Dam Removal and Fish Passage Improvement Influence Fish Assemblages in the Penobscot River, Maine

Diadromous fish restoration actions on the Penobscot River (Maine, USA) include the removal of the two lower-most dams and represent one of the most significant conservation efforts in the United States.  This work also included improvements to fish passage at remaining barriers, including Milford Dam which is now the most downstream dam on the river.  We assessed fish assemblages before (2010-2012) and after (2014-2015) river modifications using boat electrofishing surveys and a stratified-random sampling design.  Subsequent to restoration, river herring (Alosa spp.) passed through the new fish elevator at Milford Dam and spawned in newly available habitat upstream.  Prior to restoration most anadromous fishes, including herrings, were restricted to the reaches below Veazie Dam, the former lower-most dam.  We no longer observe American eel Anguilla rostrata concentrating below Veazie Dam, which suggests a more even distribution throughout the watershed. We also observed a substantial reduction in lacustrine species in former impoundments (e.g. golden shiner Notemigonus crysoleucas) and a reduction in the relative abundance of smallmouth bass Micropterus dolomieu, an invasive habitat generalist that typically dominates the assemblage in most reaches.  Our results demonstrate the potential for large dam removal projects to restore populations of anadromous fishes and riverine fish assemblages.