Dam Impact Analysis on Atlantic Salmon Recovery in the Penobscot River, Maine

Atlantic salmon populations in Maine are listed as endangered under the Endangered Species Act (ESA), and dams have been identified as a major contributor to their historic decline and current low abundance. Under the ESA, federal agencies must ensure their actions, such as the licensing of hydroelectric dams by the Federal Energy Regulatory Commission, do not preclude population recovery. To help meet this requirement, we developed a model to quantitatively evaluate the impact of federally licensed dams on Atlantic salmon dynamics. We examined the Penobscot River, a large river system in Maine that produced approximately 75% of all US adult Atlantic salmon returns in recent years. This highly modified river has multiple hydroelectric facilities that reduce passage rates for downstream migrating smolts and upstream migrating adults on both mainstem and major tributary reaches. We developed a life-stage specific model to compare the population dynamics of the current state of passage success to a hypothetical scenario of 100% passage success (i.e., no dams). Downstream passage survival distributions were generated for each dam using site- and facility-specific attributes, as well as biological and river flow data, accounting for both direct mortality and indirect secondary effects, such as increased passage time. Empirical field data were used to verify these distributions and also to develop upstream passage survival distributions. This general approach allowed for the development of more accurate passage distributions for dams with and without prior data. Model outputs include probability density functions for smolt and adult abundance, dam-induced losses at each hydroelectric facility, and natural mortality losses at key life stages. This model will allow the National Marine Fisheries Service to develop dam passage survival standards for downstream and upstream migrating salmon that will not significantly reduce the recovery of the species. The model will also help prioritize future passage improvement efforts to maximize the benefits to the Penobscot River Atlantic salmon population.