Variações nas assembléias de peixes de um reservatório tropical ao longo do ciclo nictimeral em ambientes providos ou não com abrigos artificiais

Abstract

Lajes reservoir was built a hundred years ago, and is a "type-lake" reservoir with an area of approximately 30 km2, on the slopes of the Sea Mountains, with inflow contribution of small streams, lacking major routes for fish migration and a defluence of 20 m3/sec through the turbines of a hydroelectric plant. In this relatively closed system, we evaluated changes in fish assemblages along the daily cycle (sunset, night, sunrise and day) in two areas with different levels of physical structure (presence or absence of artificial shelters). We hypothesized that the fish community has greater richness and abundance in areas with the presence of artificial shelters, and that diel changes determine structure of the fish assemblage in this “type-lake” reservoir. Fish sampling were carried out monthly, between September 2005 and August 2006, using gill nets (30 m long × 2.5 high, with mesh size ranging from 1.5 to 7 cm between adjacent knots). A total of 192 samples (450m2 of nets / sample) was performed, collecting 1934 individuals (1230 in the sheltered area and 704 in the not sheltered area). Metynnis maculatus, a non-native characiforme, was the most abundant species, accounting for approximately 25% of the total number of fishes. The community structure did not differ between the two areas, although a higher number of fishes and a higher number of M. maculatus (P < 0.05) were found for the area with artificial shelters. Other species (e.g. Astyanax parahybae, Astyanax bimaculatus and Loricariichthys castaneus) had higher abundance in the area with shelters but no significant differences were detected between the two areas. Loricariichtys castaneus was the only species with higher numerical and biomass abundance in the area lacking shelters. Changes in structure of the fish assemblages were observed along the diel cycle in both, sheltered and not sheltered areas, with the period of the day differing from the other periods. The number of fishes, number of species, Margalef richness and Shannon Diversity were significantly higher during the night e lower during the day for both areas. Astyanax parahybae had the most contribution to the average similarity for the night period, whereas Geophagus brasiliensis was the characteristic species for the period of the day in both areas. Cichla kelberi showed preference for the sunrise and sunset periods. Seasonal changes in environmental condition, although not particularly relevant for structuring fish community, had two more defined periods; the Spring-Summer, coinciding with the highest rainfall, and Autumn-Winter, coinciding with the dry season. Metynnis maculatus and G. brasiliensis were typical species of the Spring-Summer. Astyanax parahybae (in the area provided with shelters) and C. kelberi (in the area lacking shelters) were typical species of the Autumn-Winter, according to SIMPER. Besides rainfall, water level were more important factors structuring fish community than the environmental variables of temperature, dissolved oxygen, pH and conductivity, although responses were species-specific to different environmental variables. The hypothesis of higher abundance in the area provided with shelters was accepted, although richness and structure community did not change between the two areas. Moreover, the hypothesis that diel changes are important for structuring fish assemblages was accepted, being good predictors of fish abundance and richness in this “type-lake” reservoir.