Expecting the Unexpected: Combined Effects of Dam Removal and Re-Allocation of Hydropower on Atlantic Salmon Smolt Migrations

We evaluated Atlantic Salmon (Salmo salar) smolt survival in the lower Penobscot River, Maine USA through the main-stem of the river and an alternative migration route, the Stillwater Branch.  We conducted research prior to removal of two main-stem dams and doubling of hydropower generation in the Stillwater Branch.  Survival and proportional use of migration routes in the lower Penobscot were estimated from multistate models using six years of acoustic-telemetry data (1,669 smolts) and two years of radio-telemetry data (190 smolts).  Few migrants (12%) used the Stillwater Branch, and mean survival through two operational dams therein was high (1.00 and 0.97).  Survival at Milford Dam, which will remain in the main-stem of the river, was relatively low (0.91), whereas survival through two dams that were removed was relatively high (0.99 and 0.98).  Smolt survival could decrease in the Stillwater Branch with the addition of two new powerhouses while continuing to meet the fish passage standard of 96% survival.  These data suggest that the effect of removing two dams in the main-stem is expected to be minimal for smolt survival based on high survival at those locations.  Milford Dam, therefore, offers the best opportunity for improving smolt survival in the lower river.