Determination of patterns of ichthyofauna cooccurrence in the Parana Riverbasin, area of influence of the Itaipu reservoir

An investigation of the existence of groups of co-occurrent fish species in the Itaipu reservoir and in its area of influence and evaluation of inters pecies relationship are presented. The identification of significant positi ve and negative inter-relationships between pairs of species was made possi ble by means of data regarding the presence and absence of the most frequen t species in the habitats studied. To rest the hypotheses that two species are distributed independently the chi 2 distribution was applied. With rega rd to positive interactions three affinity groups were registered. Group I is made up of 10 species associated to similarity level higher than 0.95. T hey are characterized by constancy of sampling in low to moderate water vel ocity, where the development of juveniles predominantly occurs. Groups II a nd III are made up of 3 species each and their similarities are higher than 0.95 and 0.99, respectively, Species of Group II have a distribution area larger han those of Group III and occur in all the sampled environments. Em phasis has also been given to aspects of dominance and abundance among the most similar species according to their feeding habits or behaviour in each co-occurrent group. Concordance among species in habitat preference was fo und only in one group (Group I).