Effects of Dam Removal on Assemblage Composition and the Interactions of Fishes in the Penobscot River, Maine

Dams and their impoundments disrupt river habitat connectivity to the detriment of migratory fishes. Removal of dams improves riverine connectivity and lotic habitat which benefits these fishes along with resident fluvial specialist species. Restoration efforts on the Penobscot River (Maine, USA) are among the largest recently completed in the United States, and include the removal of the two lower-most dams and improvements to fish passage at several remaining barriers. Here we describe initial and potential future changes to fish communities in the Penobscot River associated with these restoration efforts. We assessed fish assemblages in the mainstem river and several major tributaries before (2010-2012) and after dam removal (2014-2016) using boat electrofishing surveys and a stratified-random sampling design. In total, we sampled 303 km of shoreline and captured 107,335 individual fish from 39 species. Similarity indices suggest that the most pronounced changes in fish assemblage composition occurred in reaches that underwent both habitat and connectivity changes (i.e. directly above removed dams). The newly connected reaches became more compositionally similar, as demonstrated by an average increase of 31% in similarity scores. The similarity score changes in these reaches were driven by increasing access for anadromous fish and decreasing abundances of slow-water specialist species. For example, we observed a marked reduction in lacustrine species in former impoundments. We also found all anadromous species in greatest abundance below lower-most dam during each respective sampling period. River herrings Alosa spp. passed through the new fish elevator at the new lower-most dam and spawned in newly available habitat upstream, as evidenced by presence of juveniles in our samples. Our results demonstrate the potential for large dam removal projects to restore both fluvial and anadromous fish assemblages. We also examined the current and future impacts of rebounding river herring populations on Smallmouth Bass. Here we describe the diet and growth of Smallmouth Bass collected from different areas of the Penobscot River watershed and project changes to annual growth associated with increasing access to juvenile river herring prey using bioenergetics modeling. We collected 765 Smallmouth Bass throughout 2015, examined the stomach contents of 573 individuals, and found notable differences in diet between river reaches with common seasonal trends. Juvenile river herring composed an average of 19% (SE = ±6%) of stomach contents by mass from Smallmouth Bass collected in the freshwater tidal area but were observed only rarely in the diets upstream. We used von Bertalanffy growth models to examine potential difference in growth among reaches and found overlapping 95% credible intervals for all estimated growth parameters in each area, with the exception of the freshwater tidal reach where the average asymptotic length was the largest (425 mm TL). Results from bioenergetics models suggest that increasing consumption of juvenile river herring will likely lead to increases in seasonal growth throughout the watershed as river herring populations continue to rebound. Our results provide new insight to both the predator-prey dynamic of these fish in a large river and the implications of anadromous river herring population recovery in systems where Smallmouth Bass has been introduced