The potential of the native characid, Brycon orbignyanus, to help to control some invasive fish species currently present in the Paraná River basin was addressed through microcosm experiments. Juveniles of three invasive (Cichla piquiti, Oreochromis niloticus and Ictalurus punctatus) and two native (Astyanax altiparanae and Prochilodus lineatus) fish species were offered as prey to B. orbignyanus in 300-l aquarium trials with three habitat complexity treatments (0%, 50% and 100% structure). Prey survival was much more variable among species (I. punctatus < C. piquiti < O. niloticus P. lineatus A. altiparanae) than through time, and B. orbignyanus predation was clearly mediated by habitat complexity, with lower prey survival in low-complexity treatments. The lower survival observed for C. piquiti could be partially explained by its less active behaviour and its preference for a surface microhabitat and less structured areas. Prey size also affected predation rates, as for the three less consumed species the largest individuals were significantly preyed on. Our results suggest that preserving and restoring the populations of B. orbignyanus might help to control some fish species introduced to the Paraná River basin, particularly I. punctatus and C. piquiti.

Effects of water level fluctuations on body condition of Geophagus brasiliensis were studied in a 30 km 2 Brazilian oligotrophic reservoir. Physiological condition (K) and gonadosomatic index (GSI) were compared according to water level (low and high). Females' best conditions were associated to higher resources availability during high water, since gonad development did not change between low and high water. Males' condition did not change between water levels, while the highest gonad development occurred in low water. Females presented higher reproductive investment than males, which allocated most of energy for somatic development. This strategy could be a mechanism to undergo the stress caused by oligotrophic characteristics of the reservoir enhanced during low water level.Efeitos do nível da água na condição de Geophagus brasiliensis foram analisados em um reservatório oligotrófico. A condição fisiológica (K) e o índice gonadossomático (IGS) foram comparados entre os níveis da água (baixo e alto). Melhores condições de fêmeas foram associadas a maiores disponibilidades de recursos no nível alto, já que o desenvolvimento gonadal não variou. Não foram registradas diferenças na condição de machos, contudo maiores valores de IGS ocorreram no nível baixo. Fêmeas apresentaram elevado investimento reprodutivo, enquanto machos investiram mais no desenvolvimento somático. Tal estratégia pode ser um mecanismo para suportar o estresse causado pelas características oligotróficas do reservatório, intensificadas durante o período de níveis baixos da água.

ABSTRACT. Diet of CieMa fIIonoeulus (Bloch & Schneider) (Osteichthyes, Cichlidae) in Lajes' Reservoir, Rio de Ja neiro, Brazil. The diet of Cichla monoculus (Bloch & Schneider, 1801) in Lajes's Reservoir, a major impoundment in Rio de Janeiro State, Brazil, was assessed, trom tishes collected in 1994, 1996 and 1999/2000. Gut contents in individuaIs was analyzed by the index of relative impoItance (IRl) which deals with numerical, gravimetrical and frequency of occurrence. Cichla l11onoculus showed a strong pisc ivorous habits feeding on Cichlidae, Characidae and Pimelodidae, in decreas ing order of importance, with a remarkable cannibalism on yo ung-ot~the-year. Others minor items in the diet were Mac/'obrachium sp. and Odonata. Changes in feeding composition varied with reservoir's zones and seasons, with higher diversity in Autumn and peaks of cannibalism in lower zone during Spring/SlIl11l11er. Overall, only one third of fi sh species composition in the reservo ir are predated by C.

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. monoculus 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.Key words: food habit, diet, Characiforms, reservoirs. RESUMO