1. University of Minho, Centre of Mathematics (CMAT) , Braga 4710-057 , Portugal
2. University of Minho, Centre of Mathematics (CMAT) , Guimarães 4800-058 , Portugal
3. Portuguese Institute for the Sea and Atmosphere (IPMA), Division of Modelling and Management of Fishery Resources , Algés 1495-165 , Portugal