| Summary: | Ontogenetic diet changes (prey species richness and size) in juveniles of white sea catfish (Genidens barbus) were tested in three southern Brazilian estuaries: Mampituba (298120S), Tramandaı´ (308020S), Chuı´ (338440S). Cluster analysis revealed that white sea catfish juvenile populations in the three estuaries are composed of two feeding groups. These two feeding groups are coincident with a bimodal size–age distribution of the juveniles of white sea catfish. In small catfish (5 to 10 cm TL) copepods were the most numerous prey (Chuı´¼ 86.66%N, Tramandaı´¼ 85.52%N and Mampituba ¼ 52.34%N). In large catfish (10 to 20 cm TL) the most abundant and frequent prey was fish (Chuı´: 73.19%N and 74.56%FO; Tramandaı´: 85.92%N and 73.33%FO; Mampituba: 52.34%N and 61.54%FO). The Morisita overlap index among small and large fish was low in all estuaries; high values of Morisita’s similarity index were observed among same size catfish groups. In all cases, no differences were observed among prey bio-volume curves of same size predator groups(small, F ¼ 0.41, P ¼ 0.65; large, F ¼ 2.19, P ¼ 0.11). In all estuaries, prey size increased significantly with increasing predator size. The 90th regression quantile estimated with most precision the predator–prey size relationship.
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