Distribution, Relative Abundance, and Length of Sculpins in the Elwha River Watershed Following the Removal of Two Hydroelectric Dams

Abstract

Dam removal often shows positive effects for many fish species, especially migratory species. In contrast, information regarding the effects on less migratory species is lacking. The Elwha River, Washington had two hydroelectric dams removed from 2011 to 2014, which resulted in the loss of two reservoirs. Because of glacial history, the Elwha River system has a depauperate freshwater fish fauna, including just two sculpin species (coastrange sculpin [Cottus aleuticus] and prickly sculpin [C. asper]). Both species exhibit a similar migratory life history strategy, which includes a planktonic larval stage that drifts downstream to a nursery area (e.g., lake, large river, or estuary), followed by an upriver movement pattern. Due to these characteristics, Elwha River dam removal and loss of the reservoirs may have had important effects on these two sculpin species. We sampled sculpins with longitudinal electrofishing surveys in 2018 and 2019 to evaluate their current distribution, relative abundance, and body size. These metrics were generally similar to other river systems in the Pacific Northwest, with fewer and larger migratory sculpins upstream. Additionally, we compared our findings to pre-dam surveys. After dam removal, sculpins in upstream reaches were less abundant and had a smaller range of sizes. This suggests the population characteristics of sculpins in the Elwha River has changed, but it is unclear how indirect effects, such as anadromous salmonid (Salmonidae) increases, may influence their overall abundance. This study also provides a baseline to monitor sculpin population changes in the future.