Distribution patterns of indigenous freshwater fishes in the Tagus River basin, Spain

Classification and ordination methods used to examine the internal complexi ty of the Mediterranean Tagus River catchment based on fish distribution re vealed that it is not a homogeneous biogeographical unit. The indigenous fi shes analyzed in this study are distributed through the basin forming geogr aphical communities (chorotypes), some of which are associated with environ mental factors like river morphology, water quality or geographical locatio n. Nevertheless, 40% of the variation in species occurrence remains unexpla ined by either environmental or geographical variables, suggesting that his torical factors may influence the freshwater fish distribution patterns. Th ree main biogeographical areas, delimited by significant boundaries, were i dentified. Two of them are identified as the upper and the middle-lower bas ins of the Tagus River catchment; the third corresponds to the Alagon River and seems to be linked to historical factors of the catchment.