Effects of reduced-impact logging on physical habitat and fish assemblages in streams of Eastern Amazonia

Reduced-impact logging (RIL) has been considered as the main activity of forestry management in the Amazon. However, little is known of the effects of RIL on the region's ecosystems or in their biodiversity, and such information would be useful to inform forest management. Accordingly, this study aimed to investigate the effects of RIL on physical habitat and fish assemblages in Eastern Amazonia streams. We predicted that changes caused by RIL affect stream habitat, leading to a loss of diversity and changes in species composition with a predominance of generalist species. We sampled 13 streams in basins dominated by native vegetation without evidence of logging activity (unlogged areas) and 23 in areas that had been logged at different times between 2001 and 2011. Stream physical habitat structure was characterized using a standardized protocol, and the abundance, richness and composition of fish assemblages were assessed. Biotic and abiotic data were compared between unlogged and logged areas, and among streams in areas with different logging years. The relationship between the stream habitat structure and fish assemblages was assessed using a distance-based linear model using a forward procedure, followed by application of the Akaike information criterion. Variations in tree canopy cover, channel morphology, water temperature and dissolved oxygen and conductivity accounted for most of the differences in habitat characteristics between unlogged and logged areas. A total of 53 fish species was collected, belonging to 20 families and 6 orders. Fish abundances did not vary consistently between streams in unlogged areas and those that had undergone RIL, and showed minor evidence of species-specific changes in response to logging years. Likewise, there were no obvious differences in species richness, composition and biomass between logged and unlogged sites, regardless of logging years. RIL in Eastern Amazonia is performed in compliance with technical guidelines, but this practice has resulted in changes in stream physical habitat. Nonetheless, in the headwater streams that we sampled, the effects of RIL on fish assemblages were minor.