REGIONAL AND LONGITUDINAL PATTERNS OF FISH COMMUNITY STRUCTURE IN THESEINE RIVER BASIN, FRANCE

The fish communities of 371 sites from 4 natural regions of the Seine River basin were studied. The sites were located from small to medium size rivers (catchment area : 5 to 3895 km(2)). We examined the differ ences between local communities according to river size (estimated by catchment area) and region. In the Seine River basin, fish communities follow a general organisation rule: total species richness increases with river size and importance of limnophilic species versus rheophili c ones increases from upstream to downstream. However, fish communitie s show differences of total species richness, species richness of repr oductive groups and species composition between the four natural regio ns of the basin. Particularly, river size and regional organisation of environmental factors interact on species composition of communities and several regional patterns of longitudinal changes of fish communit ies are identified. The origin and range of regional differences of fi sh communities are discussed according to historical and environmental factors.