Fish assemblage composition in relation to environmental gradients in Portuguese reservoirs

Patterns in fish taxa composition were examined in Portuguese reservoirs to assess whether distinct types of assemblages exist. Furthermore, the environmental variables most tightly associated with assemblage-level patterns were identified. Classification of the fish taxa-by-reservoirs matrix revealed four major groups. Groups were separated by means of canonical correspondence analysis according to (by decreasing correlation with canonical axis): air temperature, annual rainfall, surface elevation, trophic state, mean depth and reservoir placement within the basin. In addition to these variables, stocking practices and predation likely contributed to assemblage composition. Assemblages in group 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 rare. Assemblages in group C were also characterised by nase and barbel, but in contrast to group B, trout was frequent, whereas bass was rare and pumpkinseed was absent. Group D assemblages consisted exclusively of trout and chub ( Leuciscus sp.). The relationships observed parallel patterns detected in natural lakes to a certain extent, despite the critical human influence and the probable instability of some of the patterns detected. La structure des peuplements piscicoles a été étudiée afin d'établir si des assemblages existaient et afin d'identifier les facteurs du milieu qui leur étaient les plus étroitement liés. La matrice de classification des taxons par réservoir révèle quatre grands groupes. Les groupes ont été séparés au moyen de l'analyse canonique des correspondances (par corrélation décroissante) selon la température de l'air, les précipations annuelles, la surface maximale des plans d'eau, l'état trophique, la profondeur moyenne et la situation du plan d'eau dans le bassin hydrographique. En complément de ces variables, les pratiques de stockage et la prédation contribuent aussi à la composition de la structure ichtyologique. Les assemblages du groupe A sont distincts par la présence permanente du ≪black-bass≫ ( Micropterus salmoides) et l'absence de truite ( Salmo trutta et/ou Oncorhynchus mykiss), du hotu ( Chondrostoma sp.) et du barbeau ( Barbus sp.). Les assemblages du groupe B incluent invariablement