Movement, survival, and delays of Atlantic Salmon smolts in the Piscataquis River, Maine, USA

Movement, delays, and survival of hatchery Atlantic Salmon Salmo salar smolts were evaluated through the Piscataquis River, a tributary of the Penobscot River in Maine, USA. We explored the effects of the river’s four dams (Guilford, Dover, Browns Mill, and Howland dams) from 2005 to 2019. During this period, the downstream-most dam (Howland Dam) transitioned from full hydropower generation to seasonal turbine shutdowns and later was decommissioned with the construction of a nature-like fish bypass in 2016. We estimated survival through open-river reaches and at each dam using acoustic telemetry (n = 1,611). Dams decreased survival, with per-river-kilometer (rkm) apparent survival averages of 0.972, 0.951, and 0.990 for Guilford, Dover, and Browns Mill dams compared to a per-rkm survival of 0.999 for open-river reaches. Turbine shutdowns increased survival at Howland Dam (to around 0.95), which was further increased by the nature-like fish bypass (0.99). We used radiotelemetry in 2019 (n = 75) and demonstrated that approximately one-third of the fish used the bypass, while the remaining fish used alternative routes. Smolts successfully passing the three upstream dams had lower apparent survival through Howland Dam than smolts that were released upstream of Howland Dam. Although smolts passing Browns Mill Dam had high survival, the dam caused extended delays, with median delay times surpassing 48 h in most years. Most of the delays caused by Browns Mill Dam occurred after fish had passed the dam and may indicate a sublethal effect of passage. Overall, while survival through Howland Dam has improved, passage and delays at the three upstream dams in aggregate represent a critical impediment to the effective use of the high-quality spawning habitat found upstream.