Summary :This study investigates the ecology of monogenean gill parasites of Astyanax altiparanae Garutti & Britski, 2000 and Rhamdia quelen (Quoy & Gaimard, 1824) in a stretch of the São Francisco Verdadeiro River, Paraná, Brazil. Statistical and ecological indices were used to examine observed levels of parasitism in relation to host and environmental characteristics. A. altiparanae and R. quelen had infestation intensities of 2.8 and 23.1 parasites per fish, respectively. The only significant environmental influence was observed at the upstream station for R. quelen. For both host species, parasitized and non-parasitized individuals presented similar weight-length relationships. Parasitized individuals had dispersed K n values indicating abnormal conditions. The low levels of parasitism observed in this study suggest that the environment is relatively undisturbed. Additional studies should compare these two species and their respective parasites following completion of the hydroelectric headquarters planned for construction in this stretch of the São Francisco Verdadeiro River. Résumé : RELATIONS HOTE-PARASITE DE MONOGÈNES PARASITES DES BRANCHIES D'ASTYANAX

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