FISH ASSEMBLAGE COMPOSITION IN RELATION TO ENVIRONMENTAL GRADIENTS INPORTUGUESE RESERVOIRS

Patterns in fish taxa composition were examined in Portuguese reservoi rs to assess whether distinct types of assemblages exist. Furthermore, the environmental variables most tightly associated with assemblage-l evel patterns were identified. Classification of the fish taxa-by-rese rvoirs matrix revealed four major groups. Groups were separated by mea ns of canonical correspondence analysis according to (by decreasing co rrelation with canonical axis): air temperature, annual rainfall, surf ace elevation, trophic state, mean depth and reservoir placement withi n the basin. In addition to these variables, stocking practices and pr edation likely contributed to assemblage composition. Assemblages in g roup A were distinguished by the permanent presence of largemouth bass (Micropterus salmoides) and the absence of trout (Salmo trutta and/or Oncorhynchus mykiss), nase (Chondrostoma sp.) and barbel (Barbus sp.) . Group B assemblages invariably included bass, barbel and nase, while pumpkinseed (Lepomis gibbosus) was frequent and trout was extremely r are. Assemblages in group C were also characterised by nase and barbel , but in contrast to group B, trout was frequent, whereas bass was rar e and pumpkinseed was absent. Group D assemblages consisted exclusivel y of trout and chub (Leuciscus sp.). The relationships observed parall el patterns detected in natural lakes to a certain extent, despite the critical human influence and the probable instability of some of the patterns detected. (C) Ifremer/Elsevier, Paris.