To what extent are the fish compositions of a regulated river related to physico-chemical variables and habitat structure?

Abstract

Habitat structure and physico-chemical variables determine distribution of fish species at local scale. We aimed to determine influences of physico-chemical variables and habitat descriptors on fish assemblage structure in a regulated river in Rio de Janeiro State, Southeastern Brazil. The tested hypotheses are that assemblages are determined by these features at the local scale, and that increased flow during the wet season increases fish abundance and richness because increased habitat availability. Samplings were conducted during the dry and wet seasons in four river reaches (upper, middle-upper, middle-lower and lower), each one covering an extension of c.a. 1,000 m. Fish assemblage differed among the four river reaches because of different habitat structure. Three physico-chemical variables (turbidity, depth and transparency), four habitat descriptors (type of substrate, riparian cover, shelters and type of mesohabitat), erosion and anthropogenic influences were the most significant factors that determined fish assemblage structure. Seasonal changes in assemblage were less conspicuous irrespective of changes in physico-chemical variables. The hypothesis that habitat structure determines local fish assemblage structure was accepted, but only slight increase in fish abundance was found in the wet season. Overall, physico-chemical variables played a lesser role in structuring fish assemblages compared to habitat structure at the local scale.