Fish assemblage subjected to strong anthropogenic stress: The case of the barra bonita reservoir, tietê river basin, São Paulo, Brazil

Abstract

The Barra Bonita reservoir (S=310 km2; z =10,1 m) is an ecosystem subjected tostrong anthropogenic stress. It is located in the central part of the Tietê river basin - SãoPaulo State, characterized by the widespread urbanization, industrial development andintensive agricultural use of the soil. The Barra Bonita reservoir has high social-economicimportance due to hydropower generation, navigation and fisheries. From a limnologicalpoint of view, it is classified as polymictic and eutrophic. The purpose of this study wasto characterize the actual fish assemblage in the reservoir and, by means of the speciesabundancerelationship, value its organization in relation to the disturbances caused bythe anthropogenic impact. For the fish sampling we chose 24 sites located in threedifferent habitats: reservoir shoreline, mouth of tributary and centre. Samples were takenin two periods of the year: dry season (winter, August-September 2003), and rainy season(summer-February 2004). Fish sampling was standardized by using 10 gillnets with meshsizes ranging from 3 to 12 cm between opposite knots and funnel traps. At each point wealso measured some morphological, physical-chemical and environmental variables.Multivariate analyses (three way-anova and ancova) were employed to, respectively,point out space and temporal variations in the Catch per Unit Effort in weight (CPUEW)and to detect the importance of morphological, physical-chemical and environmentalvariables on Shannon species diversity (H' in number and weight). A total of 35 species,belonging to 14 families and 4 orders, were caught. Fish assemblage is composed ofsmall body-sized species, with wide feed flexibility and high reproductive compensation. The superposition of the biological cycles of the fishes with the hydrologicalmanagement of the reservoir, suggests that only those with multiple spawn and/orparental care have success. The CPUEW distribution shows significant statisticaldifferences between seasons, zones and habitat, indicating the presence of transversalmigration from the centre to the shoreline and mouth of the tributary habitats, especiallyin the rainy season. The variables that influenced the diversity of the fish assemblagewere: depth, transparency and landscape for H'N and H'W; temperature (H'N);conductibility and macrophytes (H'W). The correlation of diversity with the depth wasnegative, showing that the tributary mouths and the shoreline habitats of the reservoir arethe most explored by the ichthyofauna. The environmental variables selected(surrounding landscape and macrophytes beds) act respectively as external feedingsupport and shelter/nursery habitat, revealing the essential importance of the structuralenvironmental complexity for the diversity maintenance in reservoirs. © 2009 Nova Science Publishers, Inc. All rights reserved.