Abundances and biological traits of the juveniles salmon sampled in the survey of Salmon abundance Indices in the Oir river (France)

Study Extent

Oir river, CdSandre=I9280600 Latitude between 48.6840 and 48.5985; longitude between -1.2949 and -1. 0994 Source latitude and longitude : 48.6667 -1.1138 Last confluence point latitude and longitude : 48.6305 -1.2943 Oir river is located in the south of the Manche department, the Oir is a tributary of the Sélune, small Normandy coastal river flowing into the Bay of Mont Saint-Michel. The course of the Oir, long 21 km, east west, has an average gradient of 11 ‰. Its watershed of 87 km², dominated by sedimentary schist land and corneal including some granitic enclaves. The water is near neutral and reasonable quality but with high nitrate levels. On this basin, human activities are mainly from agriculture (meadows, fodder crops, dairy cattle). The upstream Sélune is not accessible to migratory fish. They are blocked to fifteen kilometers from the sea by a dam. Oir river, with its 12 Km accessible to migratory fish spawning tributary is the most productive of the network

Sampling

Sampling is conducted at 11 sites spread over the mainstem and over the 2 main tributaries in the area of the Oir hydrographic network colonized by Atlantic salmon. Since 2016, 1 site is sampled over the network colonized by Atlantic salmon to vérify this limit. The sites are the same every year and are visited once a year in the beginning of autumn (September).

Method steps

1. Juvenile salmon are caught by electrofishing. Until 2021, a 400 Hz pulsed current generator and then a direct current generator were used and the following procedure is followed: 1 Two large dip-nets with metal frames are placed facing the current, lying on the bottom, and kept at a fixed position.
   2 The anode scans an area of about 4-5 m upstream of the dipnet in the vein of water filtered by them. 3 Fish attracted by the anode and shocked go down into the nets guided by the electrode and driven by the water flow. 4 If necessary, the fish stuck on the bottom or in aquatic vegetation are recovered by a small hand net. 5 - Individuals are transferred into a bucket previously filled with water. The whole team moves laterally several meters (the carrier of the martin fisherman taking care not to step on the area he will next explore with the anode) out of the area that has just been disturbed by the electric field, then steps 1-5 are repeated. When one bank is reached, the progression is a few meters upstream. 5 The sampling at a given site stops after 5 minutes of actual fishing time - ie during which the electric field is applied in the water - measured directly on the counter of the electric fishing equipment. Juvenile abundance is quantified by the number of 0+ captured for a unit effort (5 minutes of fishing under the conditions specified above).
2. Age is verified from scale reading according to standard methods described by Baglinière & al (2022).

1. Salmo salar
   common name: Saumon atlantique, Atlantic Salmon rank: species

The Oir river in Normandy (France). http://services.sandre.eaufrance.fr/Courdo/Fiche/client/fiche_courdo.php?CdSandre=I9280600