Green River Juvenile Salmonid Production Evaluation: 2009 and 2010 Annual Report

This report provides the 2009 and 2010 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. Additional objectives were to estimate the 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 27 through July 15 in 2009 and from January 19 through July 14 in 2010. During these periods, the trap fished 87% and 92% of the time, respectively. Production (abundance) estimates of juvenile migrants were derived for Chinook, coho, and steelhead. Abundance of juvenile pink salmon was estimated in 2010 only (Table 1).

TABLE 1 - Catch and production (abundance) estimates for juvenile salmonids leaving the Green River in 2009 and 2010. Data represent freshwater production above the Green River screw 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 2009 outmigration (2008 brood) was estimated to be 2.1%, yielding a basin-wide production estimate of 269,277 juveniles. Egg-to-migrant survival of the 2010 outmigration (2009 brood) was estimated to be 5.7%, yielding a basin-wide production estimate of 263,466 juveniles.

Juvenile migrant Chinook in the Green River are predominantly sub yearlings. Outmigration timing of sub yearling Chinook was bimodal in both years. In 2009, fry (<45-mm fork length) represented 61% of all sub yearling migrants and peaked between March 30 and April 5, whereas parr migrants (45+ mm fork length) peaked between June 15 and 21. In 2010, fry migrants represented just 10% of total sub yearling migrants and peaked between March 29 and April 4. Parr migrants peaked between June 7 and 13 in 2010.