2001 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, 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. 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 2001, the second year of this project and the second and final year of preconstruction baseline monitoring for the AWS project. Study objectives in 2001 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, we operated a floating screw trap on the mainstem Green River (river mile 34.5). A portion of all downstream migrating juvenile salmonids were 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. Nightly migration was estimated by dividing the nightly catch by the estimate of trap efficiency.

Over the 182-day January 31 to August 1 trapping period, over 56,000 juvenile chinook were captured. From this catch and our estimates of trap efficiency, we estimated a total of 728,000 age 0+ wild chinook migrated past our trap in 2001.

During both years of operation (2000 and 2001), 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 smolts that migrated from May through June. We have observed this timing distribution in other rivers monitored in western Washington.

Relating our estimate of age 0+ chinook production to the number of eggs estimated to have been deposited above the trap results in an egg-to-migrant survival estimate of 5.3%.

We estimated 497,000 age 0+ chinook were produced below the trap for a total 2001 Green River age 0+ chinook natural production of 1.23- million. This estimate of lower river production was made by assuming the egg-to- migrant survival of chinook eggs deposited below the trap was equal to that measured above the trap and that the natural production from Big Soos Creek was the same as was estimated in 2000.

In addition to age 0+ chinook, we also estimated 55,000 wild and 132,000 hatchery coho smolts, and 15,000 wild and 45,000 hatchery steelhead smo lts migrated past the trap. No cutthroat smolts were captured in the trap in 2001.

Since most hatchery fish released upstream of the trap were adipose- marked, we were able to estimate survival to the trap of marked chinook, coho, and steelhead. Assuming that hatchery fish are trapped at the same rate as wild fish, we estimated a 67% survival of Keta Creek coho, 15% survival of Keta Creek and Palmer Ponds steelhead, combined, and 0.22% survival of age 0+ Keta Creek chinook released upstream of Howard Hansen Dam.

Over the two years of baseline monitoring related to the AWS project, age 0+ wild chinook production upstream of the trap ranged from 535,000 in 2000 to 728,000 in 2001. Variation in the proportion of chinook migrating as fry and smolts occurred over the two years with 53% of the production migrating as fry in 2001 compared to 68% in 2000. Since more of the production migrated as fry in 2000, seasonal chinook migration timing was earlier that year. The median migration point occurred nearly a month later in 2001 compared to 2000. Results from monitoring production in other rivers suggests that production levels and migration timing measured from just two years of baseline monitoring in the Green River likely underestimates the true variability in these components of chinook life history.