Green River Juvenile Salmonid Production Evaluation: 2011 Annual Report

This report provides the 2011 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 natural-origin Chinook in the Green River. Genetic samples were collected on 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 2010 and contributed to the 2011 juvenile production. 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 number of juvenile migrants and associated variance were derived using a Bailey estimator.

The trap was operated from January 29 through June 30, 2011. During this period, the trap fished 92% of the time. We estimated the freshwater production (juvenile abundance) of Chinook (sub yearling) and coho salmon. Because of channel configuration and flow conditions at the trap site, we were unable to effectively capture steelhead smolts and were unable to produce a production (abundance) estimate for this species (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 2011. 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 and a basin-wide production was derived by applying survival estimated 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 2011 outmigration (2010 brood) was estimated to be 8.0%, yielding a basin-wide production estimate of 445,718 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 51% of all sub yearling migrants and peaked in mid-March , whereas parr migrants (45+ mm fork length) represented 49% of the migration and peaked in late May.