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

Study Extent

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

On the Bresle, sampling is conducted at 14 sites spread along the mainstem, in the area of the hydrographic network colonized by the European eel.

Method steps

1. Fish capture was carried out with a portable 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. In the case of a watercourse of reduced width, only one is used. 2. The anode scans an area about 2-2.5 m upstream of the dip-nets in the vein of water filtered by them. The electric current is applied for a duration of 30s, with a circuit cut after 15s and for a duration of 2s.
   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. The electric field is variable, adapted to the conductivity of the water. In the areas where there are many roots or stones, the operator may have to fish longer in order to attract the fish out of their hiding place. 4. Individuals are transferred into a bucket previously filled with water. 5. The whole team moves from bank to bank 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 and the team goes the other way. 6. 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 tip of the caudal fin (1 millimeter precision). Fish are then weighed (0.2 g precision). All the fish sampled are then released in their original site of capture.

The Bresle river in Normandy (France). http://www.sandre.eaufrance.fr/geo/CoursEau/G01-0400