Ces dernières années, un ensemble de méthodes et techniques d'étude a été mis au point ou amélioré à partir de recherches réalisées sur les populations sauvages de salmonides. Dans un premier temps, ces nouvelles techniques peuvent être considérées comme des outils disponibles pour les gestionnaires; ce sont : la mise en place et l'homogénéisation de critères scalimétriques pour la détermination de l'âge des salmonidés migrateurs; la détermination du sexe par sérodiagnostic et à partir de critères morphométriques; l'estimation de la production en smolts de saumon à partir de l'habitat et des caractéristiques des peuplements automnaux. Dans un deuxième temps, l'application de certaines de ces méthodes dans le domaine écologique a permis d'obtenir des éléments essentiels pour la connaissance et/ou la gestion des populations sauvages de salmonides.
Research on the wild salmonid populations in Brittany and Lower Normandy: possible applications in their management
Turing the fast few years some new techniques for the study of wild populations of salmonids have been developed. The following procedures are intended to provide.
1) an improved method for determining age by scale-reading, particularly with migratory salmonids (Atlantic salmon and sea trout). A standard zone for removing scales, the presence of three different zones on each scale (river, run-out, sea) and some criteria for scale interpretation have all been defined. These criteria are mainly concerned with the structure of the scale during the sea-life stage and reproduction. On the other hand, a spawning mark has been determined for both sedentary and migratory salmonids.
2) an means of sex determination. Two techniques have been proposed :- the first one is based on the detection by immunoagglutination of the serum vitellogenin of females in active gametogenesis. This method concerns females only; it does not allow a distinction to he made between non spermating males and immature salmonids. It is a very reliable technique which can be used in field conditions, but only by experienced researchers;
- the second technique is based on the sexual dimorphism of the relative length of the upper jaw bone. It is very reliable (8 % to 12 % error) and can also be applied in field conditions, but only for large fish (fork length > 40-45 cm).
3) a way of estimating Atlantic salmon smolt production. This technique is based on :
- the estimation of a 0+ and 1+ salmon population in the whole river from data of density obtained in sectors representative of the different types of habitat ;
- the presence of growth bimodality in a 0+ salmon population in autumn, given the following hypothesis : one yearold smolts belong to the upper mode ;
- the estimation of mortality for the whole winter season.
This technique has been used in the River Oir and compared with a smolt-trapping experiment conducted for two years. A comparison of the results shows that :
- the one year-old smolts belong to the upper mode of bimodality;
- when based on data on habitat characteristics, this technique can be used in field conditions to evaluate the smolts with sufficient accuracy.
All three techniques are immediately available as useful methods for fisheries management. At a second stage, they can also serve in ecological research to advance knowledge on which to base further study and improved management of wild salmonid populations.
- wild populations,