Life stage‐specific marine migration and spatial behaviour of sea trout (<i>Salmo trutta</i>) from two coastal rivers

The current study utilised data from acoustic telemetry to assess movement traits important for the conservation of sea trout (Salmo trutta), providing an evidence base for future management of the species in the marine environment. The inclusion of sea trout (Salmo trutta) in Annex III of the EU Nature Restoration Law (Regulation [EU] 2024/1991) marks a significant step in recognising the species' ecological importance and the urgent need to understand its marine space use. In total, 255 sea trout (n = 170 post-smolts, n = 54 finnock) from two coastal rivers were monitored between April 2018 and September 2021, with a high level of migration being observed between both natal and non-natal estuaries and open coast zones. The key findings highlight clear differences in dispersal, residency and space use both within and between post-smolts and finnock (sea trout in their first year after smolt migration). Post-smolts exhibited greater dispersal distances and longer residency in the marine array, while finnock displayed strong seasonal trends, particularly in open coastal areas, suggesting waterbody type-specific movement behaviours. On average, post-smolts dispersed farther than finnock, but average dispersal for all tagged fish was approximately 20 km from the natal estuary, confirming localised marine space use. The average duration spent at sea was 83 days for post-smolts that returned to their natal river in the same year. Tagged sea trout travelled cumulative distances of up to 276 km within the marine array during the study period. The results demonstrates that sea trout marine movement has a seasonal element, with natal estuaries acting as a key transitional zone before broader marine dispersal and highlights the importance of non-natal estuaries during summer months. The mixing of sea trout from multiple rivers in shared marine areas (e.g. estuary & inshore coastal zones) highlights the need for management at a regional scale rather than a single river approach. The marine movement traits presented here provide essential insights to support policymakers and managers to make informed decisions that promote sustainability of sea trout populations.