Diversity of the estuarine fish assemblages on Crabs Island, Maranhão State

Abstract

Patterns in the diversity of estuarine fish on Crabs Island were studied in order to evaluate spatial (three creeks)  and seasonal (rainy and dry seasons) variations. Three-monthly, from November, 2002 to April, 2004, a non-selective fish  sampling was carried out using gill nets as fish weirs to block the creeks’ entrance. The estuary presented an assemblage  of fishes composed by 32 species distributed over 8 orders, out of which 87.5% were classified as “constant” and 12,5%  as “accessory”. Spatially,  Anableps anableps, Sciades herzbergii, Bagre bagre, Cathrorops spixii e  Genyatremus  luteus predominated in the three creeks surveyed. The number of species, Margalef’s richness and Sanders’ rarefaction  curves showed higher values in the upper side of the Island (farther from Mearim River’s mouth), mainly during the rainy  season. The composition of Crabs Island’s ichthyofauna was characterized by a gradual changing in the predominance of  the main species in the creeks. The large proportion in the samples of juveniles of all species suggests that this estuarine  zone is mainly used for feeding and growth activities. Differences of physiography throughout the length-wise extent of  the creeks do not coincide with shifts in the fish assemblages, implying that environmental factors associated with water  salinity should account for changes in their structure.