Baseline fish inventory of the Trombetas River rapids, a drainage of Southeastern of the Guiana Shield, Brazil

Abstract

The Trombetas River basin, a clearwater tributary of the left bank of the Amazon River, originates in the southern portion of the Guiana Shield and flows into the state of Pará, Brazil. Upstream of Cachoeira Porteira, the river flows through a rocky channel with steep gradients and sandy-rocky substrates, forming numerous rapids and waterfalls. Despite the recognized biological importance of Guiana Shield rivers, comprehensive fish surveys remain scarce. Here, we conducted a rapid assessment of the fish community inhabiting rapids, primarily using diving-based collection methods, to document species diversity and community structure in a poorly explored environment. We collected 302 specimens, representing 53 species, 43 genera, 14 families, and seven orders. Siluriformes was the most species-rich order, followed by Characiformes, Cichliformes and Gymnotiformes. At family level, Loricariidae showed the highest species richness, followed by Cichlidae, Anostomidae and Apteronotidae. We also report the first occurrence of Asterophysus batrachus (Siluriformes, Auchenipteridae) in the Trombetas River and range extensions of the strictly rheophilic genera Hypancistrus (Siluriformes, Loricariidae) and Teleocichla (Cichliformes, Cichlidae). The rapids remain preserved mainly due to their location within protected areas, such as the Trombetas State Forest and traditional community territories. However, the proposed Cachoeira Porteira Hydroelectric Dam poses a significant threat to these habitats and their associated fauna, as this stretch of the Trombetas River encompasses one of the last remaining natural rapid systems in the Amazon Basin.