Inter-annual dispersal stability within the Atlantic salmon metapopulation from the Bay of Biscay

Natural biocomplexity, characterized by genetic and phenotypic diversity, enhances the resilience of metapopulations by buffering against environmental fluctuations and stochastic events. This diversity acts as a natural bet hedge, where different populations may thrive under varying conditions, ensuring the overall stability and survival of the species. Regarding migratory salmonids, the portfolio effect is particularly important due to their complex life cycles and reliance on diverse and interconnected habitats along their ontogenic migration. To date, little is known about the temporal variability of dispersal movements, both in terms of intensity and direction. Yet, dispersal behavior is expected to vary over the next decades, in response to the overwhelming stressors (biotic and abiotic, from natural and anthropogenic sources) that salmonids are facing. In this study, we investigated Atlantic salmon ( Salmo salar ) dispersal patterns on a regional scale in western Europe, i.e. the Bay of Biscay, using otolith chemistry (Ba/Ca, Sr/Ca, and 87 Sr/ 86 Sr). Our results display significant connectivity between salmon subpopulations, particularly that of the Adour, which appears to be both a source (emigration rate: 1.3 ± 1.0 %) and a sink (immigration rate: 4.9 ± 4.3 %) for related populations. Although changes in Atlantic salmon dispersal could be expected with the acceleration and intensification of global change effects, our study shows very stable philopatric and dispersal rates over the last decade. Combined with genetic studies, our observations suggest that the metapopulation structure may help in the stability of the system, probably via a demographic but also perhaps genetic rescue effect. • Salmon population from the Adour acts as both a sink and a source for adjacent, smaller populations. • Homing and dispersal patterns of Salmo salar have remained stable in the southern Bay of Biscay for the past decade. • Metapopulation structure may help buffer against environmental stochastic events and promote overall stability.