Tributaries create habitat heterogeneity and enhance fish assemblage variation in one of the largest reservoirs in the world

The Three Gorges Reservoir is one of the largest reservoirs in the world. Previous studies uncovered longitudinal changes in fish assemblages among riverine, transitional, and lacustrine zones, but whether they vary at small-scale habitats and how tributaries contribute to fish conservation is poorly understood. We selected three neighboring tributaries with different habitat characteristics within and among tributaries and hypothesized that the small-scale habitat heterogeneity within or among tributaries could shape spatial and temporal variation in fish assemblages. We analyzed spatial (among tributaries, pelagic versus benthic, lower versus upper reach) and seasonal patterns of fish assemblage and their relationship with the environment. Piscivorous, limnophilic, and mid-pelagic species dominated the fish assemblage in different habitats. Fish assemblage structure varied spatially between lower and upper reaches and among tributaries. Fish assemblages also varied seasonally, mainly related to environmental variability (i.e., water depth, water temperature, pH, specific conductivity, riparian slope, and vegetation coverage). Catch per unit effort was similar between pelagic and benthic habitats but exhibited horizontal and seasonal changes. The most influential environmental variables were water depth, water temperature, dissolved oxygen, and pH. Our results supported the hypothesis, and indicated that we should protect tributaries that provide important areas for fish diversity in large reservoirs.