Fish Community Response to a Small-Stream Dam Removal in a Maine Coastal River Tributary

Sedgeunkedunk Stream, a third-order tributary to the Penobscot River in Maine, historically has supported several anadromous fishes including Atlantic Salmon Salmo salar, Alewife Alosa pseudoharengus, and Sea Lamprey Petromyzon marinus. Two small dams constructed in the 1800s reduced or eliminated spawning runs entirely. In 2009, efforts to restore marine-freshwater connectivity in the system culminated in removal of the lowermost dam (Mill Dam) providing access to 4.7 km of lotic habitat and unimpeded passage into the lentic habitat of Fields Pond. In anticipation of these barrier removals, we initiated a modified before-after-control-impact study, and monitored stream fish assemblages in fixed treatment and reference sites. Electrofishing surveys were conducted twice yearly since 2007. Results indicated that density, biomass, and diversity of the fish assemblage increased at all treatment sites upstream of the 2009 dam removal. No distinct changes in these metrics occurred at reference sites. We documented recolonization and successful reproduction of Atlantic Salmon, Alewife, and Sea Lamprey in previously inaccessible upstream reaches. These results clearly demonstrate that dam removal has enhanced the fish assemblage by providing an undisrupted stream gradient linking a small headwater lake and tributary with a large coastal river, its estuary, and the Atlantic Ocean.