Green River Juvenile Salmonid Production Evaluation: 2017 Annual Report

This report provides the 2017 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 18 through June 29, 2017. During this period, the trap fished 93% of the time. We estimated the freshwater production (juvenile abundance) of Chinook (subyearling), coho, pink and steelhead. (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 2017. Data represent freshwater production above the juvenile trap, which is located at river mile 34.5.

^a This figure includes 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.

^c Unable to distinguish between natural origin and hatchery production.

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 2017 outmigration (2016 brood) was estimated to be 12.86%, yielding a basin-wide production estimate of 2,389,934 juveniles. Included in this estimate was an estimate of 60,493 Chinook migrating from Big Soos Creek. 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. The fry (< 45 mm fork length) represented 78% of all subyearling migrants and peaked in early-February and mid-March, and late April. Parr migrants (>45 mm fork length) represented 22% of the migration and parr migration peaked in the first week of May.