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INTRODUCTION
Spatial and temporal changes in physical-chemical environmental characteristics strongly influence the structure of fish assemblages within estuarine ecosystems (Pessanha & Araújo, 2003), and other factors, such as predation and competition relationships, seem to act at a small scale (Kennish, 1990; Jung & Houde, 2003). In these environments, the fauna is highly dynamic due to interactions between species-specific physiological limitations and life strategies. The salinity, temperature, dissolved oxygen, turbidity and nutrient concentrations are the main abiotic factors controlling the abundance, distribution and composition of the fish communities in tropical and subtropical estuaries (Blaber, 2000; Rueda & Defeo, 2003). Thus, an investigation of the environmental affinities of species and of their distribution in space and time is a basic step toward conservation and sustainable use planning. This is especially relevant for Baía da Babitonga because the estuarine area and surrounding mangrove forests in this region have been assigned a high-priority status for conservation, and management measures, such as the establishment of a marine reserve, are currently being developed (MMA, 2007).
The role of environmental variables on the structure of fish assemblages in Brazilian estuaries is still poorly understood (Garcia et al., 2001; Araújo et al., 2002; Bouchereau & Chaves, 2003; Barletta et al., 2005; Chagas et al., 2006; Azevedo et al., 2007). Published studies mainly deal with demersal species that live in deep areas (>3 m), leaving the environmental affinities of species that predominantly inhabit shallow water areas unclear. As elsewhere, logistical and financial constraints have traditionally impeded the determination of the environmental factors and interactions that most influence the distribution and structure patterns of Brazilian intertidal fish communities. Consequently, studies on fish assemblages in shallow water areas have minimized the spatial dimension in their analyses, obscuring patterns, variability scales and the interpretation of causal effects (Jung & Houde, 2003).
Baía da Babitonga is a subtropical estuary located near the southern extreme of the Brazilian zoogeographical province (sensu Briggs, 1995). It offers a particularly good opportunity for analysing the effects of environmental factors on species relative abundance and fish community structure in the intertidal areas of the south-west Atlantic Ocean. Physically, its shore is dominated by low energy shallow water areas (< 1.5...