1. Royal Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands, Centre for Environment, Fisheries and Aquaculture Science (CEFAS), Remembrance Avenue, Burnham-on-Crouch, Essex CMO 8HA, UK, and Netherlands Institute for Fisheries Research (RIVO), P.O. Box 68, IJmuiden, The Netherlands