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Measuring the restoration of connectivity between fish populations following the levelling of the Sélune dams

Coordinator(s)

Simon Blanchet and Guillaume Evanno

Scientific context

The presence of obstacles or dams in watercourses leads to fragmentation of populations of aquatic organisms by preventing them from moving to one side or the other of the obstacle. This also applies to non-migratory fish species, which are more or less partitioned by the presence of dams. This fragmentation has a direct impact on the genetic diversity of aquatic populations.

Scientists have developed a fragmentation index, the F-index, to quantify the effect of river fragmentation on populations of aquatic organisms. The removal of the dams on the Sélune provides an exceptional opportunity to measure the fragmentation effect induced by large dams on several fish species, and to quantify the reduction in this effect following the removal of the dams.

Subject(s) of study

The project focuses on 5 species of fish found in the Selune valley: minnow, stone loche, chub, european bullhead and gudgeon.

Objectives

The aim of the research project is to measure the effect of fragmentation on 5 species of fish in the Selune, considering two phases:

1. Before the last dam removal - to understand the effect of dams on the genetic differentiation of the 5 fish species.
2. After the removal of the dams - to assess the speed and conditions for restoring connectivity between the populations of the 5 species in question.

Methods

The research project is based on the measurement of a fragmentation index, the F index, developed by Prunier et al (2020). This index quantifies the fragmentation effect of a dam on populations of aquatic organisms in a standardised way, taking into account the age of the dams and the size of the populations in question.

To do this, populations located downstream and upstream were collected by electrofishing. The fin samples collected from each of the 5 species are genetically analysed using microsatellite markers. The sampling campaigns will be carried out 3 times: before (2022), 1 year (2024) and 2 years (2025) after the last dam is removed.

Laboratories involved

• SETE - Theoretical and Experimental Ecology Station, CNRS, Moulis
• DECOD - UMR Ecosystem Dynamics and Sustainability: from source to ocean, INRAE/Institut Agro/Ifremer, Rennes
• U3E - Experimental Ecology and Ecotoxicology Unit, INRAE, Rennes