Responses of fish assemblages to subtle elevations in headwater streams in southwestern Amazonia

This study aimed to characterize species composition and abundance of fish assemblages of low-order streams of the Machado River Basin and to identify the main environmental factors associated to fish assemblages along the elevational gradient. Fishes were collected by seine and dip nets, along an 80-m stretch of 71 streams. In each site, we recorded physical, chemical, and land-water ecotone variables. We collected 22,702 specimens of 91 species. Species richness and abundance varied negatively with elevation. Ordination and similarity analyses detected that streams located in higher and middle elevation ranges differed in species composition from those in the lower elevation range. Almost one-third of the recorded species were selected as indicator species. Species of the families Trichomycteridae, Cichlidae, Gymnotidae, and Characidae were closely associated with the land-water ecotone and structural characteristics of lower elevation range, whereas several other Characidae species were more abundant in middle elevation range, where aquatic vegetation cover and conductivity values were higher. By assuming that composition and relative abundance of fish respond to environmental changes, hydrological characteristics may be viewed as environmental filters for stream assemblages by ultimately selecting the pool of species able to reach for specific elevation ranges and meet their biological requirements.