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

Dams and their impoundments disrupt river habitat connectivity to the detriment of diadromous fishes.  Removal of dams is assumed to reestablish species distributions and abundance, but this is infrequently tested.  Restoration efforts on the Penobscot River (Maine, USA) are among the largest currently underway in the United States, and include the removal of the two lower-most dams and improvements to fish passage at remaining barriers.  We assessed fish assemblages of the Penobscot River, Maine before (2010-2012) and after dam removal (2014) using boat electrofishing surveys and a stratified-random sampling design.   We found all anadromous species in greatest abundance below the current lower-most dam.  River herrings Alosa spp. passed through the new fish elevator at this dam and spawned in newly available habitat.  We observed a reduction in lacustrine species in former impoundments (e.g. pumpkinseed Lepomis gibbosus and golden shiner Notemigonus crysoleucas) and greater American eel Anguilla rostrata abundance throughout the main stem.  Our results demonstrate the potential for large dam removal projects to restore diadromous fish assemblages.