1. INRA, UR 546 Biostatistics and Spatial Processes (BioSP); F-84914 Avignon France
2. Institute of Marine Research; Postboks 6494 9294 Tromsø Norway
3. Statistics, ur 1052; Domaine Saint Maurice 67, Allée des chênes CS 60094; F-84143 Montfavet cedex France
4. Centre d'Etudes Biologiques de Chizé, Centre National de la Recherche Scientifique; F-79360 Villiers en Bois France