1. Fish Biology Group, University Field Station, University of Glasgow, Rowardennan, Glasgow, G63 0AW
2. Fish Biology Group, Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, G12 8QQ
3. SOAEFD Freshwater Fisheries Laboratory, Faskally, Pitlochry, Perthshire, PH16 5LB, Scotland, U.K.