Regional differences in salmon post-smolt migration routes can explain variation in individual growth at sea

• We developed an individual based model of salmon post-smolt migrations in the Northeast Atlantic Ocean. • The model is coupled with a hydrodynamic model and dynamic prey fields to investigate how the marine environment affects salmon growth and survival. • The optimal migration strategy for the post-smolts was to migrate away from shallow, coastal regions and to follow surface currents. • The first weeks at sea is a critical period for post-smolts leaving rivers in southern and western Norway as the available prey abundance is low. • A rapid migration to the central or northern Norwegian Sea is associated with faster post-smolt growth. The low number of Atlantic salmon sampled at sea makes it necessary to use alternative methods to understand their migration routes and interactions with the marine environment. In this study, we use a newly developed individual based model to simulate the ocean migration of Norwegian post-smolts during their first summer in the sea. The model is coupled with a high-resolution hydrodynamic model and dynamic prey fields to investigate how interactions between post-smolts and the dynamic marine environment in the Northeast Atlantic Ocean affect salmon growth and survival. The migration strategy leading to the best fit between the simulated geographic distribution and catches of post-smolts at sea, is to migrate away from shallow, coastal regions and to follow surface currents. Post-smolts originating from the middle parts of Norway can quickly reach the good feeding areas west of the Norwegian continental shelf, while post-smolts leaving rivers further south must start the marine life-stage migrating through the North Sea, where the prey abundance is low, before reaching the feeding areas in the Norwegian Sea. The results indicate that the first weeks at sea is a critical period for post-smolts as the available prey abundance is low, especially for smolts leaving rivers in southern and western Norway. A rapid northward migration to the central or northern Norwegian Sea is associated with faster growth for smolt emigrating from rivers flowing into the North Sea. The presented model is a first attempt to couple salmon and their marine prey in a spatiotemporal model covering the Northeast Atlantic Ocean.