Towards a better taking into account of the intraspecific biodiversity for the management of native Mediterranean brown trout (Salmo trutta L.) populations on the Dranses basin in Haute-Savoie (France).

An interdisciplinary study of the management of brown trout (Salmo trutta L.) populations was undertaken in the Dranses river system in Haute-Savoie (France) after a genetical study indicating the survival of populations belonging to the Mediterranean autochtonous lineage (MED). In this system, stocking was practiced during a century with fry issued from domesticated trout belonging to the Atlantic lineage (ATL). The study aimed to evaluate and to try to change management practices towards a better taking into account of the autochtonous MED component in this system. The usual stocking practices with high densities (1.5/m(2)) of ATL juveniles have been estimated in a large scale experiment using otolith fluoromarking. Practiced with similar densities (around 1.5/m(2)) the contribution of stocking estimated at juvenile stages was significantly higher when fed fry were used than when unfed sac fry were used. The respective global contribution values were 43% (fed fry) and 7% (sac fry) at 0(+) stage and 20% (fed fry) and 0% (sac fry) at 1(+) stage. With densities fivefold lower, rehabilitation stockings with MED showed a rather good contribution with low fluctuations between 0(+) (21%) and 1(+) (30%) stages. Whatever the stocking practice, the vast majority (70 to 100%) of 1(+) juveniles was supplied by natural recruitment. Consequently to the results of genetical analysis and to the evaluations of stocking practices, the stocking at high densities with ATL domesticated fed fry was discarded in the whole system. A part of the system (the upstream zone of the Dranse d'Abondance River) was placed as a sanctuary zone without any stocking. Elsewhere a temporary rehabilitation stocking was practiced with low densities of MED fed fry, the contribution of which was studied during four years in the catches by anglers in 5 zones and for 3 cohorts. The age classes 2(+) and 3(+) represented the vast majority (77 to 90%) of the catches by anglers. For the three cohorts surveyed, the contribution of marked trout to the total of catches was similar at age 2(+) and 3(+). For the three age classes 2(+), 3(+) and 4(+), percentages of marked trout in the catches by anglers did not vary significantly from one cohort to another. For the three age classes gathered and for the different angling seasons the percentage of marked trout in catches by anglers varied between 10 and 30 % according to the different zones. The results concluded to adapt the management practices allowing a better taking into account of the intraspecific diversity of the brown trout.