1. Netherlands Institute of Ecology (NIOO-KNAW), Droevendaalsesteeg 10, 6708 PB Wageningen, The Netherlands
2. Amhara Region Agricultural Research Institute, P.O. Box 527, Bahir Dar, Ethiopia. Present address: FAO, Sub Regional Office for Eastern Africa, P.O. Box 5536, Addis Ababa, Ethiopia
3. Zoological Sciences Program Unit, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia
4. Aquaculture and Fisheries Group, Wageningen Institute of Animal Sciences (WIAS), Wageningen University, Marijkeweg 40, 6709 PG Wageningen, The Netherlands