Influência de três tipos de barragens sobre atributos ecológicos da ictiofauna no Rio Paraíba do Sul, RJ, Brasil

Abstract

Influences of three types of dams on fish community structure along a river from Southeastern Brazil were investigated. Comparison of fish assemblages between above- and below-dam habitats was studied in order to detect possible differences in fish community between these habitats in two seasons (wet and dry). The three types of dams and associated fishways used as models were: (i) total blockage, (ii) partial blockage/side channel and (iii) total blockage with fish ladder. The tested hypothesis was that there are differences in fish assemblage composition between above- and below-dams habitats. We expected an impacted fish assemblage with the highest differences in fish fauna between up and downriver habitats in (i), intermediate in (iii) and less evident in (ii). Additionally, secondary investigations were made using models separately. When we investigated (ii) partial blockage, an impoundment with permanent hydrologic connection and environmental homogeneity between patches, we found that these attributes were not able to avoid the negative impact of river fragmentation on fish assemblage. The above- and below-dam had different fish assemblage composition and structure. Migratory fish species such as Pimelodus maculatus, Pimelodus fur and Prochilodus lineatus were the most affected, and probably are prevented to perform upriver migrations. On the other hand, only adapted species such as Hoplias malabaricus and Hoplosternum littorale successfully colonized the reservoir (above-dam). Therefore, for this investigation, we conclude that the presence of a hydrologic connectivity alone certainly does not guarantee an ecological connectivity and, consequently, fish passage facilities should be monitored and managed to evaluate and improve their functionality and efficiency. Another research was made in (i) total blockage to evaluate the possible impacts of this impoundment, considering two contrasting environmental conditions: wet/summer and dry/winter seasons. A major shift in biotic and abiotic parameters occurred due to contrast between above- and below-dams habitats. The comparatively high habitat complexity and water flow regime of the below-dam downriver zone favored a richer and abundant fish assemblage, especially in the wet season. In this period, the waters cover part of the riparian vegetation, increasing habitat availability and nutrient input. Additionally, the obstruction of upriver migrations of reophilics, mainly in wet season, such as Leporinus copelandii, Pimelodus fur, Pimelodus maculatus and Prochilodus lineatus increases shoals below dams. Although the reservoir represents a simplified ecosystem, highly influenced by non-native top predator species (Cichla kelberi and Plagioscion squamosissimus), cyclic processes (e.g. water level fluctuations and flood pulses) seems to be involved in the structuring of fish assemblages. The five selected environmental variables, although highly influenced by the impoundment, explained 55% of spatio-temporal patterns, defined mainly by turbidity (downriver) temperature, and dissolved oxygen (reservoir). Visual-predators fishes, seasonal changes and eutrophication process were linked to these variables, respectively. Finally, our main hypothesis was accepted but our expectations were not fully matched. Differences of fish assemblage between above- and below-dams habitats were highest in (i) total x blockage. However, partial blockage/side channel (ii) showed higher differences in fish fauna than (iii) total blockage with fish ladder, mainly in wet season, when flood triggers biological features (e.g. dispersal movements and reproductive migration) in Neotropical freshwater fish. Moreover, species richness was higher in downriver habitats, as an effect of habitat heterogeneity and fish concentration down the dams because impairment of migration. We provide information that may contribute to management programs and future projects as well as enhance freshwater ecology data base.