1. Wageningen IMARES (Institute for Marine Resources and Ecosystem Studies), PO Box 68, 1970 AB IJmuiden, The Netherlands, and Aquaculture and Fisheries Group, Department of Animal Science, Wageningen University, PO Box 338, 6700 AH, Wageningen, The Netherlands
2. Institute of Hydrobiology and Fisheries Science, University of Hamburg, Olbersweg 24, 22767 Hamburg, Germany
3. Centre for Environment, Fisheries and Aquaculture Science, Pakefield Road, Lowestoft NR33 0HT, England, UK