Green River Juvenile Salmonid Production Evaluation: 2020 Annual Report

This report provides the 2020 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 16 through June 22, 2020. During this period, the trap fished 72% of the time. We experienced several prolonged outages, two early in the season due to a prolonged high water event in late January thru the first half of February and a third outage in March and the first half of April due to field work restrictions associated with Covid-19. We suspect that large numbers of juvenile salmonids moved downstream (possibly involuntarily) during the prolonged high-water event, especially young of the year fry. We estimated the freshwater production (juvenile abundance) of natural origin subyearling Chinook and wild pink 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 2020. Data represent freshwater production above the juvenile trap, which is located at river mile 34.5.

^a This figure includes natural-origin and unmarked hatchery-origin fish.
^b This catch is median catch date which is not adjusted for trap efficiency and therefore 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 2020 outmigration (2019 brood) was estimated to be 1.66%, yielding a basin-wide production estimate of 95,525 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 79% of the natural subyearling migrants and peaked in the early February. Parr migrants (>45 mm fork length) represented 21% of the total abundance and their migration peaked two times, once in mid-February and again in early June.