Variações temporais cíclicas nas assembléias de peixes de uma praia continental e uma insular na baía de Sepetiba

Abstract

Bays are semi-closed coastal systems that have beaches whose environmental characteristics, location and degree of anthropogenic influences determine fish assemblages that recruit in these systems and are modeled by seasonal e short term cyclical variation. Two sandy beaches in the outer zone of Sepetiba bay were sampled throughout seasonal, daily and tide cycles: one, Itacuruçá beach, located in the continental margin near to intense anthropogenic activities, and the other, CRCEN beach, located in the Marambaia Island with restricts access. The aim of this study was to test the hypothesis that habitat characteristics are determinant of the composition, richness and structure of assemblages and that temporal cyclical variation results in species replacement to optimize the resources use. Twenty four hauls were carried out during the 24 hour cycle in each season at each site. Beach seine were used in the sampling at 1.5 m maximum depth. At each sampling occasion, the environmental variables of water temperature, salinity, condutivity, dissolved oxygen, transparency and depth were taken. Samplings on seasonal basis using beach seines at the maximum depth of 1.5 m were carried out and environmental variables of temperature, salinity, conductivity, dissolved oxygen, transparency and depth were recorded at each sample occasion. The CRCEN beach showed the higher number of species (60), individuals (35.793), weight (18.867,14 grams), Margalef richness (D), Shannon-Wiener diversity (H ) and Evenness (J) when compared with Itacuruçá beach (37 species, 8,985 individuals e 15,785.74 grams). The beaches showed high dissimilarity in species composition and relative abundance (78,05%). Dominant species at Itacuruçá beach (> 1% total number of individuals), in decreasing order were Eucinostomus argenteus, Atherinella brasiliensis, Diapterus rhombeus, Eucinostomus gula, Anchoa januaria and Anchoa tricolor and at CRCEN beach, Harengula clupeola, A. januaria, Sardinella brasiliensis, Brevoortia aurea, Mugil liza, A. tricolor, Micropogonias furnieri, Larimus breviceps, D. rhombeus, Cetengraulis edentulus and A. brasiliensis. Seasonality was the main factor to influence richness and fish abundance in both beaches, with higher number of species, individuals and weight during Summer and Autumn at Itacuruçá (warm season), and higher number of species in Autumn and Winter (warmer and colder seasons, respectivally) at CRCEN. Seasonal variation was also detected in composition of the ichthyofauna of the two beaches, with CRCEN beach showing a more conspícua separation, with species typical for each season been replaced throughout the year, while in Itacuruçá beach been dominated by some abundant species all year round. Short time variation showed lesser influence on fish assemblages, with the majority of species no showing a defined patter consistent with photoperiod, time of the day or tide. Short time differences (daily, tides and period of the day) were detected at CRCEN beach only, with the number of species being higher during the day, at 15 hours (3 p.m) and the number of individuals being higher during the day, when large schools of H. clupeola, M. liza and C. edentulous were caught. Tide variation showed minor influence in ichthyofauna of the two beaches. Depth and temperature were the environmental variable to show higher correlation with fish species, considering the two pooled beaches. Preference by shallower waters was explained, mainly by habitat selection for the main abundant species, while preference for warmer waters was explained, by seasonal peaks of abundance during the highest temperarture period. The higher richness and abundance of juveniles fishes at CRCEN beach may be associated to its geographic position near to sea connection (reproduction grounds), habitat characteristics with shallow and less transparent waters, higher structural complexity and better conservation status due to restrict access and distance from urban and industrial centers.