Newaukum River Smolt Production, 2023

This report provides the results from the 2023 juvenile salmonid monitoring study on the Newaukum River main stem near Centralia, Washington. 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 coho salmon (O. kisutch), steelhead, and Chinook (O. tshawytscha). Based on the location and timing of the study, the results reflect juveniles that completed their freshwater rearing phase in habitats upstream of a rotary screw trap operated in the main stem Newaukum River.

To meet the study objectives, a 1.5-meter (5–foot) rotary screw trap was deployed near river kilometer 9.35 (river mile 5.8) of the main stem Newaukum River from March 9 to July 10, 2023.

Coho outmigrants were predominately of the yearling (or “1+”) age class (95.3%). Scale age data indicated that there was a small 2+ year-old component of the coho out-migration (3.9%) that started near the middle of March. Scale age data also indicated that there was a small subyearling (“0” age class) component of the coho out-migration (0.8%) that started in early June. The average fork length of all outmigrant coho was 111.7 mm (± 11.9 mm standard deviation SD). The average fork length of known subyearlings was 94.0 mm (± 2.6 mm SD), known yearlings was 111.8 mm (± 10.6 mm SD), and two-year-old outmigrants averaged 121.7 mm (± 11.6 mm SD). Abundance of wild coho outmigrants in 2023 was estimated to be 75,630 (95% CI: 63,042 – 92,116) with a coefficient of variation (CV) of 9.8%.

Steelhead outmigrants ranged from Age-1 to Age-3 based on scales (54.5%, 39.4% and 6.1% for Age-1 through Age-3, respectively), indicating three different juvenile life histories. Outmigrant timing was concurrent for all age classes. The average fork length for all measured steelhead was 140.0 mm (± 29.5 mm SD). Fork length averaged 127.8 mm (± 21.3 mm SD) for Age-1, 165.1 mm (± 16.3 mm SD) for Age-2, and 176.8 mm (± 10.0 SD) for Age-3. Abundance of wild steelhead outmigrants was estimated to be 11,443 (95% CI = 8,028 – 17,169) with a CV of 20.1%.

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, the goal was to estimate the subyearling smolt (> 45 mm fork length) component of the Chinook out-migration that generally occurs from March – July. Fork length of all Chinook subyearlings increased steadily throughout the trapping period and averaged 41.8 mm (± 3.1 mm SD) and 84.6 mm (± 5.6 mm SD) in the first and last sampled week of trapping, respectively. Approximately 89.8% of the total catch of wild Chinook outmigrants were > 45 mm. Abundance of wild Chinook subyearling smolt outmigrants in was estimated to be 64,472 (95% CI = 59,777 – 73,289) with a CV of 10.7%.