Abstract
Dams on rivers are known to facilitate the colonisation and spread of aquatic alien and native invasive species, but the actual mechanisms involved are poorly understood. Since the construction of the Solina Dam on the upper San River system in Poland, European perch (Perca fluviatilis) have expanded their distribution into the headwaters of this river system, becoming a native invader. In this study, we assessed the spread of perch in detail over time upstream of the Solina Reservoir, and used otolith trace element microchemistry to determine the spawning and larval rearing locations of perch in the catchment upstream of the dam. Extensive sampling over several years across the catchment upstream of the Solina Reservoir confirmed the widespread occurrence of perch into the headwaters of the tributary river systems, with smaller size classes dominating locations closer to the Solina Reservoir. Despite perch being widely distributed upstream of the Solina Reservoir, otolith microchemical analysis indicated the populations from various reservoir tributaries mostly shared the same spawning and larval rearing habitat, most likely the Solina Reservoir. Our results suggest that reservoirs can facilitate the colonisation of river systems by providing a critical habitat element that would be otherwise missing from riverine landscapes, i.e., an extensive and productive pelagic larval rearing environment. This research shows that the impacts of large dams can extend many kilometers upstream from the river reaches directly affected by the resulting impoundment.