UNSATURATED FISH ASSEMBLAGES IN RIVERS OF NORTH-WESTERN FRANCE - POTENTIAL CONSEQUENCES FOR SPECIES INTRODUCTIONS

Introductions of freshwater fish into French rivers have been carried out with increasing frequency in the last decades. Thus, the potential impact of these introductions is of major concern for biologists and fishery managers. Knowledge of the degree of saturation of a community provides an important basis for understanding how the community react s or will react to the introduction of new species. In this paper, we compare local and regional freshwater fish species richness in similar sizes tributaries and coastal rivers located in the same biogeographi c region (North-Western France). We show 1) that species richness is g reater for tributaries than for coastal rivers on both local (station) and regional (catchment) scales, and 2) that, whatever the rivers stu died (tributaries or coastal rivers), there is a significant, positive linear relationship between local species richness and regional speci es richness. These findings suggest that local, and probably regional, freshwater fish communities in North-Western French rivers are unsatu rated and thus, that major impacts on the community, such as short-ter m extinctions, are not to be expected. However, competition for food a nd space, predation, introduction of exotic parasites and diseases, co uld also affect some native species.