Oir River

Until recently, the upper reaches of the Sélune River were inaccessible to migratory fish, as they were blocked approximately 15 kilometres from the sea by two dams. Only two tributaries located further downstream were suitable for spawning: the Beuvron River and Oir River. The latter is the most productive spawning tributary in the Sélune River system.
When the two dams were removed in 2021, the entire Sélune River once again became accessible. In the Oir River, upstream and downstream fish migration is monitored throughout the year using traps at the Cerisel mill in the town of Ducey.

Monitoring activities for migratory fish are the subject of an annual report:

Reports : 2018 - 2017 - 2016 - 2014 - 2013 - 2012 - 2011 - 2010 - 2005

The Oir River, which is 21 km long and flows east to west, has an mean gradient of 11‰. It drains a catchment of 87 km², that consists of sedimentary and metamorphic schist terrain with a few granite outcrops. The water is nearly neutral and has acceptable quality, but contains high nitrate concentrations. Most human activities in the catchment are agricultural (grassland, fodder crops, dairy cattle).