Green River Juvenile Salmonid Production Evaluation: 2012 Annual Report

This report provides the 2012 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 in the Green River. Tissue samples were collected from a significant portion of the juvenile Chinook migrants captured over the season as part of a project to estimate the number of adult Chinook that returned to the Green River in the fall of 2011 via Genetic Mark Recapture.. This work is part of the Sentinel Stock Program, an effort to improve the accuracy of the adult Chinook escapement estimates for rivers across Puget Sound. Additional objectives were to estimate the number of juvenile migrants produced by other salmonid species and to describe life history characteristics of all juvenile migrants. 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 24 through July 12, 2012. During this period, the trap fished 87% of the time. We estimated the freshwater production (juvenile abundance) of Chinook (sub yearling), coho, chum and pink. Because of channel configuration and flow conditions at the trap site, we were unable to recapture any of the marked steelhead smolts that had been released for trap efficiency trails, so no production estimate was calculated.

Chinook salmon spawn above and below the juvenile trap and a basin-wide production was derived by applying estimated survival above the trap to spawning below the trap (main-stem and above the Big Soos Creek weir). Egg-to-migrant survival of Green River Chinook for the 2012 outmigration (2011 brood) was estimated to be 6.0%, yielding a basin-wide production estimate of 146,909 juveniles.

Juvenile migrant Chinook in the Green River are predominantly sub yearlings. Outmigration timing of sub yearling Chinook was bimodal. The fry (<45-mm fork length) represented 47% of all sub yearling migrants and peaked in mid-March , parr migrants (45+ mm fork length) represented 53% of the migration and peaked in early June.