Chehalis River Smolt Production 2018

This report provides the 2018 results from the juvenile salmonid monitoring study on the Chehalis River main stem near Rochester, WA. The primary objective of this study is to describe the freshwater production of salmon and steelhead in the Chehalis River. Specifically, we describe the abundance, timing, and diversity (body size, age structure) of juvenile outmigrants for wild Chinook salmon, coho salmon, 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 84 (river mile 52) of the main stem Chehalis River.

To meet the study objectives, a 2.4 meter (8-foot) rotary screw trap was operated near river kilometer 84 (river mile 52) of the main stem Chehalis River from March 19 to July 22, 2018.

Chinook outmigrants were subyearlings. Fork length of Chinook subyearlings increased steadily throughout the trapping period with an average of 45.8 mm (± 0.8 mm, standard deviation) and 101.4 mm (± 6.3 mm) in the first and last week of trapping, respectively. Roughly 73% of the total catch of wild Chinook subyearling outmigrants (>45mm) occurred from May 21 to June 24. Abundance of wild Chinook subyearling outmigrants was estimated to be 295,708 + 27,431 (standard deviation (SD)) with a coefficient of variation (CV) of 9.3%. This estimate did not include Chinook fry, which outmigrate immediately following emergence from the gravel and prior to the trapping period.

Coho outmigrants were both yearling and subyearlings. Scale age data indicated the subyearling component of the coho outmigration started sometime near the middle of May and that prior to this date all outmigrants were one year of age. Fork length of yearling outmigrants averaged 125.6 mm (+25.4 mm) whereas fork length of subyearling outmigrants averaged 95.4 mm (+5.8 mm). Roughly 79% of the total catch of wild coho outmigrants occurred from April 30 to May 20. Abundance of wild coho outmigrants was estimated to be 304,806 + 43,998 (SD) with a CV of 14.4%.

Steelhead outmigrants were one, two, and three years of age. Fork length averaged 157.4 mm (± 11.6 mm) for one-year olds, 174.0 mm (± 23.1 mm) for two-year olds, and 194.8 mm (±23.1mm) for three-year olds. Roughly 81% of the total catch of wild steelhead outmigrants occurred from April 30 to May 20. Abundance of wild steelhead outmigrants was estimated to be 32,058 + 15,864 (SD) with a CV of 49.5%.