STRUCTURE OF FISH ASSEMBLAGES ALONG ENVIRONMENTAL GRADIENTS IN FLOODPLAIN LAKES OF THE ORINOCO RIVER

Fish assemblages of temperate lakes are structured primarily by an int eraction between piscivory and a small number of environmental variabl es, but tropical floodplain assemblages have often been viewed as unpr edictably structured. We tested the predictability of fish assemblage structure in floodplain lakes of the Orinoco River, Venezuela, in rela tion to 22 variables describing environmental variation at the microha bitat, habitat, and supra-lake levels. Fish species abundances were es timated through electro-fishing surveys of 20 lakes in three regions f or the early and late dry seasons of each of two consecutive years. Ca nonical correspondence analyses indicated that assemblage structure wa s predictably related to only four descriptors of lakes: transparency, conductance, depth, and area. Discriminant function analyses revealed that transparency (''clear'': Secchi transparency >20 cm; or ''turbid '': Secchi transparency less than or equal to 20 cm) was tightly assoc iated with the numerical density of six major taxa (82% classification accuracy) and the numerical density of piscivorous species (89% accur acy). Depth and area probably derived their significance from causal r elationships to transparency and availability of cover, whereas the in fluence of conductance arose incidentally through an association with biogeographical zonation. Mantel tests indicated that similarity in st ructure of assemblages was not strongly related to the distance betwee n lakes. Transparency was a remarkably reliable predictor of species c omposition. Fish with sensory adaptations to low light were dominant i n turbid lakes, whereas visually oriented fishes predominated in clear lakes; seasonal change involved decline in the proportion of visually oriented fishes concomitant with a decline in transparency. The effec t of transparency on assemblage structure was probably mediated by the relationship of transparency to visibility of prey. The structuring o f Orinoco fish assemblages by piscivory, under the influence of transp arency as controlled by depth and area, contrasts with previous views emphasizing random assemblage variation in neotropical floodplain lake s and extends the applicability of a conceptual model originally devel oped for temperate lakes.