1. Department of Fisheries and Marine Biology, High-Technology Cenrre N-5020 Bergen, Norway;
2. Norwegian College of Fisheries, University of Tromsf, P.O. Box 3083 Guleng, N-9001 Troms, Norway
3. SINTEF Automatic Control, N-7034 Trondheim NTH, Norway
4. Institute of Marine Research, P.O. Box 1870 Nordnes, N-5024 Bergen, Norway (revised August 1991).