Assemblage structure is related to predictable patterns of species composition, richness, and abundance. Stream habitat heterogeneity can be approached by two markedly hydrological sorts, riffles and pools, mainly featured by current, depth and substrate. Riffle and pool effect was studied on fish assemblages of three headwater terra firme rainforest streams in Urubu River basin. Central Amazonia, by 30-minute diurnal bank-side observational sampling intervals in which specimens were identified and counted. Twenty four habitats were chosen, eigth by stream, four riffles and four pools, for sampling in four trimestral periods in 1996 and 1997. The observations resulted in 754 specimens of 19 species. The most abundant species were: Pyrrhulina brevis Steindachner, 1975, Hemigrammus gr. ocellifer, Hyphessobrycon aff. melazonatus Durbin in Eigenmann, 1908, Hyphessobrycon aff. heterorhabdus (Ulrey, 1864), Iguanodectes variants Géry, 1993, Nannostomus marginatus Eigenmann, 1909. and Aequidens pallidus (Heckel. 1840). Riffles comprised 215 specimens of 15 species, and pools 537 specimens of 17 species. Segregation of assemblages through riffles and pools was explained by a trend of greater species richness and abundance in pools. This pattern might be related to several factors such as higher resource availability and inactivity of nocturnal predator in pools.
Amazonia; fish assemblage structure; riffles and pools; species richness; composition, and abundance; bank-side observational sampling