Spatial Organization of Fish Communities Along a Large River System: The St. Lawrence River (Canada)

Hosting a variegated number of fish habitats, Large River System (LRS) comprise diversified fish communities. However, their spatial organization within theses complex ecosystems remains largely unknown. The objective of this study is to describe the spatial pattern of fish communities along the St.Lawrence River(SLR) using the yearly standardized governmental fish surveys. Spatial patterns of fish communities were estimated using complementary traditional diversity indices, multivariate analyses and finally, biological/taxonomic traits to infer functional diversity indices. From 1995 to 2012, a total of 280 475 individuals (77 fish species) have been sampled within 2 386 sampling sites. Two distinct fish assemblages were targeted in each site: the littoral (71 species) and the pelagic community (47 species). The most diversified regions defined using  Simpson’ index were not always associated with elevated values of spatial heterogeneity of fish communities (i.e. Bray Curtis dissimilarity). On the other hand, the main channel is acting as a barrier isolating fish communities on each shores of the SLR. Our results, based on standardized data and robust indices, provide a real vision of fish communities’ organization in a LRS. Reflecting the quality of the SLR ecosystem, it’s a new light for policies of preservation and restoration of this resource.