Combining biotelemetry and genetics provides complementary insights relevant to the management and conservation of a freshwater predator (Esox lucius) living in brackish lagoons

To inform the management of wild fish populations, it is equally important to understand both the ecological connectivity of habitat patches, apparent at annual and seasonal scales, and the genetic connectivity, emerging at evolutionary scales across generations. Ecological connectivity indicates the potential for rapid recolonization upon local depletion, while genetic connectivity informs about the conservation needs related to the evolution of subpopulations and ecotypes in metapopulations. We combined acoustic biotelemetry and pooled-genome sequencing to study a northern pike (Esox lucius) population as a model of a freshwater piscivore that inhabits a network of shallow brackish lagoons in the southern Baltic Sea. We found limited ecological connectivity among genetically similar subpopulations of pike, suggesting a metapopulation structure characterized by discrete local subpopulations with infrequent migrations between them. Connectivity of different lagoons increased during spawning, suggesting directed spawning migrations to either freshwater rivers or low salinity patches in connected lake-like bays. Spawning site fidelity to either brackish or freshwater spawning sites was observed, further contributing to the reproductive isolation of certain subpopulations. The genetic population structure aligned with salinity gradients and geographical distance and was significant between pairs of rivers draining into the lagoon network, but it was unrelated to ecological connectivity. The results collectively suggest that local subpopulations may not rapidly replenish upon local depletion and that even weak connectivity among subpopulations was sufficient to maintain genetic homogeneity across lagoons with similar salinity levels. Effective management and conservation of species forming metapopulations, such as the coastal northern pike studied here, necessitate localized approaches that adapt fishing mortality to local abundance and promote access to specific habitats, especially rivers, during spawning to conserve the entire genetic biodiversity and foster resilience of the metapopulation.