Evaluation of juvenile silver perch Bidyanus bidyanus (Mitchell) nutritional needs using high- and low-protein diets at two feeding levels S Harpaz 1 , D Sklan 2 , I Karplus 1 , A Barki 1 & Y Noy 3 1 Department of Aquaculture, Agricultural Research Organization, The Volcani Center, PO Box 6, Bet Dagan 50250, Israel 2 Department of Animal Sciences, Faculty of Agricultural, Food & Environmental Quality Sciences, The Hebrew University of Jerusalem, PO Box 12, Rehovot 71200, Israel 3 Milobar Central Feed Mill, M.P. Ashrat 25235, Israel Correspondence: S Harpaz, Department of Aquaculture, Agricultural Research Organization, The Volcani Center, PO Box 6, Bet Dagan 50250, Israel Abstract Juvenile silver perch Bidyanus bidyanus (Mitchell) of 3.2 g average weight were reared under controlled conditions on diets containing 23.4% and 40.6% protein. Diets were administered at 3% or 6% of the ®sh biomass per day. Both the protein concentra- tions and the feeding level of the diet in¯uenced growth and proximate composition of the ®sh at the end of the 100-day growth trial. The energy requirement for these ®sh for maintenance was found to be 81.9 kcal 3 BW ±0.8 and, for each unit of energy retained per metabolic body weight, 7.05 needed to be supplied, whereas for each gram of protein retained per metabolic body weight, 5.79 g were required. The results indicate that it is possible to obtain the same weight increment when feeding a 23.4% protein diet at a level of 6% body mass day ±1 compared with only 3% body mass day ±1 of a 40.6% protein diet. This study provides the ®rst estimation of nutrient requirements for juvenile silver perch, and these estimations require further re®nement. Introduction The omnivorous freshwater silver perch Bidyanus bidyanus (Mitchell), which is endemic to Australia, is considered to have high potential for aquaculture because of its rapid growth under a variety of conditions (Rowland, Allan, Clark, Hollis & Pontifex 1994; Rowland, Allan, Hollis & Pontifex 1995; Kibria, Nugegoda, Lam & Fairclough 1997) and its ready acceptance of arti®cial feeds (Rowland & Barlow 1991; Kibria, Nugegoda, Fairclough & Lam 1998a,b). Initial evaluation of the nutritional needs of this ®sh has been carried out by a number of researchers (Anderson & Arthington 1989). Allan & Rowland (1991) studied the growth of juvenile silver perch on diets with different levels and sources of protein. Their results show that, of the three protein levels tested, 20.7%, 35.7% and 49.0%, the best growth coupled with the lowest feed conversion ratio was obtained with feed containing 35.7% protein. With the increase in intensive aquaculture of this species (Gooley & Rowland 1993; Rowland 1995; Kibria et al. 1997), more ef®cient dry diets will be required as food constitutes the principal operating cost in the production of ®sh. Initial tests carried out in order to replace the ®sh meal component have shown that replacement of all the ®sh meal and oil in the silver perch diets with the same protein levels resulted in slower growth (Allan, Rowland & Parkinson 1993). Some guidelines for the development of arti®cial diets for silver perch have been summarized by Allan (1995), and these indicated that the diet for this ®sh can be based on the same criteria used for other omnivorous ®sh, e.g. channel cat®sh. In order R # 1999 Blackwell Science Ltd 603 Aquaculture Research, 1999, 30, 603±610