Green River Juvenile Salmonid Production Evaluation: 2019 Annual Report

This report provides the 2019 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 23 through July 12, 2019. During this period, the trap fished 87% of the time. We estimated the freshwater production (juvenile abundance) of subyearling Chinook (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 2019. Data represent freshwater production above the juvenile trap, which is located at river mile 34.5.

^a This figure includes unmarked hatchery and natural 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 2019 outmigration (2018 brood) was estimated to be 10.90% yielding a basin-wide production estimate of 1,320,791 natural-origin juveniles.

Juvenile migrant Chinook in the Green River are predominantly subyearlings. Outmigration timing of natural origin subyearling Chinook was multimodal. The fry (≤45 mm fork length) represented 86% of the natural subyearling migrants and peaked in the second half of March. Parr migrants (>45 mm fork length) represented 14% of the total abundance and their migration peaked 3 times, in mid-April, mid-May and again in late June.