Understanding the Impacts of River Network Fragmentation on Freshwater Fish Species ^†

Longitudinal connectivity of freshwater systems allows upstream/downstream movements of aquatic migratory species and can be disrupted by natural or artificial barriers. Multiple obstacles, partially or totally blocking upstream movements, along the river promote cumulative non-additive impacts on freshwater fish. In addition to limiting fish migration, barriers can also affect habitat quality downstream by creating changes in flow regime, sediment and nutrient transport, and water temperature. Practically all European river corridors were already substantially modified by the building of dams and other barriers, resulting in at least 1.2 million instream barriers in 36 European countries. The impact of river network fragmentation is progressively intensifying, with the construction of additional barriers, and by the interaction of exiting barriers with other human-induced pressures such as water abstraction. Hence, it is important to assess the impacts of barriers on the structural and functional longitudinal connectivity of riverine systems and the consequences for fish species, assemblages, and diversity, to improve conservation measures and ecosystem management to halt biodiversity loss. The goal of this thesis project is to understand the impacts of riverine longitudinal connectivity loss, regarding potamodromous and diadromous fish species across Europe. For that, the research will be focused on: (i) determining connectivity loss caused by large dams in Europe, using graph theory analysis; (ii) understanding if we are over or underestimating the effects of artificial barriers by comparing the effects of natural to artificial fragmentation—regarding waterfalls, dams and smaller barriers, and the upstream water reservoir; (iii) distinguishing the effects of different types of barriers; and (iv) determining if functional resilience of fish communities can be improved by restoring connectivity. We expect that the fragmentation of freshwater systems will greatly reduce structural and functional longitudinal connectivity within river basins, which will ultimately have effects on fish species and communities, leading to a decrease of functional resilience.