Variação da ictiofauna ao longo de um trecho longitudinal do sistema rio Paraíba do Sul - reservatório da UHE do Funil e adaptação do Índice de Integridade Biótica

Abstract

The Hydroelectric Power Plant of the Funil reservoir is an important impoundment in the Rio de Janeiro State, being the major antropic interference in the Paraíba do Sul River, for hydroelectric power generation purposes. This study aimed to assess changes in fish assemblages structure and their spatial and temporal variation, and their relation with their physico-chemical environmental variables along the longitudinal gradient in the stretches Paraíba do Sul river Funil reservoir. Furthermore, the Index of Biotic Integrity (IBI) was adapted for this system. Standardized monthly samples were carried out between October 2006 and September 2007 in four zones: 1 river upstream the reservoir; 2 upper reaches of the reservoir; 3 lower reaches of the reservoir, and 4 river downstream the reservoir. Four sites were sampled within each zone. At each site, three gill nets (30 m per 2,5 m) with different mesh size (2.5 cm, 4.5 cm and 6.5cm between opposite knots) were used, amounting an average area of 900m2 of net per zone. Physico-chemical variables (temperature, percentage of oxygen saturation, pH, conductivity and redox potential) were measured in each fish sampling occasion at the four sites within the four zones. A total of 4,548 specimens and 35 species were collected. The introduced species (Metynnis maculatus, Piaractus mesopotamicus, Plagioscion squamosissimum, Cichla monoculus e Oreochromis niloticus) were more abundant in the reservoir zones, corresponding to 22% (zone 2) and 23% (zone 3) of the total number of fishes. The wet season showed the higher values of Capture per Unite Effort in both number (CPUEn) and biomass (CPUEb) for all zone, although significant differences (p<0.05) have been shown for the river zones (1 and 4). A trend for increased number of carnivores individuals and species along the longitudinal gradient from zone 1 to zone 3 was observed, accompanied by a decreasing in the number of herbivores. Higher percentage of oxygen and pH were recorded in the reservoir, compared with the river during both season. The zone 4 had lower pH and oxygen saturation and higher redox potential, while the river zone upstream the reservoir showed higher oxygen saturation and conductivity. The IBI was adapted for the system, assuming the denomination of Index of Fish Assemblages for Reservoir (IFAR). A total of candidates 24 metrics of fish assemblages were tested for the river zones (1 e 4) and 23 metric for the reservoir. Throughout all studied months, no zones was classified as having acceptable quality. According to RFAI seasonal averages (wet and dry) it was possible to depict along the longitudinal profile, a decreasing in environmental quality from river upstream the dam to reservoir, with a slight increased trend in the river zone downstream the reservoir, at zone 4; Significant differences in RFAI scores were detected between the lower values at zone 3 compared to the other zones, although all of them were classified as impacted. The results suggests that there is a decreasing trend in environmental quality along the longitudinal profile river-reservoir (zone 1 to zone 3). Water quality downstream the dam change according to hydroeletric plant operations, and increased environmental quality indicate by RFAI was linked to other features such as habitat complexity, determining fish assemblage structure.