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

Study Extent

Bresle river, CdSandre=G01-0400 Source latitude and longitude : 49.6909, 1.7498 Last confluence point latitude and longitude : 50.0644, 1.37 The Bresle is a small coastal river that flows into the English Channel, in the commune of Le Tréport. 68.1 km separate in a straight line its source, in the commune of Abancourt in Oise (60) from its mouth. However, the particular hydrology of limestone substratum rivers, as well as the multiplication throughout the history of mills and other structures along its course, more than doubles the length of the actual watercourse. The river drains a watershed of nearly 748 km², composed mainly of a chalky substrate dating back to the upper Cretaceous. The basin has historically been marked by the glass industry, but also by the paper industry, as well as by gravel extraction. Agricultural practices that were once mainly focused on dairy farming and grazing are now shifting towards the production of maize, protein crops for livestock, and potatoes. Migratory salmonids can now reach the municipality of Saint-Germain-sur-Bresle, just over 50 km from the sea.

Sampling

In the Bresle, sampling is conducted at 8 sites spread along the mainstem in the area of the hydrographic network colonized by Atlantic salmon. The sites are visited in 3 years, 2 in the first year, 4 in the second and 2 in the third year.

Method steps

1. Age is verified from scale reading according to standard methods described by (Baglinière and Le Louarn, 1987) and (Richard and Bagliniere, 1990).
2. Fish capture was carried out with a backpack electrofishing equipment. The same gear was used since the beginning of the data collection, namely a Martin Pêcheur from DREAM Electronique, It is tuned to produce a pulsed DC with a 400 Hz frequency, 250-300 volts and a square-waveform between 4 and 10% duty cycle. Fishing is operated according to the following steps : 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.

1. Salmo salar
   rank: species

The Bresle river in Normandy (France).