Green River Juvenile Salmonid Production Evaluation: 2008 Annual Report

This report provides results from the juvenile salmonid production study conducted on the Green River in 2008. The primary objective of this project was to estimate natural-origin production of Puget Sound Chinook in the Green River. Additional objectives were to evaluate environmental variables contributing to Chinook survival and to estimate the production of other Green River salmonids. Juvenile salmonids were captured in a five-foot screw trap located at river mile 34.5 (55 rkm). Production estimates were made using a time-stratified mark-recapture approach that relied on release and recapture of marked fish throughout the period of outmigration. A Petersen-Chapman estimator was used to calculate juvenile migration and its variance during each time stratum. The sum of all strata provided the total production estimate.

The trap was operated from January 23 through July 15, 2008. The trap fished 92% of the time during this period. Mark and recapture efficiency trials were conducted throughout the trapping period. Production estimates of natural-origin salmonids were derived for subyearling Chinook and pink salmon and a combined natural and hatchery-origin estimate was derived for chum salmon (Table 1). Low catches and recapture rates prohibited production estimations for other species.

TABLE 1. Catch and production estimates for juvenile salmonids in the Green River, 2008. Data are for juveniles migrating from above the Green River screw trap, river mile 34.5.

Egg-to-migrant survival of the 2007 brood of Green River Chinook was estimated to be 3.40%. A basin-wide production estimate of 546,756 natural-origin sub-yearling Chinook was based on production measured above the trap and 3.40% survival applied to spawning below the trap and to spawners passed above the weir in Big Soos Creek.

Timing of the Chinook outmigration was bimodal. The earlier migration represented 63% of total abundance and peaked between March 11 and 16. The later migration represented 37% of total abundance and peaked between June 2 and 8. The first peak was composed of recently emerged juveniles (i.e., "fry", 40-45-mm FL) whereas the second peak was mostly subyearlings that had reared in the river for several months (i.e., "parr" , 46 to 106-mm FL).