Recolonization in a devastated Corsican river: Respective contribution of wild and domestic brown trout.

The present study was carried out in the Abatesco River basin on the Eastern coast of Corsica. In September 1989, this small river basin was devastated by a storm spate. In the main river bed, the destruction of the riparian vegetation and the perturbation of the habitat led to a nearly complete loss of the benthic fauna and thus of the brown trout (Salmo trutta L.) populations. About 20 000 domestic fingerlings (5 cm mean length) were stocked during the spring of 1990 in the main river and its tributaries. In subsequent years, stocking practices were limited to the head of the basin. Fishing was prohibited until March 1993. Two sites were sampled five times from September 1989 to September 1993 for the evaluation of trout densities. In June 1993, 30 individuals were sampled in the Abatesco to investigate allozyme polymorphism. Recolonization required at least two years. Although the exact influence of stocking practices was difficult to evaluate, we found that brown trout restoration was mainly due to the populations of the tributaries, which had been less disturbed by the spate. The genetic analyses showed that, in June 1993, 92% of the trout carried the wild allele LDH-5*105 and 72% the endemic Corsican allele LDH-3*40, which is generally only found in basin head populations. Thus, this study has shown that the wild population was primarily restored by the surviving individuals, particularly those from the tributaries that escaped the spate.