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

Study Extent

Nivelle river, CdSandre=S52-0400 Latitude between 43.2426 and 43.3912; longitude between -1.4799 and -1.6694 Source latitude and longitude : 43.2173 -1.5231 Last confluence point latitude and longitude : 43.3854 -1.6639 The Nivelle is a 39 km long coastal stream of Pays Basque. It has its source in Spain and flows into the Bay of Biscay at Saint-Jean-de-Luz. Its drainage basin has a surface area of 238 km2, and is dominated by marly-calcareous formations. It is essentially agro-pastoral with more than 50% of the land area made of moors (Dumas, 2003). The oceanic climate, mild and wet (1700 mm year-1 in St-Pe´e-sur-Nivelle), provides a mean annual discharge of 5.4m3 s-1 downstream from the confluence of the main tributary, the Lurgorrieta, and 7 m3 s-1 at the mouth. The water, neutral to slightly alkaline, is of good quality upstream from Saint-Pée-sur-Nivelle, but deteriorates downstream.

Sampling

In the Nivelle, sampling is conducted at 10 sites spread along the mainstem and tributaries. The sites are visited once a year at the beginning of autumn (end of September to early October)

Method steps

1. 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 of about 2-2.5 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. 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. Eels 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. Overall the fishing method is modeled on the principle of De Lury fishing (Fisheries by stock depletion). Each year, an identical sector with an area close to 100 m2 is inventoried using this method. Juvenile abundance is quantified by the number of individuals captured for a unit effort (Two or three fishing under the conditions specified above, depending on the quantity of individuals captured).
2. 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). All the fish sampled are then released in their original site.

The Nivelle river in Basque Country (France). http://services.sandre.eaufrance.fr/Courdo/Fiche/client/fiche_courdo.php?CdSandre=S52-0400