TECHNICAL REPORT OF THE SCALLOP PROJECT CRUISES. III. CRUISES IV (MAY, 1996) TO XIV (MAY, 1997)

Abstract

Main technical results and original data of the Scallop Project Cruises IV (May, 1996) to XIV (May, 1997) are presented in this Report. The aims of the cruises were: a) to sample scallops for growth, mortality, recruitment and stock assessment analysis; b) to study environmental characteristics in the scallop beds, named, temperature, salinity, dissolved inorganic nutrients and chlorophyll-a profiles in the water column, phytoplankton and microphytobenthos composition, and grain size, percent of organic matter and calcium carbonates in the sediment. Contrasting with the homogeneus thermal profiles observed during the autumn and winter, a strong termocline developed between October and April, as resulting from the intrusion of the Southern Atlantic Central Water (SACW) on the inner shelf. Lowest salinities, around 30, were observed in some winter months, whereas values above 36 were found in the other periods. Phosfate, nitrite, nitrate and chlorophylla concentrations were highest in the bottom water, and the highests concentrations of chlorophylla were coincident with the SACW intrusion. Silicate concentrations did not show any seasonal or vertical pattern. Both beds showed little changes in the sediment grain size (mostly fine to very fine sands). On the other hand, percent of organic matter and calcium carbonate varied seasonally but with peaks in opposite periods, with an 8-fold increase in the organic matter content of the sediment from autumn/winter to spring/summer. After some modifications of the beam trawl structure, a good and regular efficience was attained to sample scallops. Pecten ziczac abundances in the São Francisco bed were low but relatively stable between December/95 (first cruise of the project) and May (1997). On the other hand, Bom Abrigo bed was completely collapsed between December/95 and May/96, showing catch rates of 1 ind.hr-1. Considering the extremely fragile condition of the resource, its suggested that emergencial actions be implemented in order to prevent an irreversible colapse of the resource in Southern Brazil