ORIGINAL ARTICLE Utilization of dipeptide/protein based diets in larval and juvenile Koi carp – post-prandial free amino acid levels K. Kwasek 1,2 , Y. Zhang 2 and K. Dabrowski 1,2 1 School of Environment and Natural Resources, The Ohio State University, Columbus, OH, USA, and 2 Faculty of Environmental Protection and Fisheries, University of Warmia and Mazury, Olsztyn-Kortowo, Poland Introduction The substitution of live food by formulated diets during the early stages of fish development has been a major objective in nutrition research for several years. Formulated feeds are expected to reduce costs and increase success of juvenile fish production. The larval digestive tract is not Keywords carp, dietary peptides, amino acids Correspondence Konrad Dabrowski, School of Environment and Natural Resources, The Ohio State University, 2021 Coffey Road, Columbus, OH 43210, USA. Tel: (614) 292-4555; Fax: (614) 292-7432; E-mail: dabrowski.1@osu.edu Received: 29 July 2008; accepted: 1 September 2008 First published online: 11 November 2009 Summary We have shown previously that diets in which the nitrogen portion is based on synthetic dipeptides (PP) resulted in weight gain by rainbow trout alevins when free amino acid (FAA) based diets did not. However, the protein-based diet used as a control in the previous study resulted in a significantly better performance of fish than peptide- or FAA based diet fed fish. Therefore, the objectives of our study were (i) to test how stomachless fish respond to peptide-based diets, (ii) to evaluate PP and protein-PP mixture diets and (iii) to examine if post-prandial response to FAA concentrations in the fish body can be used as an indicator of the availability of dietary amino acid sources. The first experiment was conducted with a 4-day old Koi carp (Cyprinus carpio) larvae and included groups fed a casein–gelatin (CG) based diet, a commercial diet and live Artemia nauplii. In the second experiment, fish fed live Artemia for 3 weeks (69 ± 12 mg) were placed in glass aquaria in triplicate per dietary treatment. Four diets provided equivalent amounts of nitrogen in the form of a CG, a PP, a 50% PP and 50% CG (PP50) mix and as FAA. Fish were fed at 1 and 3 h intervals, and the survival and growth were monitored during the second and third weeks of experiments 1 and 2 respectively. Following the completion of feeding, juvenile carp were sampled prior to feeding and 3 h after a meal. Whole body FAA analyses were carried out. We observed marginal suitability of the CG diet for larval Koi carp, as earlier indicated in common carp. Juvenile Koi carp fed the CG diet achieved 236 ± 19 mg, whereas PP50, PP and FAA diet fed fish grew to 140 ± 37, 70 ± 8 and 73 ± 5 mg respectively. Free amino acids in the fish body, and in particular indispensable amino acids (IDAA), were excellent indicators of dietary availability. The pres- ent experiment shows that a dietary 1:1 ratio of protein to synthetic PPs results in better growth, survival and whole body IDAA concentrations in relation to FAA- and PP-based diets confirming earlier reports on the use of synthetic PP-based diets, although this diet does not match the performance of complete protein, CG-based diet. DOI: 10.1111/j.1439-0396.2008.00877.x Journal of Animal Physiology and Animal Nutrition 94 (2010) 35–43 ª 2009 The Authors. Journal compilation ª 2009 Blackwell Verlag GmbH 35