Juvenile Coho Salmon Passage at the Elwha River Dams: A Comparison of Short and Long Term Survival Estimates

摘要

In 1984, Fisheries Assistance Office (FAO) Olympia began a study of juvenile coho salmon passage survival of the Elwha river dams. Results of their work were intended for use in a larger effort aimed at restoring anadromous salmonids to the upper Elwha River basin. The FAO passage study incorporated both short and long-term estimation of coho passage survival. The short-Term estimate was intended for immediate use in project planning, while the long-term esimate was intended to evaluate any latent mortality due to dam passage and also serve as a check on short-term estimation methods. An FAO report by Wunderlich and Dilley (1985) described in detail the short-term mortality study as well as initiation of the long-term survival study based on coded-wire-tagging (CWT). Briefly, four test groups of marked (freeze-branded) coho smolts were released in Lake mills over the spring outmigration in 1984 (Figure 1). Eight control groups of marked coho smolts were also released below the Elwha Dam on a weekly basis over the outmigation period. Test and control groups were recaptured one and half river miles downstream of Elwha Dam in an inclined plans scoop traps. Recapture rates of control groups were used ot define trap efficiency, which in turn was used to estimate short-term servival of test groups passing the Elwha dams.