Rapid Recolonization and Life History Responses of Bull Trout Following Dam Removal in Washington's Elwha River

Abstract

AbstractConnected river systems confer advantages upon migratory salmonids by facilitating greater life history expression and, ultimately, population persistence. In Washington's Elwha River, two impassable, century‐old hydroelectric dams were removed in the world's largest dam removal project. Dam removal restored access to 130 km of habitat and reconnected headwater, estuarine, and marine areas. We used multi‐year radiotelemetry data to characterize migratory patterns of federally threatened Bull Trout Salvelinus confluentus after dam removal. Bull Trout rapidly responded to barrier removal and were the first among the Pacific salmonids to move into the newly accessible headwaters. Spatial extent and mean distance traveled upriver increased annually, and adults reached the headwaters (64 km) within 3 years of dam removal. Telemetry data revealed that Bull Trout migrated between the river and its estuary (up to 168 km), and spawning migrations occurred in consecutive years. Relocations of Bull Trout in the tidal zone and the presence of bright, silvery colored adults were indicative of marine residence. Length at age of Bull Trout significantly increased after dam removal, and fish captured before and after dam removal were larger at similar ages in comparison with migratory populations throughout their range. We described the diet of Bull Trout captured in the river and estuary after dam removal. Bull Trout primarily consumed Pacific salmonids Oncorhynchus spp., Threespine Sticklebacks Gasterosteus aculeatus, and sculpins Cottus spp., and the prevalence of sculpins in their diet decreased from the estuary to upstream areas. As the length of Bull Trout increased, they progressively consumed larger prey items. In this historic dam removal project, Bull Trout were early and rapid recolonizers and displayed migrations throughout the reconnected river and its estuary. This study is among the first to evaluate Bull Trout responses to dam removal and establishes a baseline to assess future recovery of this threatened species in the Elwha River.