Combining movement ecology and genetics to understand the ecology and evolution of a freshwater top predator (northern pike, Esox lucius) that has colonized brackish lagoons in the southern Baltic Sea: implications for management and conservation of weakly connected metapopulations

To inform management and conservation of fishes, understanding both ecological connectivity of habitat patches, apparent at the year-to-year scale, and genetic connectivity, emerging at the long-term evolutionary scale, is equally important. Ecological connectivity provides an indication of the potential for rapid recolonization upon local depletion, while genetic connectivity informs about the conservation needs related to genetically differentiated subpopulations. We combined acoustic biotelemetry and pooled-genome sequencing to study a northern pike ( Esox lucius ) population as a model of a freshwater top predator 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 increased during spawning, suggesting directed spawning migrations to either freshwater in tributaries or low salinity patches in connected lake-like bays. Pronounced spawning site fidelity to either brackish or freshwater spawning sites was observed, contributing to reproductive isolation. Genetic population structure aligned with salinity gradients and geographical distance but was unrelated to ecological connectivity. The results collectively suggest that local subpopulations may not rapidly replenish upon local depletion, yet even weak connectivity among them was sufficient to erode genetic differences over time. Effective management and conservation of species forming metapopulations, such as the northern pike studied here, necessitate localized approaches that adapt fishing mortality to local abundance and promote access to specific habitats during spawning to conserve the entire genetic biodiversity and foster resilience of the metapopulation.