1. Molecular Ecology and Fisheries Genetics Laboratory, School of Biological Sciences; Bangor University; Deiniol Road Bangor LL57 2UW U.K.
2. Institute of Marine Research; P. O. Box 1870 Nordnes NO-5817 Bergen Norway
3. Department of Biology, Sea Lice Research Centre; University of Bergen; P. O. Box 7803 N-5020 Bergen Norway
4. School of Biological Sciences; University of East Anglia; NR4 7TJ Norwich U.K.