Trade-offs between ontogenetic changes and food consumption in Brazilian silverside Atherinella brasiliensis from two tropical estuaries.

Salinity variation in estuarine environments influences the distribution of fish species as well as the availability of food resources to be used by them. This study examines the effect of the range of salinity on the trade-off between growth and feeding intensity of Atherinella brasiliensis from two tropical estuaries (positive and hypersaline). To investigate the effects of salinity, we hypothesized that hypersalinity negatively affects foraging intensity, consumption and prey selection by the Brazilian silverside, leading to differences in body condition. Sampling was carried out using the beach seine method in two areas of the estuaries (upper and lower zone) during rainy and dry periods. A total of 2549 stomachs (1124 for the positive estuary and 1425 for the hypersaline estuary) were examined, and the results indicated a dissimilarity of 92.7% of the diet between environments. In the positive estuary, there was more predation on Calanoida, Gastropoda, Hymenoptera, Ceratopogonidae larvae and Decapoda larvae, while Alga and plant-material characterized the diet in the hypersaline estuary. Significant correlations between the volume of food and salinity were observed in both estuaries. The vacuity index indicated that hypersaline environments presented higher contributions of semifull stomachs, indicating an intense consumption of algae. On the other hand,in the positive estuary, these values were less intense, but the stomachs were always with animal items. The variation found for both environments reinforces the effect of salinity on the physiological mechanism of the populations once the higher proportions of filled stomachs in the hypersaline environment indicate the need for constant and high ingestion of prey to guarantee the pronounced energy expenditure with osmoregulation.