2007 Skagit River Wild Salmon Production Evaluation Annual Report

The Skagit River juvenile salmon trapping project, initiated by the Washington Department of Fish and Wildlife in 1990, began with the goal of estimating natural coho smolt production and was later expanded to estimate production of natural-origin juvenile Chinook and to enumerate other juvenile salmonid migrants. Results from this project contribute to the fishery management of coho and provide information on the recovery status of Puget Sound Chinook and steelhead, both listed under the Endangered Species Act.

Scoop and screw traps operated on the Skagit River are located 17-miles upstream of the river mouth. Traps were operated from January 19 through July 25, 2007. The study objectives were to estimate migration abundance and associated confidence intervals for sub-yearling Chinook and yearling coho, investigate the relationship between environmental variables and inter-annual variation in egg-to-migrant survival of sub-yearling Chinook, and document juveniles of additional salmonid species migrating from the Skagit River.

Chinook production estimates were made using a stratified mark-recapture approach. Coho production estimates were made using a pooled mark-recapture approach. For each time strata, juvenile migration and associated variance is based on a Petersen estimator calculated from the number of fish that were marked and released, the number of unmarked fish that were recaptured, and the number of marked fish that were recaptured. Catches of unmarked fish included estimated catch during trap outage periods.

An estimated 2.2 million ± 0.27 million (95% C.I.) natural-origin sub-yearling Chinook migrated during the trapping period, with an additional 4,661 Chinook estimated to have migrated prior to the start of trap operation. Egg-to-migrant survival of the 2006 brood of Skagit River Chinook was estimated to be 3.9%.

The 2007 outmigration of juvenile salmonids included 747,490 ±78,324 (95% C.I.) naturalorigin coho smolts as well as catches of 137,829 natural-origin chum fry, 3 pink fry, 925 naturalorigin steelhead smolts, and 232 Dolly Varden/bull trout smolts. Catches also included Chinook yearlings, coho fry, sockeye fry and smolts, steelhead adults, cutthroat smolts and adults, and trout fry and parr.

Egg-to-migrant survival of Chinook was among the three lowest survivals observed in eighteen years of data compiled from the Skagit River. This trend was attributed to high flows during vulnerable egg incubation periods in fall 2006.