Newaukum River Smolt Production, 2019

This report provides the 2019 results from the juvenile salmonid monitoring study on the Newaukum River main stem near Centralia, 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 Newaukum 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 (O. mykiss). 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 9.35 (river mile 5.8) of the main stem Newaukum River.

To meet the study objectives, a 1.5 meter (5–foot) rotary screw trap was operated near river kilometer 9.35 (river mile 5.8) of the main stem Newaukum River from March 13 to July 12, 2019.

Chinook outmigrants were subyearlings. 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, our goal was to estimate the subyearling (> 45mm fork length) component of the Chinook outmigration that generally occurs from March – July. Fork length of Chinook subyearlings increased steadily throughout the trapping period with an average of 55.0 mm (± 2.7 mm, standard deviation SD) and 78.4 mm (± 6.5 mm SD) in the first and last sampled week of trapping, respectively. Roughly 88% of the total catch of wild Chinook subyearling outmigrants were > 45mm. Abundance of wild Chinook subyearling outmigrants was estimated to be 277,109 ± 33,482 SD with a coefficient of variation (CV) of 11.8%. Coho outmigrants were both yearling and subyearlings. Scale age data indicated the subyearling component of the coho outmigration started near the middle of May and that prior to this date outmigrants were primarily one year of age, or yearlings. Fork length of yearling outmigrants averaged 105.0 mm (± 12.9 mm SD) whereas fork length of subyearling outmigrants averaged 75.5 mm (± 7.9 mm SD). Abundance of wild coho outmigrants was estimated to be 51,228 ± 3,820 SD with a CV of 13.1%.

Steelhead outmigrants were one, two, and three years of age. Fork length averaged 140.0 mm (± 21.6 mm SD) for one-year olds, 162.9 mm (± 15.3 mm SD) for two-year olds, and 180.4 mm (± 19.4 mm SD) for three-year olds. We were not able to produce an accurate or precise estimate this season due to low recaptures and not trapping over the entirety of the steelhead outmigration.