PELICICE, Fernando Mayer; AGOSTINHO, Angelo Antonio; THOMAZ, Sidinei Magela. Fish assemblages associated with Egeria in a tropical reservoir: investigating the effects of plant biomass and diel period. Acta Oecologica, Paris, v.27, no.1, p.9-16, 2005.Studies investigating fish/macrophyte associations in the neotropics are rare. Aquatic vegetation enhances structural complexity of the environment, potentially influencing predator prey relationships. The present work characterizes fish assemblages associated with beds of Egeria, a submerged rooted macrophyte, in Rosana Reservoir, Brazil. The main subject of this study was to investigate effects of plant biomass and diel period on fish assemblage attributes (density, total weight, taxa richness and diversity), testing the hypothesis that macrophyte density and diel period greatly influences fish assemblage structure. Fishes were sampled with a 1-m2 throw trap at morning, midday and dusk, at six sites. The assemblages were primarily composed of small-sized species (e.g. Characiformes) and juveniles, with few large species (Gymnotiformes and Cichlidae). Around 95% of the individuals were captured in habitats with medium and high vegetation biomass. Fish assemblage attributes differed significantly with respect to macrophyte biomass. Highest values of these attributes were observed in maximum plant biomass, in contrast with trends observed in previous works, where higher values (mainly of fish density) were found in intermediate plant coverage. Oxygen, temperature and pH were within normal ranges, therefore, variation in assemblage attributes is assumed to be explained primarily by plant density. No pattern of diel variation was observed, perhaps reflecting sedentary characteristics of these taxa. Results from this study illustrate that procedures to reduce macrophyte density in reservoirs of the Paraná-Tietê-Paranapanema systems, may cause strong alterations in the fish assemblages, resulting in lower fish density, biomass and species richness of taxa utilizing Egeria beds.