Understanding the Impacts of Dams on up and Downstream Passage - Lessons from Atlantic Salmon in the Penobscot River

Perspectives generated by salmon researchers and managers working in the Pacific and Atlantic are understandably disparate based on diversity of species and trajectories of historic and contemporary exploitation.  The basic issues remain starkly familiar; quality and accessibility of habitat.  In the Penobscot River, impoundment construction at the turn of the 19^th century decimated runs of Atlantic salmon long before Pacific salmon passage was threatened.  The planned removal and bypass of three dams on the Penobscot River has generated great optimism for the recovery of this Atlantic salmon stock. Ongoing studies of smolt (acoustic telemetry) and adult passage (PIT arrays) indicate that benefits of restoration may be very specific to life history stage and site.  Adult passage at the two lowermost dams being removed (Veazie and Great Works Dams) and Howland Dam (to be bypassed) is extremely poor (median rates of 64 - 75%).  For downstream migrating smolts, the two lower dams are not associated with high mortality but two dams that will remain after restoration (Milford and West Enfield Dams) are associated with high loss (15-25%).  These results underscore the need to target both upstream and downstream passage in order for restoration to succeed.