The competitive interactions between two species of characins (Astyanax fasciatus and Astyanax bimaculatus) were investigated by studying their horizontal distribution, relative abundance and feeding habits in littoral and limnetic zones of Americana Reservoir, southern Brazil. Differences in horizontal distribution of the species mainly as adults led to their partial habitat segregation. Most adult A. bimaculatus were littoral dwellers, whereas A. fasciatus adults were distributed from inshore to offshore waters, but with most in the latter region. Both are faculative zooplanktivores in the reservoir, feeding mostly on planktonic crustaceans, insects (chiefly chironomids), some higher plant material, detritus and even fish scales (A. bimaculatus). Food overlap coefficients, considering adult and juvenile stages of both species are large in dry as well as wet seasons, although in the dry season the overlaps are significantly reduced for juveniles of both species and for juveniles of A. bimaculatus versus adults of A. fasciatus. Both species prey more on cladocerans than on cyclopoid copepods, and the adults selectively take larger forms (Daphnia gessneri and Moina micrura) in the reservoir (as in previously reported enclosure experiments). Juveniles prey heavily on Bosmina longirostris. These two formerly riverine species may reduce competition, in the reservoir, by partial habitat segregation in older stages, a mechanism to avoid competition, which may have existed in their previous interactive history in rivers.
