority of Chinook fry (≤ 45 mm fork length) outmigrate 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 50.3 mm (± 3.2 mm, standard deviation SD) and 92.1 mm (± 7.6 mm SD) in the first and last sampled week of trapping, respectively. During this time roughly 98% of the total catch of wild Chinook outmigrants were (> 45 mm). Abundance of wild Chinook subyearling outmigrants was estimated to be 118,834 ± 7,513 SD with a coefficient of variation (CV) of 6.3%.

The coho outmigrants quantified were yearlings. Scale age data indicated the presence of two age classes, one- and two-year-olds. The average fork length of age one yearling outmigrants was 115.1 mm (± 11.5 mm SD) and age two yearling outmigrants averaged 131.3 mm (± 17.8 mm SD). Abundance of wild coho outmigrants was estimated to be 12,932 ± 1,656 SD with a CV of 12.6%.

Steelhead outmigrants were one, two, and three years of age. Fork length averaged 149 mm (± 15.7 mm SD) for one-year olds, 162 mm (± 15.3 mm SD) for two-year olds, and 210.5 mm (± 55.4 mm SD) for three-year olds. Abundance of wild steelhead outmigrants was estimated to be 9,736 ± 2,857 SD with a CV of 27.6%.

Table 1. Abundance of Chinook, coho, and steelhead outmigrants that completed their freshwater rearing phase upstream of river kilometer 151.7 (river mile 94.3) of the Upper Chehalis River.