1. Fisheries Ecology Division; Southwest Fisheries Science Center; National Marine Fisheries Service; NOAA; 110 Shaffer Road Santa Cruz CA 95060 USA
2. Department of Applied Math and Statistics (SOE2); University of California; 1156 High Street Santa Cruz CA 95064 USA
3. Department of Mathematics; University of Bergen; P.B. 7800 N-5020 Bergen Norway