Relative abundance, population size structure and diet composition and similarity were examined over 5 years for the nine most abundant catfish (Siluriformes) species captured in the Apure-Arauca River fishery centred around San Fernando de Apure, Venezuela, the largest freshwater fishery in the Orinoco River Basin. Based on size classes obtained by the fishery, all nine catfishes were almost entirely piscivorous. Four species that are entirely restricted to main channels of the largest rivers (Brachyplatystoma flavicans, Brachyplatystoma jurunse, Brachyplatystoma vaillanti and Goslinia platynema) fed predominantly on weakly electric knifefishes (Gymnotiformes) and had high pair-wise dietary overlap. The other five species (Ageniosus brevifilis, Phractocephalus hemioliopterus, Pinirampus pirinampu, Pseudoplatystomafasciatum and Pseudoplatystoma tigrinum) occurred in a range of channel and off-channel habitats and were observed to feed on a variety of characiform, siluriform and gymnotiform prey. Diet overlap also was high among these habitat-unrestricted species, but overlap between the channel-restricted and unrestricted species was low. Within each of the two groups, species were divided into approximately equally sized subgroups based on differences in body size distributions. The two most morphologically similar species, P. fasciatum and P. tigrinum, further subdivided prey based on the vertical stratum occupied by prey species (benthic v. midwater). The two most morphologically dissimilar, channel-restricted species, B. juruense and G. platynema, also diverged in frequency of consumption of benthic and midwater prey.
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