Individual Variation in Diet of Four Fishes in Shallow Tropical Estuarine Areas: Comparisons Between Seasons and Habitats

For the development of conservation and management strategies, scientists often define a ‘typical’ individual from a population and act as if all members of that group are basically the same, ecologically speaking. However, studies have found that resource use can vary among individuals from the same population, often due to their life stage, and that this variation helps reduce intraspecific competition among them. In this study, the feeding habits and the degree of individual specialization in the use of food resources, determined by the individual specialization index, were analyzed for the populations of the Brazilian silverside (Atherinella brasiliensis), mojarra (Eucinostomus argenteus), and two anchovy species (Anchoa tricolor and Lycengraulis grossidens) in different estuarine habitats. A total of 3855 specimens were collected from vegetated and unvegetated areas during the rainy and dry seasons of 2014. Overall, the highest density and biomass were registered in unvegetated areas. Density differed significantly among sites for A. brasiliensis, while for anchovies it differed significantly among seasons. Species exhibited spatial segregation in feeding, with anchovies feeding primarily in the water column, while A. brasiliensis and E. argenteus were benthic feeders. Despite using similar items, benthic species showed little dietary overlap, coexisting through differential use of prey proportions. A. brasiliensis, E. argenteus and A. tricolor were generalists, with the first two showing high intrapopulational diet variation (low IS values), whereas L. grossidens was a specialist with narrower niche breadth and lower individual specialization. Corroborating the Niche Variation Hypothesis, generalist species had a higher degree of individual specialization, which may be related both to an attempt at resource partitioning and to reducing intraspecific competition.