Abundances and biological traits of the juveniles salmon sampled in the survey of Salmon 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 20 sites spread along the mainstem, 6 along the main tributary (i.e. the Lurgorrieta) and 2 distributed on 2 smaller tributaries. All sites are located 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). Sampling events by site and by year are indicated in table 3. 8 sites sampled in the first years of the survey were abandoned afterwards.

Method steps

1. 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.
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 Nivelle river in Basque Country (France). http://services.sandre.eaufrance.fr/Courdo/Fiche/client/fiche_courdo.php?CdSandre=S52-0400