Catchment-wide survival of wild- and hatchery-reared Atlantic salmon smolts in a changing system

We developed a hierarchical multistate model to estimate survival of Atlantic salmon (Salmo salar) smolts in the Penobscot River, USA, over a decade during which two mainstem dams were removed from the catchment. We investigated effects of (i) environmental factors, (ii) rearing history, and (iii) management actions, including dam removal, turbine shutdown, and installation of new powerhouses. Mean +/- SD smolt survival per kilometre was higher through free-flowing reaches of the catchment (0.995 +/- 0.004.km(-1)) than through reaches containing dams that remain in the system (0.970 +/- 0.019.km(-1)). We observed maximum survival between 12 and 17 degrees C and at intermediate discharges (1200 m(3).s(-1)). Smolt survival increased concurrent with dam removal and decreased following increases in hydropower generation. The greatest increase in smolt survival followed seasonal turbine shutdowns at a dam located on the largest tributary to the Penobscot River, while other shutdowns had little influence. Our model provides a useful tool for assessing changes to survival of migratory species and will be useful for informing stocking plans to maximize numbers of smolts leaving coastal systems.