Demersal fish assemblages and its environmental determinants in tropical Atlantic coastal waters
One of the fundamental aims of ecology is to identify the key mechanisms and processes that control changes in species' richness and abundance over time and space. Environmental drivers of coastal fish assemblages may be qualitatively different to temperate and sub-tropical areas due to coastal characteristics and the local effects of seasonal variation in biophysical conditions, mainly related to large seasonal fluctuations in precipitation. We tested how density, diversity and species composition of tropical coastal demersal fish assemblages in northeast Brazil are affected by seasonality (dry and wet seasons), depth and seabed complexity (high, intermediate, low). Specifically, we evaluated whether demersal fish abundance, assemblage composition and species richness vary seasonally along a depth gradient in coastal areas with contrasting shoreline characteristics and bottom topography. Data was collected over a year by experimental fisheries using gillnets. We used a GLM with Poisson distribution measured the effect of the variables on the Simpson diversity index. A Jaccard coefficient was used to determine the relationship between similarity of community composition and the environmental variables (region, period and depth). These values were then correlated using an ADONIS test. Hierarchical clustering was applied to data from dry and wet sampling periods.
During all samples, 88 species and 38 fish families were recorded. The most frequent families were the Sciaenidae (15,9%), Carangidae (11,4%) and Haemulidae (8%). The largest catches were in the intermediate region in the 20 m depth zone (Table I). The wet season produced a greater number of individuals (58%) and more species that are exclusive. The GLM with Poisson distribution showed a significant relationship between number of species to depth (P=0.032) and especially driven by the high complexity region (P=0.002), that had significantly more exclusive species. The high complexity region also indicates relations with depth (P=0.002) and dry period (P=0.017) (Table II). The Simpson index does not indicate significant relations with the environment variables (P>0.05). No significant latitudinal or depth gradients were observed in species' diversity or abundance. However, in shallow waters (up to 10m depth) there was a strong influence on fish assemblages of the proximity to reefs and to a major river estuary (the São Francisco river mouth). At deeper depths, fish composition was more sensitive to seasonal and spatial changes. We concluded that pluviometry or salinity variability must be priority variables to be considered in conservation policies and management programs in tropical coastal waters.