The lake sink in <scp>Atlantic</scp> salmon smolt downstream migration

The objective was to estimate the movements and survival of Atlantic salmon smolts migrating downstream through a river-lake system consisting of two large, interconnected lakes (areas 58.2 and 13.2 km², minimum crossing distance 29 and 16 km). Whereas downstream migration in running waters is much studied, limited research in small lakes and reservoirs suggests that smolt survival may be lower when moving through still water. Across two migration seasons, wild and hatchery-reared Atlantic salmon smolts were radio tagged and released in the inflowing river upstream the lake. Tracking of smolt individuals indicated major losses during lake migration. Among 300 radio-tagged smolts, about 90% or more perished during lake migration with loss rates about 3%-4% per kilometre. Migration time through the lakes varied considerably by smolt individual. We conclude that Atlantic salmon downstream migration through lakes may represent major population sinks, on a scale similar to marine losses, but likely depending on exposure (travel distance, time, navigational complexity) and predation risk. This additional cost for salmon populations upstream of lakes makes them more vulnerable and less resilient to environmental stressors. They therefore require special management attention.