Diversity of sea trout marine migration routes in the English Channel

Abstract Anadromous brown trout ( Salmo trutta ), also called sea trout, migrate from freshwater nursery and spawning grounds to the sea to maximise their growth and fecundity. Sea trout may undertake this marine migration several times in their life cycle, repeatedly facing significant natural or anthropogenic pressures on their migration path. Using data storage tags, we reveal for the first time the migratory paths and behaviour of post-spawning sea trout from three rivers (two in England and one in France) draining into the English Channel. Our results show a longitudinal gradient in sea trout migratory timing, with individuals from the western population reaching the sea earlier and residing longer in the marine environment than the eastern populations. Overall, sea trout dispersed widely within the English Channel, and each population used distinct marine areas up to the Celtic and North Seas. Within populations, individuals followed one or two directional migration routes that enabled us to identify seven main migratory corridors. Most sea trout spent 80% of their marine phase within 45 km of the shore, with a greater tendency to utilise waters further away from the shoreline at the midpoint of their marine migration. Individual tag data also show that having prior migratory experience influences horizontal behaviours. Finally, individuals from France were geolocated in English waters and vice versa, underpinning the necessity for international management of sea trout around the English Channel.