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Mobile species are particularly affected by artificial barriers requiring large investments to restore connectivity. However, few large-scale, long-term studies have investigated the ecological outcomes of restoring connectivity for these species. Our study, spanning 15-20 years, quantified response trajectories, which represent temporal trends following disturbance, of three native salmonids colonizing 20 km of protected habitat following restoration of fish passage at Landsburg Dam, Cedar River, WA, in 2003. Built in 1901, the dam blocked the upriver movement of native anadromous coho and Chinook salmon and nonanadromous mountain whitefish for 102 years. Restoration effectiveness was also assessed by comparing temporal trends in freshwater productivity of juvenile coho and Chinook salmon in the Cedar River after restoration to a nearby undammed subbasin. We also compared summer densities of juvenile coho and Chinook salmon, and mountain whitefish above the dam measured a decade after restoration to undammed reference systems. Anadromous salmon and nonanadromous mountain whitefish populations increased linearly or nonlinearly following restoration. The positive, asymptotic response represented by adult Chinook salmon counts indicates a slowing in population recovery rate, plateauing a decade after restoration. In contrast, annual abundance of adult coho salmon increased at a constant rate, indicating additional capacity 15 years post-restoration. Salmonid compositional diversity, driven largely by juvenile coho salmon, also increased nonlinearly, plateauing in a decade. We observed substantial spatial variation in the temporal response, as juvenile coho salmon and mountain whitefish population expansion slowed linearly with upstream distance from the restoration site. There was evidence that some of the annual variation in salmonid biomass in summer was a result of discharge variability in winter and spring, with biomass declining as flow variability increased. Species reintroduction and establishment had no discernible effect on stream-rearing salmonids living above
Mobile species are particularly affected by artificial barriers requiring large investments to restore connectivity. However, few large-scale, long-term studies have investigated the ecological outcomes of restoring connectivity for these species. Our study, spanning 15-20 years, quantified response trajectories, which represent temporal trends following disturbance, of three native salmonids colonizing 20 km of protected habitat following restoration of fish passage at Landsburg Dam, Cedar River, WA, in 2003. Built in 1901, the dam blocked the upriver movement of native anadromous coho and Chinook salmon and nonanadromous mountain whitefish for 102 years. Restoration effectiveness was also assessed by comparing temporal trends in freshwater productivity of juvenile coho and Chinook salmon in the Cedar River after restoration to a nearby undammed subbasin. We also compared summer densities of juvenile coho and Chinook salmon, and mountain whitefish above the dam measured a decade after restoration to undammed reference systems. Anadromous salmon and nonanadromous mountain whitefish populations increased linearly or nonlinearly following restoration. The positive, asymptotic response represented by adult Chinook salmon counts indicates a slowing in population recovery rate, plateauing a decade after restoration. In contrast, annual abundance of adult coho salmon increased at a constant rate, indicating additional capacity 15 years post-restoration. Salmonid compositional diversity, driven largely by juvenile coho salmon, also increased nonlinearly, plateauing in a decade. We observed substantial spatial variation in the temporal response, as juvenile coho salmon and mountain whitefish population expansion slowed linearly with upstream distance from the restoration site. There was evidence that some of the annual variation in salmonid biomass in summer was a result of discharge variability in winter and spring, with biomass declining as flow variability increased. Species reintroduction and establishment had no discernible effect on stream-rearing salmonids living above
Restoration of movement corridors is a key management action used to address threats to migratory and other mobile species. Yet, we lack restoration effectiveness studies that allow for species to reestablish naturally (i.e. without supplementation) following habitat reconnection that capture all phases (dispersal, growth, and regulation) of recovery, and that takes an ecosystems approach. We investigated the natural recovery of migratory anadromous Coho salmon following habitat reconnection across a 5‐km section of Rock Creek, a forested tributary of the Cedar River, Washington, United States, 3 km upstream of Landsburg Dam. The dam blocked upstream fish movement for 102 years until the completion of a fish ladder in 2003. We also evaluated the response of non‐migratory trout, which are closely related to Coho salmon. Juvenile Coho salmon natal to the Cedar River dispersed into Rock Creek for rearing until spawning there in 2007. After restoration, juvenile Coho salmon density (fish/m2) increased 18‐fold, approaching an asymptote (i.e. regulation phase) a decade later. Coho salmon recovery in Rock Creek was spatially variable, however, slowing with distance from the site of restoration. Trout density was also higher after restoration relative to before, likely due to several mechanisms, including increased capacity resulting from the reestablishment of marine organic matter subsidies delivered by spawning anadromous fish. Our study demonstrates that migratory species can recover naturally after the restoration of habitat connectivity and associated movement corridors. Furthermore, our results suggest that such actions can also benefit nontarget species by reestablishing key ecosystem links driven by the target species.
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