DYNAMIQUE TEMPORELLE DE LA CONTRIBUTION DU REPEUPLEMENT EN STADES PRÉCOCES DE TRUITE (SALMO TRUTTA L.) DANS LA POPULATION EN PLACE ET DANS LA PÊCHE SUR LE DOUBS FRANCO-SUISSE.

Abstract

The technique of otoliths fluoromarking has been used to evaluate at a large scale the practices of brown trout ( them as sac fry) for three cohorts 1994-95-96 in a part of the River Doubs having a length Salmo tr utta L.) stocking (540 000 - 570 000/year, most of of 40 km and fragmented in three zones (upstream, median, downstream) by three hydroelectric dams. For each of the three cohorts, there is a rather high contribution (23 to 60% according to the zone) of the stocked fry to the automnal 0+ standing population. The mass marking of all the stocked fry allowed to show that the dynamics of the contribution of the two components of recruitment (natural or stocked) differed. Indeed, the contribution of stocking decreased with time when measured in the autumnal standing population in running parts with a depth inferior to 1.2 m. The sharp decrease of the percentage of marked trout was high between stages 1+ and 2+ for the cohort 1994 and it was earlier, between 0+ and 1+, for the 1995 cohort. In the median and downstream zones, after the stage 0+, the contribution of natural recruitment is always widely dominant in the autumnal standing population. The study showed a higher contribution of stocking among the anglers catches than in samples of the same cohort realized in lotic water the year before in autumnal standing population. The rate of marked trout is rather high (12.5 to 45.8% according to the zone) among the catches of young 2+ trout. However, the rate of marked fish decreased sharply with the age of angled trout and the main part of trout globally angled in the French-Swiss part of the River Doubs are issued from the natural recruitment