Newaukum River Smolt Production, 2021

This report provides the results from the 2021 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, run timing) of juvenile outmigrants for wild Chinook (O. tshawytscha), coho salmon (O. kisutch), and steelhead. 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 11 to July 12, 2021.

Chinook salmon in coastal Washington begin their downstream migration as Age-0 fish (fry, parr, and transitional/smolt subyearlings). Typically, the majority of Chinook fry (≤ 45 mm fork length) out-migrate when flow conditions are not suitable for smolt trapping in the Chehalis Basin (e.g., January and February). Therefore, our goal was to estimate the subyearling (> 45 mm fork length) component of the Chinook out-migration that generally occurs from March - July. Fork length of Chinook subyearlings increased steadily throughout the trapping period and averaged 54.2 mm (± 5.2 mm, standard deviation SD) and 86.6 mm (± 7.0 mm SD) in the first and last sampled week of trapping, respectively. Roughly 88% of the total catch of wild Chinook outmigrants were > 45 mm. Abundance of wild Chinook subyearling outmigrants in 2021 was estimated to be 163,146 ± 7,235 SD with a coefficient of variation (CV) of 4.6%.

Coho outmigrants were predominately of the yearling (or “1+”) age class (98.3%). Scale age data indicated that there was a small 2+ year-old component of the coho out-migration (1.6%) that started near the middle of April. Average fork length of all outmigrant coho was 113.6 mm (± 9.6 mm SD). Fork length of known yearling outmigrants averaged 113.3 mm (± 9.3 mm SD) whereas fork length of known two-year-old outmigrants averaged 114.6 mm (± 10.4 mm SD). Abundance of wild coho outmigrants in 2021 was estimated to be 57,714 ± 7,145 SD with a CV of 12.4%.

Steelhead outmigrants were predominately one (51.2%) and two (40.1%) years of age. A small proportion of steelhead were three (8.3%) and 4 (0.4%) years of age. Fork length averaged 156.1 mm (± 29.7 mm SD) for Age-1, 142.4 mm (± 22.5 mm SD) for Age-2, 166.6 mm (± 21.4 SD) for Age-3, 188.2 mm for Age-4, and 157.1 mm (± 27.4 mm SD) for all captured steelhead. We were not able to produce an estimate of abundance in 2021 due to not trapping over the entirety of the steelhead out-migration period.