DIRECT · Predicting the impacts of river reconnection, biological invasions and climate change for the conservation of endangered cold-water fishes in Europe

River fragmentation and biological invasions are driving extirpations of cold-water fishes in Europe, with extirpations exacerbated by climate change. River barriers that cause fragmentation can prevent invasive species from dispersing further upstream (by forming an impermeable invasion front), but also impede the upstream movements of cold-water fishes (such as salmonid fishes), preventing their access to cooler waters towards the headwaters. The facilitation of movements of salmonids above barriers can be achieved through building fish passes on the structure, providing an easier route upstream, but this also opens up the invasion front for the invasive species. This Action explores this conservation disjuncture between alleviating river fragmentation through fish pass construction that aims to conserve threatened populations of cold-water fishes through reconnection and access to cool waters versus the fish passes opening up the extant invasion front to a highly invasive and impacting fish. Using the River Teme, Western England, as our study system, European barbel Barbus barbus as the model invader, and brown trout Salmo trutta and endangered Atlantic salmon Salmo salar as the model cold-water salmonid fishes, our objectives are to (i) compare and contrast the spatial and thermal ecology of barbel at the invasion front versus their core range, and identify the risk of their upstream dispersal over barriers via fish passes according to phenotypic variability; (ii) quantify the ecological interactions of salmonids and barbel throughout the study river and across different population abundances; and (ii) develop novel predictive models that predict conservation outcomes by simulating how river management schemes to promote salmonid conservation might instead drive the further invasion of a highly invasive fish. The Action has high complementarity between its outstanding European researcher and his host research group with expertise in fish invasion ecology.