1. Salmon & Freshwater Team, Cefas, Pakefield Road, Lowestoft, Suffolk, NR33 0HT, U.K., and School of Conservation Sciences, Bournemouth University, Poole, Dorset, U.K.;
2. Comenius University, Faculty of Natural Sciences, Department of Ecology, Mlynská dolina B2, 842 15 Bratislava, Slovakia