O uso de praias arenosas e áreas de mangue por peixes jovens em duas baías do Sudeste do Rio de Janeiro

Abstract

Mangrove areas and bays are ecosystems that play an important ecological role by suppling vias to energy importation and exportation (migrations, predation, fisheries, etc.). The present work aims to identify variations in juveniles fish assemblages in beaches and mangrove areas to test the hypothesis that structural differences which characterize these two systems results in differences in fish assemblages. Fish samples in sandy beaches of two bays and a mangrove area in southwestern Rio de Janeiro State were taken by bi-montlhy sampling performed during the day in the bays and by monthly sampling during the day and the night in the mangrove, from August 2002 to July 2003. Standartized trawls were carried out perpendicular to the coastline at depth lower than 1.5 m, in an extension of approximately 30 meters. Sampling were performed by using a beach seine (10 m x 2,5 m, mesh 7 mm). Environmental variables of depth, transparency (except during the night), temperature, salinity, dissolved oxygen and conductivity, besides composed sediment samples were taken at each site. Sample unit were the total number of fishes caught by a trawl, with three replicates in each site. Most of fishes were young-of-the-year and immature juveniles. Ninety eight species were recorded by the pooled sampling programme, including 5 sites in the mangrove and 14 beaches in the bays, being 7 in the Sepetiba bay and 7 in Ilha Grande bay. A total of 46,444 individuals were caught, amounting to 66,206.76 grams, and comprising 38 families and 13 orders. The families Carangidae, Engraulidae, Gobiidae, Gerreidae, Sciaenidae, Mugilidae, Paralichthyidae and Tetraodontidae showed, in decreasing order, the most richeness in the Sepetiba bay, while the most diverse families in Ilha Grande Bay were Carangidae, Engraulidae, Gerreidae, Haemulidae and Sciaenidae. In the Guaratiba mangrove the most diverse families were Carangidade, Serranidae, Gobiidae, Sciaenidae, Tetraodontidae, Engraulidae, Gerreidae, Mugilidae and Paralichthyidae. Eleven species showed the frequency of occurrence higher than 10 % in the both bays: Atherinella brasiliensis, Oligoplites saurus, Eucinostomus argenteus, Trachinotus carolinus, Strongylura timucu, Anchoa tricolor, Trachinotus falcatus, Mugil liza, Hyporhamphus unifasciatus, Anchoa januaria and Diapterus rhombeus. In the Guaratiba mangrove 14 species showed frequency of occurrence higher than 10%: Atherinella brasiliensis, Eucinostomus argenteus, Gobionellus boleosoma, Eucinostomus melanopterus, Mugil liza, Sphoeroides testudineus, Diapterus rhombeus, Harengula clupeola, Anchoa januaria, Mugil curema, Synodus foetens, Citharichthys arenaceus and C. spilopterus. From a total of 98 recorded species, 87 (360 samples) were from the mangrove area and 72 (252 samples) from the two bays, being 62 (126 samples) from Sepetiba bay and 42 (126 samples) from Ilha Grande bay. 27 species occurred in both systems (mangrove and bays), and 31 were commom to the two bays. The highest specific richness in the mangrove corroborate the expectation that the most structured system influence in richness compared with the less structured sandy beaches.The higher richness in the Sepetiba bay, compared with Ilha Grande bay, which showed similar top abundant species, coincided with differences in hydrographic variables (higher temperature, condutivity, dissolved oxygen and transparency in Ilha Grande bay) and sediment nutrient (higher % of organic matter, Carbon, Nitrogen and Potassium In the Sepetiba bay) with the sediment texture not differing between the two bays. Overall, assemblages did no differ between sites in the bays, confirming the hypothesis of bay dependence and the refutation of habitat dependence, although it was only partially confirmed for the Sepetiba bay. This same hypothesis of system dependence instead the area dependence was confirmed for the mangrove.