Green River Juvenile Salmonid Production Evaluation: 2015 Annual Report

This report provides the 2015 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 14 through June 22, 2015. During this period, the trap fished 90% of the time. We estimated the freshwater production (juvenile abundance) of Chinook (subyearling) and coho (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 2015. Data represent freshwater production above the juvenile trap, which is located at river mile 34.5.

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; a screw trap fished in Big Soos Creek estimated production from that tributary. Egg-to-migrant survival of Green River Chinook for the 2015 outmigration (2014 brood) was estimated to be 8.75%, yielding a basin-wide production estimate of 506,060 juveniles. Included in this estimate was an estimate of 76,037 Chinook migrating from Big Soos Creek with an egg-tomigrant survival of 13.20%. This estimate was generated by a screw trap located just above the hatchery and operated by the Muckleshoot Indian Tribe.

Juvenile migrant Chinook in the Green River are predominantly subyearlings. Outmigration timing of subyearling Chinook was bimodal, however very few migrants remained in the system above the trap site to migrate as parr in 2015. The fry (.45 mm fork length) represented 97% of all subyearling migrants and peaked in early February, parr migrants (>45 mm fork length) represented 3% of the migration and were observed during several periods: first in February and the typical peak in late May.