Migration dynamics of acoustic-tagged hatchery and natural atlantic salmon smolts

We investigated the migration dynamics of two Atlantic salmon (Salmo salar) smolt origin groups (hatchery, n = 374; natural, n = 231) in the Penobscot River Estuary and Bay, Maine, U.S.A., during the springs of 2010-2012. Smolts were acoustic-tagged and tracked for approximately 100 km to evaluate differences in movement rate, migration duration, path choice, and survival. While the behavioral differences between origin groups were not consistent, differences in survival were evident. Using a Cormack-Jolly-Seber mark-recapture model, we found that rearing origin and river discharge were the primary factors influencing smolt survival. Survival decreased with increasing river discharge across the observed range. Hatchery smolts (cumulative survival: 0.40-0.69) presented higher survival rates than their natural counterparts did (0.32-0.64), a finding that is inconsistent with much of the literature on hatchery versus river-reared fish. We hypothesize that the collection method for natural smolts at a dam bypass facility, which involves novel handling and dam interaction, may have contributed to their lower survival, potentially masking any inherent advantage over hatchery smolts. These findings underscore the importance of minimizing dam-related stressors and informing management actions aimed at improving the survival of outgoing smolt cohorts.