Reintroduction of native salmonids affects the distribution and abundance of fish populations in the Cedar River, WA, USA

Man-made barriers block the upstream passage of migrating fishes in many river systems, and barrier removal or circumvention is a major restoration tool used throughout North America.  While it is generally assumed that such actions will benefit threatened and endangered fish, the biotic response has rarely been documented. Landsburg Diversion Dam on the Cedar River, WA had blocked upstream migration of Pacific salmon, and number of other native fish species, from 1900 until 2003, when a fish passage facility was installed.  We quantified fish density and composition in 10 reaches subsampled from 17 km of habitat for two years before and three years after barrier circumvention.  Before the fish ladder, resident trout densities increased from downstream (reach 1, 200 m above the dam) to upstream (reach 10, ~17 km from the dam) ranging from 0.01 to 0.12 fish/m². This pattern was reversed after the ladder with total salmonid density (salmon and trout) increasing from reach 1 to reach 9 approximately (~14 km from the dam), with the greatest increases occurring from reach 1 to 4 (~8 km from the ladder). The return of salmon to the Cedar River has led to a redistribution of fish largely due to the addition of juvenile coho salmon and local increases in resident trout density.