Summary 0[ Spatial patterns of freshwater _sh species at regional and local scales were inves! tigated to explore the possible role of interspeci_c interactions in in~uencing dis! tribution and abundance within communities occupying coastal streams of North! Western France[ 1[ Nine sites from nine streams situated in the same biogeographical region were sampled annually over the 5!year period from 0889 to 0884[ 2[ Similar habitats "sites# with richer regional colonization pools exhibited pro! portionally richer local communities in terms of number of species\ total density and total biomass of individuals[ Furthermore\ no negative relationships were found between density and biomass of each of the most common species and local species richness[ 3[ Results of dynamic regression models "applied to the above!mentioned species# suggest an absence of strong competition between all pairs of species[ 4[ The evidence on lack of density compensation for species!poor communities and absence of perceptible interspeci_c competition between species suggest that the communities studied are non!interactive[ 5[ Two main explanations can be advanced[ First\ the local abundance of species in the communities studied could be determined through di}erential responses to unpredictable environmental changes\ rather than through biological interactions[ Second\ as a result of historical events\ the communities studied are reduced in congeneric species which can limit\ in turn\ the in~uence of interspeci_c competition in structuring these communities[ 6[ These results underline the strong in~uence of regional processes in shaping local riverine _sh communities and minimize the possible in~uence of species interactions in governing these communities[ Key!words] density compensation\ _sh assemblages\ interspeci_c interactions\ local species richness\ regional species richness[ Journal of Animal Ecology "0887# 56\ 361Ð373

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 reacts 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 biogeographic region (North-Western France). We show 1) that species richness is greater for tributaries than for coastal rivers on both local (station) and regional (catchment) scales, and 2) that, whatever the rivers studied (tributaries or coastal rivers), there is a significant, positive linear relationship between local species richness and regional species richness. These findings suggest that local, and probably regional, freshwater fish communities in North-Western French rivers are unsaturated and thus, that major impacts on the community, such as short-term extinctions, are not to be expected. However, competition for food and space, prédation, introduction of exotic parasites and diseases, could also affect some native species.

RÉSUMÉDans l'optique de comprendre quels sont les processus qui gouvernent la structure des peuplements de poissons des cours d'eau bas-normands, une première approche descriptive est réalisée à travers l'analyse de la variabilité temporelle de ces peuplements. Les données exploitées portent sur 14 stations situées sur 10 cours d'eau de Basse-Normandie, échantillonnés de 1990 à 1996.Les principaux résultats de l'étude sont les suivants :1. on note, pour l'ensemble du peuplement, un maintien de la composition spécifique au cours du temps (persistance) ; 2. le peuplement présente une structure relativement stable ; 3. les densités moyennes des populations qui composent ce peuplement présentent, en revanche, une importante variabilité temporelle. Cette variabilité est déterminée, en partie, par les variations des débits, lesquelles sont d'autant plus importantes que l'on se situe en aval du cours d'eau ; 4. contrairement à l'hypothèse de HORWITZ (1978) (« extermination hypothesis »), le turnover des espèces n'augmente pas avec un accroissement de la variabilité du peuplement ; 5. la variabilité du peuplement augmente avec le temps.La position des peuplements de poissons des cours d'eau bas-normands sur le gradient « stochastique-déterminé » est discutée. MEDIUM-TERM VARIABILITY OF FISH ASSEMBLAGES IN STREAMS OF THE NORTH-WESTERN FRANCE. ABSTRACTIn order to outline processes governing fish assemblages in streams of the North-Western France, a first descriptive study is realized through the analysis of their temporal variability. Studied data concern 14 sites within 10 different streams sampled annually over the seven-year period from 1990 to 1996. 1. the assemblage shows a constant species composition (persistence) ; 2. the assemblage has a relatively stable structure ; 3. the mean densities of the populations are highly fluctuating. These fluctuations are explained, in part, by flow variations, which are more important in downstream areas ; 4. the « extermination hypothesis » (HORWITZ, 1978) is not supported by our data set ; 5. assemblage variability increases with time.Finally, the position of these fish assemblages along a « stochastic-deterministic » gradient is discussed.