2000 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 to estimate and monitor the production of chinook and coho salmon, steelhead, and cutthroat trout.

Beyond monitoring for ESA considerations, this study provides important information for runsize forecasting and enables assessment of recovery actions in terms of change in wild salmon production. The study will also be used to evaluate a large water storage and diversion project on the Green River (Howard Hansen Dam [HHD] Additional Water Storage [AWS] Project). This report documents our investigations during 2000, the first year of this project and the first year of pre-construction baseline monitoring for the AWS project. Study objectives in 2000 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.

In addition to the work in the Green River, this report also describes the results from our one-year juvenile chinook trapping project in Big Soos Creek. This project assessed the level of natural production resulting from hatchery parents spawning in the wild. To accomplish these objectives, two floating screw traps were operated, one on the mainstem Green River (river mile 34.5) and one just upstream of the Soos Creek Hatchery on Big Soos Creek. A portion of all downstream migrating juvenile salmonids were captured in these traps.

To estimate the capture efficiency, over the season groups of dye-marked or fin-marked fish were released upstream of the traps. Nightly migration was estimated by dividing the nightly catch by the flow-based estimate of trap efficiency.

Over the 154-day February 10 to July 13 trapping period, over 12,000 juvenile chinook were captured in the mainstem trap. From this catch and our estimates of trap efficiency, we estimated a total of 536,000 age 0+ wild chinook migrated past our trap in 2000. This estimate assumes that 76,000 age 0+ chinook migrated between an assumed January 1 migration starting date and the date that trapping began (February 10).

The trap on Big Soos Creek was operated over a 147-day period (February 1 to June 26). During this period over 90,000 juvenile chinook were captured. Based on our estimates of trap efficiency, we estimate total age 0+ chinook natural production from Big Soos Creek at 275,000.

Like other wild juvenile chinook migrations that we have monitored, the chinook migration from the mainstem Green River followed a bi-modal timing distribution. An earlier-timed �"fry” component, comprised of newly emerged fry, that migrated between February and early April was followed by a later-timed �"smolt” component, comprised of larger chinook smolts, that migrated from May through June. On Big Soos Creek, nearly all age 0+ chinook migrated as fry which, we believe, indicates nearly all parent-brood spawning occurred just upstream of the trap.

Relating our estimates of age 0+ chinook production to the number of eggs estimated to have been deposited above the traps in the mainstem Green River and in Big Soos Creek results in egg-to-migrant survival estimates of 7.3% and 3.8%, respectively. We believe differences in survival between these two streams are primarily related to the delayed release of spawners upstream of the Soos Creek Hatchery rack in 1999 which concentrated the spawning activity in Big Soos Creek to a small area and resulted in redd superimposition. There may also be differences in the quality of spawning habitat given differences in basin morphology and stream power between the Green River and Big Soos Creek, as well as differences in the genetic fitness of the Soos Creek stock.

By summing the estimated chinook production from the mainstem Green River above the trap with that from Big Soos Creek, and accounting for mainstem production below the trap, we estimate total 2000 Green River age 0+ chinook natural production at 1.08-million.

In addition to age 0+ chinook, we also estimated 33,000 wild and 203,000 hatchery coho smolts, 36,000 to 41,000 wild and 46,000 to 52,000 hatchery steelhead smolts, and 400 to 500 wild cutthroat smolts migrated past the mainstem Green River trap in 2000. We also estimate that 64,000 coho smolts were produced in Big Soos Creek.