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AIM: Life-history strategies in fish include essential parameters related to offspring survivorship, fecundity and time of reproduction, which represent adaptive traits that enable a species to deal with spatial and temporal variability of abiotic conditions. This study aimed to compare reproductive traits associated to life-history theory for three Hyphessobrycon species from two lentic environments (four natural oxbow lakes and a man-made reservoir) of Mogi Guaçu River, upper Paraná River basin. METHODS: Specimens were collected with four minnow traps between August 2005 and July 2006 to cover dry and wet seasons (three samples in each season, and three samples in each environment). RESULTS: Reproductive strategy of H. bifasciatus and H. eques, which predominated in the oxbow lakes, differed from H. anisitsi in the reservoir. Hyphessobrycon bifasciatus and H. eques were single spawners with lower fecundity and size at sexual maturity, but invested more in number of eggs per body gram, while H. anisitsi was a multiple spawner species with higher fecundity, larger size at sexual maturity and body size. CONCLUSIONS: In seasonal environments, single spawners are synchronized with the floods to maximize juvenile survivorship, while reservoirs harbors multiple spawners’ fish due to the reduced fluctuation between high and low floods resulting from dam operation. Therefore, the seasonal condition in the oxbow lakes due to the flood pulse favored single spawners’ tactic, as showed by H. bifasciatus and H. eques. In contrast, the multiple spawning of H. anisitsi seems to be related to the more stable environmental condition throughout the year provided by the dam. Life-histories reported herein to Hyphessobrycon species must be common to other characid fishes inhabiting similar environments.

AIM: In this report, we describe the population structure, feeding and reproductive aspects of the cheirodontine Serrapinnus heterodon collected in a reservoir of a small hydro power of Mogi Guaçu River (SP); METHODS: Specimens were caught bimonthly between August 2005 and July 2006 near the marginal aquatic vegetation with four traps; RESULTS: A total of 38 specimens of S. heterodon were collected (mainly during the rainy season), comprising 16 males (32-47 mm SL), 18 females (32-49 mm SL) and four individuals with indeterminate sex (33-45 mm SL). There were significant differences in length distribution between sexes, with males being smaller than females (_x males = 42 ± 4.3 mm and _x females = 45 ± 4.8 mm). Sex ratio was 1:1. Eight food items were identified in the diet, with a predominance of autochthonous items. The diet classified as omnivorous. Insects (fragments) were the most consumed item, followed by microcrustaceans (Cladocera and Ostracoda), algae (mainly diatoms) and immature insects (Diptera and Odonata). Comparing with other cheirodontines, S. heterodon has higher fecundity (mean = 1,231 ± 440 oocytes) and smaller oocytes (61 to 793 µm). Moreover, the spawning was classified as total and occurred during the rainy season which is the breeding season for the majority of fish species from the upper Paraná River basin; CONCLUSIONS: The results seem to reflect the type of environment in which S. heterodon inhabit, because the lentic environment afforded by the dam provides relatively stable conditions to maintaining this species, which uses aquatic vegetation as shelter from predators and as feeding and breeding sites.OBJETIVO: Neste trabalho, a estrutura populacional e os aspectos alimentares e reprodutivos do queirodontíneo Serrapinnus heterodon são descritos, a partir de exemplares coletados no reservatório de uma pequena central hidrelétrica do rio Mogi Guaçu (SP); MÉTODOS: Os exemplares foram capturados bimensalmente entre agosto de 2005 e julho de 2006 com armadilhas do tipo covo, próximos da vegetação aquática marginal; RESULTADOS: Dentre os 38 exemplares amostrados (a maior parte durante o período chuvoso), 16 eram machos (32-47 mm), 18 fêmeas (32-49 mm) e quatro indivíduos com sexo indeterminado (33-45 mm). Os machos foram significativamente menores do que as fêmeas (_x machos = 42 ± 4,3 mm; _x fêmeas = 45 ± 4,8 mm; p < 0,05) e a proporção sexual encontrada foi 1:1. Oito itens alimentares foram identificados na dieta de S. heterodon, sendo os itens de origem autóctone predominantes. A dieta desta espécie foi classificada como onívora. Insetos (fragmentos) foi o item mais consumido, seguido por microcrustáceos (Cladocera e Ostracoda), algas (principalmente diatomáceas) e formas imaturas de insetos (Diptera e Odonata). Em comparação com outros queirodontíneos, S. heterodon tem fecundidade maior (média = 1.231 ± 440 ovócitos) e ovócitos menores (61 a 793 µm). Além disso, a desova foi classificada como total e ocorre durante o período chuvoso, que corresponde à estação reprodutiva para a maioria das espécies de peixes da bacia do alto rio Paraná; CONCLUSÕES: Os resultados parecem refletir o tipo de ambiente em que S. heterodon vive, pois o ambiente lêntico propiciado pelo reservatório, oferece condições relativamente estáveis para a manutenção desta espécie, que utiliza a vegetação aquática como abrigo contra predadores e como sítio de alimentação e reprodução.