Green River Juvenile Salmonid Production Evaluation: 2022 Annual Report

This report provides the 2022 results from the juvenile salmonid monitoring study conducted on the Green River in central Puget Sound, Washington. The primary objective of this study was to estimate the juvenile abundance of natural-origin Chinook salmon in the Green River. Additional objectives were to estimate the number of juvenile migrants and life history characteristics of other salmonid species. Juvenile salmonids were captured in a five-foot screw trap located at river mile 34.5 (55 rkm). Catch was expanded to a total migration estimate using a time-stratified approach that relied on release and recapture of marked fish throughout the outmigration period.

The trap was operated from January 19 through June 27, 2022. During this period, the trap fished 86% of the time. We experienced 8 outages -- three were to avoid large hatchery releases, one to avoid a major windstorm, and four due to the high-water events. We estimated the freshwater production (juvenile abundance) of natural origin subyearling Chinook salmon (Table 1).

Table 1. Catch, freshwater production, fork length (mm), and out-migration timing of natural-origin juvenile salmonids caught in the Green River screw trap in 2022. Data represent freshwater production above the juvenile trap, which is located at river mile 34.5.

a Includes natural-origin and unmarked hatchery-origin fish.
b Median catch date- not adjusted for trap efficiency, serves as an index of migration timing.

Chinook salmon spawn above and below the juvenile trap. A basin-wide production estimate was derived by applying estimated survival above the trap to spawning below the trap. Egg-to-migrant survival of Green River Chinook for the 2022 outmigration (2021 brood) was estimated to be 1.35%, yielding a basin-wide production estimate of 78,878 natural-origin juveniles.

Juvenile migrant Chinook in the Green River are predominantly subyearlings. Outmigration timing of natural origin subyearling Chinook was bimodal. The fry (≤45 mm fork length) represented 62.5% of the natural subyearling migrants and peaked in the late February. Parr migrants (>45 mm fork length) represented 37.5% of the total abundance and their migration peaked twice, once in early May and again in mid-June.

We were unable to identify hatchery-origin coho salmon at the trap because only a small portion of the fish released upstream of the trap received CWTs, and none were ad-clipped. Hence, our estimate of 211,750 coho salmon smolts includes an unknown mixture of hatchery-origin and natural-origin fish.