1. Centre for Environment, Fisheries and Aquaculture Science, Burnham Laboratory, Remembrance Avenue, Burnham-on-Crouch, Essex, CMO 8HA, U.K., Department of Biology and Biochemistry, Brunel University, Uxbridge, Middlesex, UB8 3PH, U.K., and Environment Agency, National Centre for Ecotoxicology and Hazardous Substances, Hawberry Park, Wallingford, Oxon, OX10 8BD, U.K.