Small scale acoustical, biological and environmental integrated study

Two acoustic assessment cruises were carried out (February, 2010 and January,2011) in three, 10 by 10 nautical mile exploratory oil blocks, previously to the start of the drilling operation, within a sardine (Sardinella brasiliensis) fishing area. Biological data were collected with zooplankton and mid water trawl hauls. Hydroacoustics with echo-integration were conducted day and night along the whole block areas to detect zooplankton and fish spatial and vertical distribution. Hydroacoustic data were obtained with an echo sounder SIMRAD EK 500 connected to two, 38 and 120 kHz, split-beam transducers. The surveys were performed along 11 parallel transects in each block, where 9 CTD (Conductivity, Temperature, Depth profiler) stations were done. The echo-integration method estimates the acoustic density of the organisms along the water column and within a given area, such density is proportional to the biological density. The method is based on the amount of backscattered energy, integrated within the acoustic beam sampling volume and for a given sampling distance. These values are converted to the parameter NASC (Nautical Area Scattering Coefficient). Average NASC values were similar and relatively low for the two cruises, with no fish schools being detected. However, we observed differences in the NASC values, with the highest densities of organisms associated with specific sectors. The CTD data revealed the presence of the shallower and warmer TW (Tropical Water) and the deeper and colder SACW (South Atlantic Central Water). Surprisingly we detected internal and significant differences in the spatial temperature distribution within each block, in the 2010 cruise. Satellite images showed that the north block (in Campos Basin) was under the effect of a front and the south blocks (in Santos Basin) under the effect of a cyclonic vortex. Local topography and the abrupt change in the coast line orientation favor the generation of meanders and vortices by the Brazil Cur- ent (BC) in the area. These structures induce upwellings along the shelf break, pushing SACW to shallower depths, approaching the lower temperature isotherms to surface. Also, cold water is pushed towards the coast and shelf water, offshore. In 2010 the highest NASC values were found in the west part of Campos Basin block, occupied by the relatively cooler waters. Two other cruises were carried on (May and September, 2012) and preliminary analyses indicate temperature gradient of up to 4°C in the Campos Basin block, in the 40 m stratum, in May cruise. Considering block dimensions and the features detected in at least two out of four cruises we can speculate that the oceanographic fronts we detected within the blocks are part of vortexes which operate in larger scales and that those features must be more common in the region than previously assumed. Also, as higher acoustic densities showed association with cold water we suggest that these vortices and meanders may affect sardine eggs and larvae survivorship in the region if shelf water is advected offshore.