Movement and mortality of Atlantic salmonkelts (Salmo salar) released into thePenobscot River, Maine

The Penobscot River is home to the last major run of Atlantic salmon (Salmo salar) in the United States. For over one century, the river has been stocked intensively. Since the late 1970s, most kelts are released to the Penobscot River estuary following spawning in a hatchery. Over that time, the proportion of the run made up of iteroparous individuals has declined to < 1%. These fish may make a disproportionate contribution to the spawning population because they are typically large and produce more eggs than virgin spawners. We radio-tagged 55 kelts that were artificially spawned at the Craig Brook National Fish Hatchery in 2015 and released them in two different locations in the river (head of tide and ~50 km upstream) to assess 1) when hatchery-spawned kelts leave the river, 2) whether release location influences river exit timing, and 3) if kelts from the upper river could successfully outmigrate through the lower Penobscot River’s hydroelectric complex. The kelts were tracked from November 2015 to July 2016. Although some fish from both release groups were documented leaving the system within one month, the majority of fish (84%) overwintered in freshwater habitat. Many (71%) of those kelts that overwintered in the Penobscot River made directed, upstream movements in November and December. However, there was no difference in upstream movement rates, timing of outmigration, or survival between the release groups. Survival to outmigration was 23.6%, which is considerably lower than kelt survival documented in Canadian and European rivers. Low survival to outmigration may have contributed to the disappearance of iteroparous individuals from the Penobscot River run of Atlantic salmon over the last four decades.