Intra and interspecific competition may influence the strength of individual specialization in resource use. We evaluated how intra and interspecific competition affects the degree of individual specialization in food resource use within sub-populations of Hoplerythrinus unitaeniatus. To do so, we identified food items from fishes collected in the Pantanal wetland. We tested whether individual specialization was related to H. unitaeniatus density (a proxy of intraspecific competition) or to the density of a potential interspecific competitor (Hoplias malabaricus) using a multiple regression. Dietary variation was related to ontogeny. Hoplerythrinus unitaeniatus density, instead of Hoplias malabaricus density, had an effect on the specialization of H. unitaeniatus individuals. A greater the density of H. unitaeniatus leads to a greater individual specialization. Our results point to a lack of specialization in H. unitaeniatus individuals. In addition, intra and not interspecific competition affected the use of food resources. Individuals of denser populations may consume secondary resources, reducing the effects of intraspecific competition and becoming more specialists.