Mise à jour 2025

This layer was created to locate sea lamprey spawning grounds in order to monitor their colonisation of the Sélune and two of its tributaries over time.

Unrestricted use subject to the following compulsory wording on all distribution documents: "Source: INRAE - U3E".

Open licence : https://www.etalab.gouv.fr/licence-ouverte-open-licence

No public access restriction

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Spawning grounds are located in the field by two operators, either by kayak (from upstream to downstream) for the Sélune, or on foot (from downstream to upstream) for the two tributaries: Le Beauvron and l'Oir. Each operator followed one half of the bed (right or left). Only clearly marked spawning grounds were geolocated using GPS (theoretical accuracy in XY: 20m).

The list of fields presented in this dataset is specified in the associated catalog.

Summary of observation conditions:

- In 2015: spawning ground counts were carried out between July 20 and 30, 2015. The main course of the Sélune was covered by kayak to cover more ground, from the Roche Qui Boit dam to Ducey. Tributaries were covered on foot, on the banks or directly in the stream depending on visibility. The Beuvron was surveyed in its entirety, from its mouth to the Valjoie fish farm, upstream of Saint James. The Oir was covered in sections with favorable habitats, from the river mouth to the Bief du Château de Montgohier. Hydrological conditions were correct, allowing good observation of the riverbed. For more information: see link “Dispersion et (re)colonisation du cours de la Sélune et de ses affluents”.

- In 2019 - 2020 and 2021 : Spawning ground counts were carried out in the first week of July 2019 and 2020, and in July and August 2021. The Sélune was surveyed from upstream to downstream by two operators in kayaks from the Roche-qui-Boit dam to Ducey in 2019 and 2020. Downstream of Ducey, habitats are largely unsuitable for sea lamprey nesting. Favourable linear sections of the Beuvron and Oir tributaries were surveyed from the confluence with the Sélune over 27.5km and 7.2km respectively. Due to unfavorable hydrological conditions in July 2021, it was only possible to survey a very small stretch of the Beuvron, with the remainder of the survey carried out in August. The results of these August counts in 2021 cannot be compared with counts carried out in July in other years: the older spawning grounds are much less visible, as they are smoothed by the current and recolonized by periphyton. The Sélune was not surveyed in 2021. For more information: link “Rapport d'activité Observatoire des biocénoses aquatiques de la Sélune 2019-2021”.

- In 2022: Spawning grounds were counted during the first week of July 2022. The Beuvron and Oir tributaries were surveyed from the confluence with the Sélune upstream for 15.5km and 3.3km respectively (upstream survey limits 25.4 and 8.1km from the confluence with the Sélune). The Sélune could not be surveyed in 2022 due to excessive turbidity caused by the emptying of the Roche qui Boit dam. For more information: link “Observatoire des biocénoses aquatiques de la Sélune - rapport d'activités 2022”.

- In 2023: Spawning grounds were counted between July 04 and 11, 2023. The Beuvron and Oir tributaries were surveyed from the confluence with the Sélune upstream for 11.1km and 2.5km respectively. The Sélune was surveyed over a total length of 21.7km, on foot on favorable riffles upstream of Saint-Hilaire-du-Harcouët, and by kayak from Saint-Hilaire-du-Harcouët to the Mazure leisure center. The Airon was surveyed on foot over a total of 5.1km on favorable riffles upstream of the Pont de Bretagne weir (Saint-Hilaire-du-Harcouët), in its entirety between the Sélune and the weir. The downstream sections of the Gueuche and Cance rivers were surveyed immediately upstream of their confluence with the Sélune. The 15km stretch of the Sélune downstream of the Mazure leisure center to Ducey could not be surveyed due to a storm on Tuesday July 04, 2023, which tripled the flow of the Sélune and tinted its water for several weeks, and the upstream tributaries for several days. For more information: link “Observatoire des biocénoses aquatiques de la Sélune - rapport d'activités 2023”.

- In 2024: Spawning grounds were counted between June 24 and July 08, 2024. A total of 49.2km of the Sélune was surveyed. The survey was carried out on foot for favorable riffles upstream of Saint-Hilaire-du-Harcouët, and by kayak from Saint-Hilaire-du-Harcouët to the Mazure leisure center. The Beuvron, Oir and Airon rivers were surveyed on foot over a total distance of 7 km, 2.76 km and 6.2 km respectively, on riffles favorable for reproduction. The Gueuche and Cance rivers were surveyed downstream, immediately upstream of their confluence with the Sélune. In all, 75.13 km of river were surveyed. For more information: link “Observatoire des biocénoses aquatiques de la Sélune - rapport d'activités 2024”.

- En 2025 : Le comptage des frayères a été effectué lors de la dernière semaine de juin et la première semaine de juillet 2025.

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