Création avec données de 2015 à 2026

This dataset makes it possible to report on the line surveyed per campaign, the number of spawning grounds identified on each line, the length of each line, and the spawning rate related to the length of the prospected line.

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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The prospected area data is reconstructed from two input data: (1) the GPS points taken by the prospecting team (including the start and end of linear prospections) and (2) the layer of hydrographic sections, which are constituents of watercourses, excerpts from the Topage DB on the right-of-way in the study area and available on the Sélune Information System.

The data is built in QGIS as follows:

1. Creation of a new layer by recovering the layer from the hydrographic sections of the IS

2. Addition of a "nb_fra" column (integer) that will contain the number of spawning grounds and leave it blank

3. Addition of a "campaign" column (text length 4) that will contain the campaign, that is, the year, and leave it blank.

4. For each year:

- Use of the lamprey spawning layer in the IS to have the upstream and downstream points

- Remove all hydrographic sections that do not match an observed spawning area

- If necessary, cut out the hydrographic sections with a long linear geometry outside the prospecting area

- group all the hydrographic sections representing a continuity of prospecting

- update the campaign

- update the spawning number in "nb_fra"

- calculate the length of the section

- calculate the spawning frequency rate on the reach

- create a GIS file per year surveyed

5. Merge the different datasets by year into a complete dataset

6. The dataset in question is loaded via a python script that feeds the information system database and recreates the necessary view(s).

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”.

- In 2025: The spawning ground survey was conducted over six days on June 23, 24, and 25, 2025, and July 4, 7, and 8. The Beuvron River was surveyed on foot along two transects on June 23, 2025. Since the section of the river between these two transects was historically considered unfavorable for sea lamprey spawning, it was not surveyed. Observation conditions were fairly good and sufficient to count the spawning grounds despite slight turbidity. The Oir River was surveyed on foot along five transects on June 24, 2025. Only those transects

with suitable habitats were targeted. Observation conditions were good and sufficient for counting spawning grounds. The Airon River was surveyed on foot over several days on June 24 and 25, as well as on July 4, 2025. Survey conditions were variable along the downstream transect stretching from the confluence with the Sélune to the Pont de Bretagne weir at Saint Hilaire du Harcouët; as water turbidity was sometimes very high, it was not possible to see all the spawning grounds. However, spawners were observed on the spawning grounds during the count. No spawning grounds were observed on the two sections upstream of the Pont de Bretagne weir (Saint Hilaire du Harcouët). The Sélune was surveyed over two days, July 7 and 8, by kayak. Observation conditions were poor due to thunderstorms in the preceding days and rain on the first day of the survey. Water turbidity was very high, making it impossible to visualize all the spawning grounds. In total, 47.3 km of waterways were surveyed.

- In 2026: The spawning grounds were counted between June 17, 2026 and July 6, 2026. The linear areas prospected are the same as those validated in 2025, namely: the linear areas downstream of the dams (downstream of the Sélune and tributaries of the Oir and the Beuvron), the linear area within the old reservoirs (La Sélune), as well as a section of linear area upstream of the dams. (The Sélune from upstream of the dams to Saint Hilaire du Harcouët and the tributary of the Airon). These linear surveys were all conducted in 2026 and are described below. Le Beuvron was prospected on foot on June 17, 2026, along two lines up to La Croisette. The portion of watercourse between these two lines having been historically considered as too unfavorable to sea lamprey spawning is no longer being explored. L'Oir was explored on foot on June 22, 2026, along five linear paths up to La Butte Pichon. On the Oir, only those shelves with favorable habitats that have been followed in previous years are prospected. Due to the high temperatures this year, the upstream shelf has been shortened by about 200m. La Sélune was explored by kayak on June 30 and July 1, 2026, along the entire linear from Saint Hilaire du Harcouët to Ducey. The Airon was explored on foot on July 6, 2026, on three lines up to the Moulin de la Vallais. Portions of streams between these lines were historically considered too unsuitable for sea lamprey spawning or difficult to access, so they are no longer prospected. The 2026 surveys were marked by several heatwaves. Water levels were very low and showed much better visibility on the riffles than in 2025.

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