Diet changes in two species of mojarras (Gerreidae) in Laguna de Términos, Campeche, Mexico

The mojarras Diapterus rhombeus and Eucinostomus gula are nektonic dominant species and play an essential role as a link in the energy transfer in the Terminos Lagoon. Thus, in order to contribute on the knowledge of their trophic dynamics, we analyzed the inter-specific and ontogenetic (intra-specific) variation in the diet of these species. A total of 156 individuals of D. rhombeus were analyzed, which consumed 16 trophic groups, whereas 134 individuals of E. gula were analyzed and these consumed 13 trophic groups. The relative importance of trophic groups (gravimetric method) showed that the overall diet of both species was determined mainly by Detritus and Algae, with slight differences between species. The asymptotic pattern of the cumulative trophic diversity indicates that the diets are accurately represented for the study area. Ontogenetically (intra-specific level), the diet of smaller individuals of D. rhombeus (with Copepoda as main food source) showed significant differences with the diets of larger individuals; whereas the diets of larger individuals showed no difference among them. Also, there was a trend in which similarity in diet was lower when the separation in the size of fish was greater, indicating a trend in decreasing intraspecific competition. In contrast, for E. gula, there were no significant differences between the size groups analyzed. Therefore, for D. rhombeus the inter-specific competition for food resources may become stronger than intra-specific competition, while for E. gula, the intra-specific competition for food resources is stronger than interspecific competition