Dispersal and Gene Flow Among Potential Spawners: Source–Sink Structure Among Populations of Anadromous Brown Trout Exposed to Multifaceted Anthropogenic Impacts

Dispersal impacts individual fitness and influences local dynamics, stability and adaptation in interconnected populations. Anadromous salmonid fishes are renowned for their precise homing and adaptations to local aquatic environments, while navigating between multiple connected habitats. However, recent studies have demonstrated considerable straying among systems, generating metapopulation dynamics among connected subpopulations or demes. Salmonids constitute valuable economic and ecological resources, yet many populations are declining due to multifaceted anthropogenic-induced disturbances. This context of reduced populations inhabiting altered environments may impact both population viability and dispersal. To explore if metapopulation processes are present among impacted neighbouring populations of anadromous brown trout (Salmo trutta), a 4-year study of individual (N = 84) dispersal behaviour (using biotelemetry) and genetic analysis was conducted in four populations, connected by an extensive (> 200 km), semi-enclosed fjord system, Sognefjorden, Norway. To estimate the demographic status of each study population, life-table matrices were built, from which a potential source–sink structure among demes could be identified. Sognefjorden brown trout formed a metapopulation consisting of multiple sink populations, primarily supplemented from a single source. Only one population exhibited intrinsic growth (i.e., λ > 1), with excess recruits in this population attributed to high survival within the fjord. Among potential spawners, dispersal movements were performed by 55% of the total population, with individual age and migration extent affecting the probability of this behaviour. Successful dispersal (straying) was performed by 25% of the total spawning population. The extensive hydroscape generated directional gene flow from the innermost to outermost populations, with the highest rates observed among neighbouring populations. Although most dispersal resulted in unsuccessful spawning events and/or was not intended for spawning (e.g., conducted for overwintering purposes), connectivity among population demes was significant. This connectivity likely enhances the overall resilience of the metapopulation to variation and shifts in contemporary conditions within the fjord.