The opening of the dams will have a considerable impact on the functioning of trout populations in the Sélune. Restoring the river's ecological continuity will modify population flows, allowing amphihaline sea trout to migrate further upstream, and native trout to move upstream and downstream of the dams. The trout population is monitored at a network of stations covering the entire Sélune basin, using an abundance index carried out every 2 years. Field campaigns follow the Vigitruite® fishing protocol, developed at INRAE for use throughout France. This index of abundance, related to the surface area of favorable habitats on the watercourse, makes it possible to evaluate the quantity of trout present in the river, resulting from natural reproduction in previous years. Field campaigns are carried out by INRAE and the Fédération d'Ille-et-Vilaine pour la Pêche et la Protection du Milieu Aquatique (FDAAPPMA35). This layer shows the locations of these study sectors by year of study : 2012, 2014, 2018, 2020, 2022, 2023, 2024 et 2025.

The re-establishment of the river continuum of the Sélune after the levelling of the dams will enable the colonization of the upstream part of the basin which was previously inaccessible to trout. As a result, monitoring of amphihaline fish populations on the Sélune is carried out on the main river and its tributaries, both upstream and downstream of the former dams. These enable us to understand the dynamics of these common trout populations before, during and after dam removal. Regarding the trouts, the inventory is carried out using the VIGITRUITE® protocol. This is a standardized protocol for estimating the abundance of juvenile trout (Salmo trutta) in rivers. It is based on sampling per unit effort, a unit fixed at 5 minutes of effective fishing, and thus delivers abundances in numbers of juveniles per 5 minutes. Abacuses for converting abundances into densities are available, and a guide to interpreting densities, from low to excellent, is proposed. Compared with traditional inventories, this protocol enables a smaller number of people to be mobilized in the field. The protocol applies to watercourses up to 8 m wide and never more than 60 cm deep. It has been validated on granite and limestone rivers in Brittany and Normandy. VIGITRUITE® specifically targets juveniles of the year (0+) and yearlings (1+). The 0+ abundance provides information on generation replacement (or recruitment), useful for population demographic monitoring. The abundance of 0+ and 1+ is also a reflection of the quality of the trutticole habitat, and of specific attacks on its integrity (pollution, sedimentation, fragmentation in particular). It is a diagnostic tool for trout populations and their river habitats. This protocol is applied in September of the years in question: every 2 years between 2012 and 2020, then every year from 2021 to 2027. The year 2020 is special because of the COVID. For more information on acquisition conditions, please refer to the Sélune biocenosis observatory reports, the links to which are provided in this document. This dataset provides juvenile trout abundance data (0+, 1+ and cumulative), by station and fishing session.

The opening of the dams will have a significant impact on trout populations in the Sélune. Restoring the river's ecological continuity will alter population flows by allowing amphihaline sea trout to migrate further upstream and native trout to move downstream and upstream of the dams. The trout population is monitored at a network of stations covering the entire Sélune basin using an abundance index, which is carried out every two years. The field campaigns comply with the Vigitruite® fishing protocol, developed by INRA and intended for use throughout France. This abundance index, relative to the area of favorable habitats in the watercourse, makes it possible to assess the quantity of young trout present in the river, resulting from natural reproduction in previous years. This table provides individual data on trout caught according to the year of study.