Composição e sobreposição alimentar de Acestrorhynchus Britskii e A. Lacustris (Characiformes : Acestrorhynchidae) do reservatório de Sobradinho, rio São Francisco (BA)

Abstract

The Acestrorhynchidae family has Acestrorhynchus as the only genus, represented by 15 species with occurrence restricted to South America. Its members possess a long and slender body, large mouth with conical and/or canine teeth, prey in a shoal formation, have piscivorous feeding habit and preference for lentic habitats such as lakes, marginal ponds and backwaters. The present study aims at characterizing the feeding habit of two species of the genus, endemic to the São Francisco River: A. lacustris and A. britskii, the way they share the available food resources from the Sobradinho Reservoir/BA, and the degree of diet overlap between them. Nightly bimonthly samplings, between November 2006 and July 2008, were conducted at different points located in the lotic, transitional and lentic stretches of the reservoir, using gillnets (12, 15, 20 and 25 mm between adjacent nodes). Stomach contents of 899 A. britskii and 476 A. lacustris were analyzed. About 70% of individuals analyzed presented empty stomachs. Their contents showed that both species are exclusive piscivores, since they preyed on more than ten fish preys. Despite the broad spectrum of food sources, few preys were constant throughout the study period. Among characiform preys, Moenkhausia costae stands out in the diet of A. britskii and Tetragonopterus chalceus for A. lacustris, as well as the participation of other foraging characiforms and the engraulid Anchoviella vaillanti, which serve as base level for the piscivore trophic chain in the reservoir. Actinopterygii (parts) and non-identified Actinopterygii were the food items with highest feeding indexes for both species, thus resulting in high food overlap between them (Cλ=0,992). The detailed analysis of fish-preys, however, showed a differentiated diet between species in relation to flooding and dry periods, and reservoir stretches, which reflected in lower food overlap (dry Cλ=0,427 and flooding Cλ=0,172; lentic Cλ=0, transitionalCλ=0 and lotic Cλ=0,434; overall overlap Cλ=0,236). Spatial and seasonal differences on the use of distinct preys between both species suggest a mechanism of food competition reduction, stressing the opportunistic character in the use of food resources, according to their environmental availability.