Sea trout entering the rivers Adour and Nivelle in south-west France at the start of the 21st century

Study Extent

The River Adour (code Q---0000 according to SANDRE database) is a large river more than 300 km in length that rises in the Pyrenees Mountains and drains a basin covering almost 17,000 km2. Three tributaries of the Adour, i.e. Nive (code Q9--0250), Gave d’Oloron (code Q---0150) et Gave de Pau (code Q---0100), are well known to local recreational anglers for the abundance of salmonid populations. The Nivelle River (code S52-0400 according to SANDRE database) is the southernmost of the four French 'Index Rivers' for salmon. As such, it is subject to intensive monitoring programmes including juvenile electrofishing surveys, trapping of returning adults and nest count surveys. The Nivelle is a 41 km long coastal stream in the Basque Country with a drainage covering less than 300 km2.

Sampling

Both river-resident and seatrout were possibly caught in the fishtrap but only sea trout were considered in this dataset. Silvered body, presence of St. Andrew's cross on the body, absence of pigmentation on the flanks, adipose fin orange-coloured and other fins transparent were some of the key criteria used to distinguish sea trout (e.g. Bagliniere et al. 2001). The data of the present dataset were obtained at seven trapping devices: for the Nivelle River – Uxondoa and Olha; for the Adour River – Puyoo, Baigts-de-Béarn, Sorde-l’Abbaye, Chopolo and Halsou. Among them, only Olha and Baigts-de-Bearn traps provided census data when operating. Depending on the site, trapping devices operated all or most of the year, and, in the latter case, trapping was planned to overlap with sea trout upstream migration peak. Depending on the distance between the trap and river mouth, the peak of migration occurred in May, June or July. When operating, traps were checked daily except when the river was in spate or when repairs to the dam were undertaken (trap disarmed). In hot conditions (higher than 20° Celsius), data were collected but no scales were taken in order to speed up the fish handling process, reduce fish stress and avoid fish mortality. Similarly, during the migration peak, scales were taken on a subsample only.

Quality Control

All field data collected by MIGRADOUR and INRAE are checked by a qualified person before they are entered into proper databases. Entries are double-checked against source documents by a second person to possibly complete or rectify the data in case of missing records and data errors.

Method steps

1. The fish were chemically anesthetized to be safely handled and visually examined.
2. The side colouration was noted (silver-coloured, light-coloured and dark-coloured). Silver body is the typical colouration of the marine fish whereas coloured sides are an early indicator of freshwater acclimation of the sea trout.
3. The presence/absence of the ectoparasite Lepeophtheirus salmonis was recorded. Attached sea lice are commonly found on salmonids in the sea and survive for at least one week after the host enters fresh water (Hahnenkamp & Fyhn 1985). The presence of the ectoparasite is thus an indicator of a recent sojourn of the host in sea water prior to its capture in the trap.
4. The fish was then placed flat on the measuring device to measure fork length (from the tip of the snout to the end of the middle rays of the tail) to the nearest 1 mm (Nivelle) or to the nearest 5 mm (Adour).
5. Scale samples were taken from fish just posterior to the dorsal fin and before the adipose fin above the lateral line using a knife or forceps. Scales were placed in a small paper envelope especially designed for storing scales. Each envelope was properly identified and slowly dried at room temperature. Whenever possible, four scales were selected for reading, each having a clear focus, exhibiting little erosion apart from spawning marks, and showing obvious annuli in both the river and sea zones. Selected scales were cleaned by soaking them for about 20 minutes in a solution (3% v/v) of sodium hydroxide, washed in distilled water and rubbed with a fine brush to remove any soft tissue or paper fibres. The selected clean scales were then mounted between a microscope slide and coverslip together with a drop of glycerine solution (20% v/v). Capture of images of scales was obtained with a microfiche reader (Canon Microfilm Scanner 300) equipped with magnification lenses EZ03 (30x-50x) and an image analysis software (CapturePerfect version 1.0) allowing a 300-600 dpi digital scanning.

The Adour and Nivelle are two French rivers draining into the Bay of Biscay. The mouth of the Nivelle River is located at Saint-Jean-de-Luz. That of Adour River has changed several times during the last centuries, being now located at Bayonne (river diversion by Louis de Foix in 1578).