Newaukum River Smolt Production, 2020

This report provides the results from the 2020 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. 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 24, 2020.

Chinook outmigrants were 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 55.7 mm (± 3.8 mm, standard deviation SD) and 91.9 mm (± 7.5 mm SD) in the first and last sampled week of trapping, respectively. Roughly 96% of the total catch of wild Chinook outmigrants were > 45 mm. Abundance of wild Chinook subyearling outmigrants in 2020 was estimated to be 129,682 ± 11,112 SD with a coefficient of variation (CV) of 8.4%.

Coho outmigrants were predominately of the yearling (or “1+”) age class (98.3%). Scale age data also indicated that there is a small subyearling (“0+”) component of the coho out-migration (1.4%) that starts near the beginning of June. Average fork length of all outmigrant coho was 114.6 mm (± 10.2 mm SD). Fork length of known yearling outmigrants averaged 105.0 mm (± 12.9 mm SD) whereas fork length of known subyearling outmigrants averaged 75.5 mm (± 7.9 mm SD). Abundance of wild coho outmigrants in 2020 was estimated to be 73,416 ± 10,042 SD with a CV of 13.5%.

Steelhead outmigrants were predominately one (47.7%) and two (50.8%) years of age. A small proportion of steelhead were three (1.5%) years of age. Fork length averaged 142.1 mm (± 20.7 mm SD) for Age-1, 175.0 mm (± 19.5 mm SD) for Age-2, 182.5 mm (± 3.5 SD) for Age-3, and 161.0 mm (± 27.5 mm SD) for all captured steelhead. We were not able to produce an estimate of abundance in 2020 due to not trapping over the entirety of the steelhead out-migration period.