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A B S T R A C TThe present study sought to develop a seabed map of the region of the Pelotas Basin using acoustic methods. A total number of 1,507,823 seabed reflectivity data, collected during six oceanographic surveys, were processed to generate a seabed map. Data processing consisted of the classification of the acoustic parameter BSBS (Bottom Surface Backscattering Strength) obtained with the Scientific Echosounder EK 500 operating at a frequency of 38 kHz. BSBS is expressed in decibels (dB), and corresponds to a logarithm of the ratio between incident acoustic energy and the energy reflected by the seabed. Four BSBS value classes, associated with different sediment types, were established. High BSBS values are associated with coarse sediments, whereas low values indicate fine sediments. A seabed reflectivity map was generated using the Natural Neighbor method to interpolate the BSBS values organized according to the pre-established classes. Four features with high BSBS values were identified at 100-200 m depth. The largest one was found in the region of Santa Marta Cape and attributed mainly to consolidated seabed and/or the presence of biodetritic material, according to comparison with maps available in the literature. Above 500m depth, there was a predominance of acoustically low reflectivity sea floor, which was attributed to the presence of muddy sediment. Considering the lack of information on the seabed at great depths, the acoustic method was shown to be an alternative tool to obtain data on seabed characteristics in these regions.

As top predators in marine environments, seabirds frequently respond to the presence of their main prey, and both predators and prey are usually associated with specific environmental features. We investigated the variability in the presence and density of flying seabirds (mostly Procellariiformes and Charadriiformes) and Magellanic penguins Spheniscus magellanicus along the southwestern (SW) Atlantic continental shelf. Five acoustic assessment surveys were conducted to determine the biomass of the Argentine anchovy Engraulis anchoita, and seabird counts and the collection of oceanographic data were conducted simultaneously with the surveys between June and October 2010. Generalized linear mixed models (GLMMs) were fitted to test the effect of anchovy density and environmental variables on seabird density. Sea surface temperature was significant for the presence of flying seabirds. Bottom water temperature and anchovy density were key variables affecting the presence and density of penguins, while bottom water salinity was also important for penguin presence. Based on Akaike’s information criterion, the most important factor explaining the density of flying seabirds was the difference between surface and bottom salinity (ΔS), while for penguin density, the most important factor was anchovy density. These results highlight that the subtropical shelf front in the SW Atlantic Ocean is a key feature influencing the aggregation of flying seabirds and confirm the close association of penguins and anchovies. Bottom water intrusion, originating from the sub-Antarctic shelf water, is an important factor explaining the presence of penguins, which tend to aggregate in areas with high anchovy densities on the SW Atlantic continental shelf.