We thank the fishermen Gilberto Lea˜o (Bosque), Zaidir (Torotama), Heli (Passinho), Adilan (Capivaras), Rui (Z3), Jorge (Mangueira) e Anderson (Va ´ rzea), without whom this work would not have been carried out. We are grateful to Prof. P.C.Vieira for his contribution in several stages of the work and for the assistance of Mr. G.F. Barenho, Department of Oceanography (FURG). Dr. G. Velasco provided valued comments on the manuscript.