Understanding the phenology of smolt migration, i.e. the timing of smolt leaving freshwater to enter the ocean, is important to better forecast the future of Atlantic salmon (Salmo salar) populations facing climate change. Smolting is a critical part of the life cycle of Atlantic salmon but it can be costly and hazardous. While migration to seawater can increase the fitness of individuals via higher growth conditions and fecundity, it comes with physiological costs and higher mortality risks. The phenology of migration is an adaptive process which can help to reduce the overall cost of migration by synchronizing favourable conditions in rivers and at sea.