NONINTERACTIVE FISH COMMUNITIES IN THE COASTAL STREAMS OF NORTH-WESTERN FRANCE

1. Spatial patterns of freshwater fish species at regional and local s cales were investigated to explore the possible role of interspecific interactions in influencing distribution and abundance within communit ies occupying coastal streams of North-Western France. 2. Nine sites f rom nine streams situated ill the same biogeographical region were sam pled annually over the 6-year period from 1990 to 1995. 3. Similar hab itats (sites) with richer regional colonization pools exhibited propor tionally richer local communities in terms of number of species? total density and total biomass of individuals. Furthermore. no negative re lationships were found between density and biomass of each of the most common species and local species richness. 4. Results of dynamic regr ession models (applied to the above-mentioned species) suggest an abse nce of strong competition between all pairs of species. 5. The evidenc e on lack of density compensation for species-poor communities and abs ence of perceptible interspecific competition between species suggest that the communities studied are non-interactive. 6. Two main explanat ions can be advanced. First, the local abundance of species in the com munities studied could be determined through differential responses to unpredictable environmental changes, rather than through biological i nteractions. Second, as a result of historical events, the communities studied are reduced in congeneric species which can limit, in turn, t he influence of interspecific competition in structuring these communi ties. 7. These results underline the strong influence of regional proc esses in shaping local riverine fish communities and minimize the poss ible influence of species interactions in governing these communities.