A toolbox to assess the spatio-temporal dynamics of Atlantic salmon recolonization after dam removal on the Sélune River (Normandy, France)
Résumé
The recolonization of the upstream part of the Sélune River catchment (Normandy, France) by diadromous fish species began in July 2022 when the second large dam was removed. A third of the watershed, which represents around 1000 km of flowing habitats, is now accessible to these species, including the iconic Atlantic salmon, after a century’s absence. Historical documents and the presence of ruins of former fisheries upstream attest of its wide distribution and abundance from the Middle Age to the 20th century. Here, we present the toolbox that has been designed as part of the Selune scientific program to track the dynamics of Atlantic salmon recolonisation. To do that, we combine traditional and modern approaches such as telemetric tracking of spawners, counts of redds, electrofishing of juveniles’ habitats, eDNA and acoustic camera monitoring. Fish tissues are also collected to further study the demography and population genetics of salmons in this area. In our presentation, we will focus on the first evidence of upstream colonisation by salmon individuals few weeks after dam removal, and roof of reproduction in the following winter 2022-2023. We will also discuss other benefits of dam removal, including thermal regime and habitat quality in the river, to show how connectivity improvement may contribute to mitigate adverse effects of climate change on Atlantic salmon populations.