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

Study Extent

Scorff river, CdSandre=J5--0220 Latitude between 47.7718 and 48.1682; longitude between -3.2497 and -3.3404 Source latitude and longitude : 48.14 -3.283 Last confluence point latitude and longitude : 47.7371 -3.3474 The Scorff river is a small coastal stream of Southern Brittany (France). The main river is 78,6 Km long including 15 km of estuary. The mean gradient is 3,6%, the annual mean discharge is 5 m3 s- and the drainage basin area is 480 km2. Human activities and land use are mainly from agriculture, with some areas made moors and forests. Atlantic salmon colonization is essentially restricted to the main river over a 50 km stretch starting at the head of tide. Reproduction of Atlantic salmon takes place mostly in the main river (Bagliniere et al., 1979).The Scorff has only three larger tributaries. Up to recent years, their colonization by Atlantic salmon was restricted their down-stream end due to mill-dams.

Sampling

In the Scorff, sampling is conducted at 42 sites spread along the mainstem and 27 spread over 12 tributaries in the area of the hydrographic network colonized by Atlantic salmon. The sites are visited once a year at the beginning of autumn (end of September to early October).

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. : 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 dip-nets 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. If necessary, the fish stuck on the bottom or in aquatic vegetation are recovered by a small hand net. 4. Individuals are transferred into a bucket previously filled with water. 5. The whole team moves laterally several meters out of the area that has just been disturbed by the electric field, the carrier of the electrofishing gear taking care of not to step on the area he will next explore with the anode. When a bank is reached, the progression is a few meters upstream. Then steps 1-5 are repeatedThe sampling at a given site stops after 5 minutes of actual fishing – i.e. the duration when 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 individuals captured for a unit effort (5 minutes of fishing under the conditions specified above). Biometric measurement of the fish are recorded after their anesthesia with benzocaïne. The fish length is measured from the tip of the mouth to the fork of the caudal fin (1 millimeter precision). Fish are then weighed (0.2 g precision) and maturity status is assessed by a gentle pressure on the belly that allows to get sperm in mature males. All the fish sampled are then released in their original site.
2. Age is verified from scale reading according to standard methods described by (Baglinière and Le Louarn, 1987) and (Richard and Bagliniere, 1990).

1. Salmo salar
   rank: species

The Scorff, a coastal river in Brittany (France)