1. Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138 and Department of Biometrics, Cornell University, Ithaca, New York 14853-7801
2. Center for Coastal Studies, Provincetown, Massachusetts 02567
3. Northeast Fisheries Science Center, Woods Hole, Massachusetts 02543
4. School of Biological Sciences, University of Wales, Gwynedd LL57 2UW, United Kingdom