Anadromous Sea Lampreys Recolonize a Maine Coastal River Tributary after Dam Removal: Ecosystem Engineers?

Sedgeunkedunk Stream, a tributary to the Penobscot River, Maine, historically supported several anadromous fishes. However, two dams constructed in the 1800s reduced or eliminated spawning runs entirely. In 2009, efforts to restore marine–freshwater connectivity culminated with removal of the lowermost dam, thus providing access to 4.6-km of lotic habitat. Because Sea Lampreys utilized accessible habitat prior to dam removal, they were chosen as a focal species with which to quantify recolonization. Migrating lampreys were marked with PIT-tags and their activity was tracked with daily surveys. POPAN models indicated a fourfold increase in the annual abundance of spawning-phase lampreys following dam removal. We also observed a greater than fourfold increase in nesting sites. Sea Lampreys disturb the stream-bed during nest construction thereby altering physical habitat and potentially affecting stream-dwelling biota. In order to address the hypothesis that Sea Lampreys serve as ecosystem engineers, we quantified stream-bed characteristics and benthic invertebrate assemblages among the mounds, pits and undisturbed reference locations of nesting sites. We sampled in early summer after spawning and again in autumn to determine if any lamprey-induced changes persisted. Persistent differences were detected for most response variables thereby suggesting that spawning-phase Sea Lampreys are ecosystem engineers in Sedgeunkedunk Stream.