Summary
The results of qualitative and quantitative analysis of the stomach contents of tambaqui, Colossoma bidens (Spix), collected in Lago do Castanho, Amazonas, were related to the measurements (standard lenght) of the specimens and to the months of the year. It is shown, with graphs and tables that there is a periodic variation in the alimentation of this fish. Fruits is constantly present during the season in which the river waters are hight, but is increasingly rare as the volume of the river water decreases. The following fruits occur most frequently: araçá — Duroia duckei Huber; cajurana — Simaba guianensis (Aubl.); capitari — Tababuia sp.; murrão — Gustavia augusta L.; muruxi — Byrsonina sp.; seringa — Hevea brasiliensis M. Arg.; seringa barriguda — Hevea spruceana M. Arg.; taquari — Mabea caudata P. et H. Planktonic crustaceans is another common food of tambaqui, predominantly during the dry season. Cladocerans, copepodes and ostracodes occur most frequently. In regard to the size of the fish specimens it was observed that the presence of fruit in the stomach contents was greater in individuals of more than 600mm and moreover that the micro-crustaceans formed part of the alimentation of all of the specimens examined. An examination of the branchial structure of tambaqui showed the fish to be adapted for feeding on plankton. The strong teeth also indicated the capacity to break the stones of fruit. These findings lead us to classify the tambaqui as omnivorous.