LAUSR

For both conservation and management purposes, it is important to identify the natal origin of migratory individuals entering a river, particularly in genetically spatially structured species like brown trout (Salmo trutta) where the migrant ecotype (called sea trout) can originate from different populations. However, little attention has been paid to the spatial distribution of non-local fish at the level of an entire river catchment. The objective of the study was to quantify the proportion of non-local sea trout entering a river catchment (i.e., straying rate) and estimate the spatial extent of their upstream migration. Here, we considered dispersal in three distinct rivers, taking advantage of ten sampling sites. Sea trout, either trapped or rod-caught, were genotyped and genetically assigned to their source populations using appropriate baselines. On the basis of 1,437 sea trout fish classified as local or non-local, we empirically demonstrate that straying rate declines in each river as distance from the coast increases in a non-linear fashion. Straying rate exceeds 50% near the mouth, and then decreases gradually to reach less than 10% 40-50 km inland. A similar spatial pattern is found in the three rivers investigated suggesting an underlying common behaviour of non-local sea trout. The data and results presented here suggest that straying in is far more constrained spatially than first expected. The majority of non-local sea trout were found within the first 25 kilometres of the estuary in the three rivers investigated. This article is protected by copyright. All rights reserved.