Movements by Adult Coho Salmon in the Lower Elwha River, Washington

The Elwha and Glines Canyon dams caused a dramatic decline in the numbers of all species of native Pacific salmonids (Oncorhynchus spp.) in the Elwha River. During the fall of 2005 and 2006, we radiotagged 49 adult coho salmon (O. kisutch) and tracked their movements between the Elwha River mouth and Elwha Dam (7.3 rkms). Half of all tagged fish were never relocated, likely due to emigration from the river. The remainder tended to migrate quickly and directly to one or two areas saturated with large woody debris and gravel, known to be high quality spawning habitat, and remain there. However, 7 of the 13 tagged fish in 2005 made multiple upstream and downstream movements prior to spawning. No tagged fish in either year migrated farther upstream than a rock weir approximately 4.9 km from the river mouth and 2.4 km downstream from the Elwha Dam, possibly indicating a migration barrier for coho salmon. We did not detect qualitative differences in migration behavior between hatchery and unknown-origin fish, but we did find that males moved slightly larger distances after tagging than females (average, 3.6 km for males, 2.5 km for females, t-test, P = 0.41). A large flow event on 6 November 2006 caused 8 of 11 tagged fish residing in the river to emigrate; none of these fish returned. Results both confirm ideas of coho salmon biology and raise concerns regarding environmental impacts on coho salmon recolonization following dam removal.