RESOLUTE · Predicting outcomes of nature based solutions and river reconnection for endangered cold-water fishes in European rivers

Climate change and river fragmentation are interacting in Europe's rivers to drive population extinctions of cold-water fishes, including those of the Salmonidae family. The effects of extreme warming in summer can be mitigated by increasing the provision of cool-water refugia through nature based solutions (NBS) that provide salmonid fishes with optimal rather than critical temperatures. However, the fishes must be able to access these refugia, which can be difficult in highly fragmented rivers, unless fish pass engineering is applied to the barriers fragmenting the river. The beneficial interactions of NBS and fish pass engineering on salmonid populations are, however, unknown. While they could facilitate behavioural thermoregulation, where fish actively move from warm areas into cooler waters (often moving >10km), fish passes also impose considerable inter- and intra-specific selectivity of fishes, often only being passable of specific size ranges and behavioural phenotypes (e.g. proactive individuals with bold, exploratory behaviours). Consequently, mitigating for warming and river fragmentation using NBS and fish pass engineering could result in considerable modifications to the diversity and distribution of fish species and phenotypes in Europe's rivers. This project aims to overcome these considerable knowledge gaps on how NBS and fish pass engineering can act in concert to provide sustainability in cold-water fishes in warming and fragmented rivers. The complementarity of an outstanding researcher in applied fish ecology with a European research group with exceptional expertise in fish spatial and trophic ecology will complete an empirical study based on fish biotelemetry methods in a natural field experiment testing relevant hypotheses within four NBS/ fish pass scenarios. The generated empirical data are then used as the basis for predictive models for simulating the outcomes of different scenarios of warming and fragmentation of global relevance.