Diet of Hypophthalmus edentatus (Spix, 1829), (Osteichthyes, Hypophythalmidae) and variations in its stock in Itaipu Reservoir

Abstract

The aim of this paper is to assess the diet of Hypophthalmus edentatus (Osteichthyes, Hypophthalmidae), also called mapará, at Itaipú Reservoir from November 1994 to October 1997 and also to compare data to the species’ diet during the first few years of the reservoir formation. The data analysis related to the density of zooplankton and phytoplankton in the environment and searched for possible relationship regarding to stock depletion shown by the species during the last few years. Stomach contents were analysed by the occurrence and volumetric methods, coupled to Food Index. Mapará diet showed an ingestion of a restricted number of feeding items consisting of plankton organisms such as cladocerans, copepods, algae and testacea amoebae in 149 stomachs from November 1994 to October 1997. Results showed that zooplankton is the chief item in mapará diet and the cladocerans are the most ingested (approximately 63.0%). No significant changes were found between sites, years and classes analysed. However, comparisons between the species’ diet in the first years of the reservoir formation and those of this study period showed significant differences, or rather a high prevalence of bigger sized cladocerans (Daphnia gessneri) and a smaller amount of Cyanophyceae algae. High concentration of nutrients after impoundment, an increase in phytoplanktinic biomass of the species are expected according to the resource variation.