| Abstract: | The Tietê River crosses the metropolitan region of São Paulo (Brazil) and receives a large amount of pollution, in contrast to Paranapanema River which runs in a less populated area. In the inferior portions of the basins of these rivers, a series of reservoirs has been built for hydroelectric energy generation which improve the water quality due to the decantation of suspended soils and aeration in the water spills. Twenty‐nine sampling sites were established along the two rivers and in each sampling site, two groups of eight gillnets were used with standardized sizes in two seasons: winter (dry season: June–September 1994) and summer (rainy season: December–March 1995). The fishes were measured and weighed. Twenty‐five parameters related to the water were recorded. PCA and cluster analysis helped to identify two groups of sampling sites. The first one corresponds to the rainy season samples, while the second corresponds to the dry season samples, so evidencing strong seasonal environmental variation. However, the dry season group is still divided into polluted and non‐polluted sites, showing that during drought there are larger effects of pollutant concentration due to the decreased flow. A total of 415 kg of fish were caught, corresponding to 8324 individuals belonging to 64 different species, in 22 families and four orders of Teleostei. The largest catches occurred in the middle Tietê River. The catches in the reservoirs of the Tietê River during the rainy season are superior to those of the dry season. On the other hand, sampling sites located in running water presented an inverse pattern with the largest catches in the dry season. In river Paranapanema, catches are lower than in the Tietê River, except in Rosana Reservoir during the rainy season. Fish diversity was shown to be a good indicator of environmental stress. In Tietê River an expressive fall in species richness was detected in the polluted stretches, with dominance of one or two species. In the intermediary stretches, the presence of rapids increases the amount of dissolved oxygen, allowing the development of the most abundant and diverse fish community of the river. Fish communities in the reservoirs of the inferior stretch of Tietê River are more diverse. On the other hand, the damming of the superior reaches of Paranapanema River provoked a decrease in diversity. O the whole, types of local fish communities detected, showing different compositions and specific abundances, revealed different ecological situations. A discriminant analysis revealed that the parameters of the water influencing the fish community were: dissolved oxygen, conductivity, nitrogen (nitrate, ammoniacal and albuminoid), phosphate, alkalinity, hardness and iron. Copyright © 2003 John Wiley & Sons, Ltd. |
