This study aims to analyze the diet and trophic ecomorphology of Odontesthes bonariensis, relating these features with species' abundance in the natural environment and in the Salto Caxias reservoir (at Iguaçu river) after the reservoir's formation. The samples were carried out before (from March 1997 to February 1998- pre) and after (from October 1998 to February 2001 - post 1 and 2) the damming. The stomach contents of 218 individuals were analyzed by occurrence and volumetric methods. O. bonariensis' diet is based on insects, scales, plants and detritus. Microcrustaceans (Cladocera and Copepoda) were the main items, allowing the characterization of the species as zooplanktivore. The species has a superior position and protractible mouth, with small teeth, creating a serrated pattern, uniformly lined side by side. There are many gill rakers, that are long and close together (typical of filtering fishes); the stomach is ill defined and the intestine is short. There was a gradual increase of O. bonariensis' abundance during the period of field study, mainly after the second year of damming. This fact seems to be closely related with the high abundance and availability of zooplankton in the dammed environment, and also with the ability of the species to exploit this resource.
Atherinidae; feeding; planktivory; morphology; freshwater
