Seaward movements and mortality of Atlantic salmon <scp><i>Salmo salar</i></scp> L. kelts in the Penobscot River, Maine

The Atlantic salmon Salmo salar L. is an endangered species in the United States, but post-spawn downstream movements remain poorly understood. We conducted a 2-year acoustic telemetry study to characterize downstream movements and to quantify apparent mortality and downstream passage of post-spawn adults (kelts) in the Penobscot River, Maine, USA (N = 112). The majority of tagged S. salar kelts (54%) exhibited a rapid movement seaward post-release instead of overwintering in the river. Salmo salar kelts that overwintered exhibited two patterns, distinguished by whether a dam was present or not. We found no relationship between post-release movement pattern, sex, body condition index or release year. Estimated apparent mortality rates were high (83%), with a greater probability of mortality occurring near the dams and release site. Apparent mortality rate did not differ by sex, body condition or year, but it was greater for individuals that moved out directly post-release and less for those that overwintered. While our results may suggest that overwintering in the river is a favourable pattern for survival, these results may be related to the closures of the designated downstream passages at the first two dams encountered by S. salar kelts. In fact, 55% of S. salar kelts passed a dam when the designated passages were closed. Altogether, these results may demonstrate the complex nature of downstream passage for S. salar kelts, particularly when having to navigate multiple dams.