Abstract
We assessed spatial and seasonal changes in the diet of Oligosarcus hepsetus in order to describe the strategy developed by this species that allows their very high abundance in Lajes reservoir, Rio de Janeiro, Brazil. Fish samplings were carried out using gill nets, deployed during ca. 12 and 24 hours, between April 2001 and May 2002. A total of 289 individuals were examined, of which 97 showed gut contents. We used the index of relative importance (IRI) to compare probable dietary shifts, and the frequency of occurrence (% OC) to analyze possible ontogenetic influences on feeding. O. hepsetus showed carnivorous habits, feeding preferably on fish and insects, the latter of which occurred in 71.0% of the guts presenting contents. O. hepsetus consumed different items along the three reservoir zones: insects (61.0% IRI) and Cichla monoculus (38.9% IRI) in the lower zone; Lepidoptera (57.0% IRI) in the middle zone; and C. monoculus (77.0% IRI) in the upper zone. Food items changed seasonally with C. nonloculus predominating in autumn 2001, and Hymenoptera and Lepidoptera in the winter. In spring almost all food was Lepidoptera (99.8% IRI), while in the summer Hemiptera dominated in the diet. In autumn 2002 Hemiptera (97.0% IRI) was dominant, in significant contrast with the previous autumn. Individuals smaller than 190 mm SL fed heavily on insects, while fishes predominated in the diet of individuals larger than 190 mm SL. Shifts in prey-capture ability among length classes suggest decreasing intraspecific competition. A higher food plasticity seems to be the strategy employed by this opportunist species, which used food resources available in the reservoir.
Highlights
Oligosarcus hepsetus (Cuvier 1829) is a very common Characidae species that inhabits almost all freshwater environments in southeastern Brazil and prefers densely-vegetated shallow microhabitats in small tributaries or littoral zones of the main rivers
Since a previous study of the Lajes reservoir had shown the fish community patterns as changing spatially and seasonally (Araújo & Santos, 2001), we investigated the diet of O. hepsetus, relating the spatial-seasonal changes of its food intake with the peculiar environmental characteristics of this reservoir
The diet was comprised by insects (56.04% index of relative importance (IRI)) and fishes (43.9% IRI): Cichla monoculus (43.0% IRI); Lepidoptera (36.0% IRI); Hemiptera (14.0% IRI); and Hymenoptera (6.0% IRI)
Summary
Oligosarcus hepsetus (Cuvier 1829) is a very common Characidae species that inhabits almost all freshwater environments in southeastern Brazil and prefers densely-vegetated shallow microhabitats in small tributaries or littoral zones of the main rivers. In Lajes reservoir, the largest impoundment in Rio de Janeiro State, O. hepsetus is one of the most successful species, playing an important ecological role within the trophic chain since the juveniles are eaten by large piscivorous fishes (SANTOS et al, 2001). Trophic ecology studies of carnivorous species are very useful to fishery monitoring and management programs, since they provide important data about regulation mechanisms in fish populations and communities (Zavala-Camim, 1996). Remarkable seasonal changes can take place in fish diets and according to area. These are primarily related to shifts in food resource composition and availability, which are associated to reproductive pulses and physicochemical changes in aquatic ecosystems. Fishdiet knowledge provides information essential to autoecological research as it supplies additional data on the trophic structure of an entire ecosystem (BasileMartins et al, 1983)
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