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Quantifying the Effects of Dams on Atlantic Salmon in the Penobscot River Watershed, with a Focus on Weldon Dam
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    Dams are a major contributor to the decline and current low abundance of Atlantic salmon in the United States. We conducted a population viability analysis to assess the effects of dams on Atlantic salmon, focusing on hydroelectric dams and the population in the Penobscot River watershed in Maine. We simulated the life cycle of Atlantic salmon, tracking the number and origin of salmon through their various life stages, especially during the smolt and adult stages when salmon directly interact with dams. This modeling approach was previously used to assess potential management actions during Federal Energy Regulatory Committee licensing activities at 5 hydroelectric dams on the Penobscot River in 2012 and to address questions about recovering the population. We have updated the model to reflect changes in the watershed and to include recent data. We estimated adult abundance and distribution to evaluate the effects of survival at dams. In addition to dam-related scenarios, we ran scenarios to look at how hatchery supplementation, including changes in the number of smolts stocked and stocking location, and increased survival in the egg-to-smolt and marine life stages affected the population. Finally, we ran a series of scenarios focused on the Mattaceunk Project, which includes Weldon Dam and is being considered for relicensing in 2019. In these scenarios, we estimated adult abundance and distribution and smolt survival and mortality. Modeled results projected the number of adults in the Penobscot River watershed and the proportion of adults located in the upper areas of the watershed to generally increase as survival at dams increased or dams were removed. Abundance declined to zero when smolts were not stocked, and survival was low during the egg-to-smolt and marine life stages. However, adult abundance increased even without hatchery supplementation when survival increased during egg-to-smolt and marine life stages. The number and location of adults varied greatly with changes in stocking location and survival during the egg-to-smolt and marine life stages. Changes in survival at Weldon Dam did not affect the number and location of adults when survival was low during egg-to-smolt and marine life stages and when most smolts were stocked low in the watershed. However, adult abundance, including above Weldon Dam, did increase with increases in survival at Weldon Dam when survival was higher during the egg-to-smolt and marine life stages and smolt stocking numbers and locations were altered. The survival of smolts above Weldon Dam also increased as dam-related mortality decreased. Our findings indicate that Atlantic salmon abundance can increase as survival at dams increases, but hatchery supplementation will be necessary to sustain the population when survival is low in egg-to-smolt and marine life stages. Increases in survival during both of these life stages will likely be necessary to attain a self-sustaining population, especially if hatchery supplementation is reduced or discontinued.

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