The intensity of intraspecific competition is expected to increase with population size. Such biological interactions usually coincide with reduced fitness characteristics (e.g. lower body condition), the narrowing of trophic niches, suggesting that intense intraspecific interactions can be modified by reducing the extent of resource use. The relationship between a proxy of population size and resource use is common in the literature for territorial species. However, whether this pattern of higher competitive pressure and effects on body condition or resource use needs to be better established for non-territorial species. We evaluated the influence of population abundance on the condition factor, and niche width of populations of Knodus orteguasae, an omnivorous and small-sized species with no territorial behavior. For this, we sampled fish in 25 sand beaches in Acre River, western Amazon. We controlled for potential effects of season and ontogeny on feeding by restricting our assessment to individuals sampled in the low water period and to adults of K. orteguasae . We quantified the food resource use by gut content analysis with the volumetric method. We explored a potential relationship between morphological and feeding variability with a Procrustes analysis. We run a linear regression between abundance and the mean of the relative condition factor (Kn) or niche width per beach to assess the effects of intraespecific competition. The Procrustes analysis indicated no correlation between morphology and food resource use. Our results show that abundance significantly reduced the mean Kn but not the niche width of K. orteguasae populations. These results imply that beaches with greater abundance tend to have fish with worse body conditions that do not explore a narrow range of resources. Our study indicates that intraspecific competition plays a role in a proxy for fitness but not resource use for non-territorial and omnivorous fish species inhabiting open environments.