| Abstract: In order to investigate trophic interactions, the diets of peacock bass (Cichla kelberi) and dogfish (Galeocharax knerii) were studied in the Corumbá Reservoir between 1997 and 2000. This dietary study was performed to assess the niche breadth of each species and to determine the degree of niche overlap during different phases of reservoir colonization. During Period I, peacock bass were absent or recorded only in low numbers; during Periods II and III, peacock bass reached high abundances in the reservoir. Interactions between the species were weak during period I, but, during Periods II and III, they were found to interact intensively. The diet overlap was highest during Period II. The niche breadth fluctuated for both species in the different phases. Greater niche breadth was observed for dogfish during periods of low peacock abundance (i.e., Period I), and the lowest niche breadth value was observed during Period II. During the same period, the peacock bass exhibited a wide foraging niche. During Period III, the dogfish showed an increase of its niche breadth, while for the peacock bass a simultaneous decrease in the niche breadth, caused by increasing rates of cannibalism, was recorded. These results show that the presence of peacock bass induces changes in the diet of dogfish, probably due to a restricted number of prey items. |