2002 Green River Juvenile Salmonid Production Evaluation

The National Marine Fisheries Service (NMFS) listed Puget Sound chinook as threatened under the Endangered Species Act (ESA) in March 1999. This listing triggered action on the part of state and local governments to develop plans and implement actions designed to restore Puget Sound chinook runs to healthy levels. An important, but often missing, component of the plans is accurate information on wild chinook abundances and the factors that limit or impact production and productivity in key wild chinook stocks. One such key stock, Green River Chinook, represents one of the largest populations of chinook within the Puget Sound Evolutionary Significant Unit (ESU). Since quantifying juvenile anadromous salmonid populations as they migrate seaward is the most direct assessment of stock performance in freshwater, a long-term wild juvenile salmon production study was initiated in the Green River in 2000 to estimate and monitor the production of chinook and coho salmon, and steelhead trout.

Beyond monitoring for ESA considerations, this study provides important information for run-size forecasting and enables assessment of recovery actions in terms of change in wild salmon production. This report documents our investigations during 2002, the third year of this project. Study objectives include estimating Green River wild chinook freshwater production, migrant size, and migration timing to evaluate the condition of the stock and to help develop a better understanding of factors influencing their production and life history.

To accomplish these objectives, a floating screw trap was operated on the mainstem Green River at river mile 34.5. Trapping with this gear began in early February and ended in mid-July. A portion of all downstream migrating juvenile salmonids was captured in this trap. To estimate the capture efficiency, over the season groups of dye-marked or fin- marked fish were released upstream of the trap. Daily migration was estimated by dividing the daily catch by the estimate of trap efficiency.

Natural chinook production was estimated at 412,500 migrants in 2002. The chinook migration followed a bi- modal timing distribution. An earlier-timed �"fry” component, comprised of newly emerged fry that migrated between January and early April, was followed by a later-timed �"smolt” component, comprised of larger chinook migrants that rear upstream of the trap before migrating between May and June. This timing distribution has been observed in other rivers monitored in western Washington. During the 2002 season, 87% of the chinook migrated as fry through April. Relating our estimates of age 0+ chinook production to the number of eggs estimated to have been deposited above the trap resulted in egg-to- migrant survival estimates of 3.4%. By accounting for chinook production below our trap, we estimated total Green River natural production at 760,500 chinook.

In addition to naturally produced chinook, we also estimated 7,200 hatchery chinook, 246,500 unmarked coho smolts, 98,000 hatchery coho smolts, 53,000 wild steelhead, and 129,600 hatchery steelhead smolts migrated past the trap. Survival- to-the-trap estimates for the 2002 hatchery releases of chinook yearlings, coho and steelhead smolts ranged from 10.6% to 70.2%: 29.5% survival of Keta Creek coho smolts, 10.6% survival of Icy Creek chinook yearlings, and 70.2% survival of steelhead smolts released from four hatchery facilities above the trap.