1. The Fish Physiology Research Group, Department of Biological Sciences, Brunel University, Uxbridge, Middlesex, UB8 3PH, U.K., and CEFAS Fisheries Laboratory, Remembrance Avenue, Burnham-on-Crouch, Essex, CM0 8HA, U.K., and Essex & Suffolk Water, Hanningfield Treatment Works, South Hanningfield, Essex, CM3 5HS, U.K.