Comparison of the lysine utilization efficiency in different plant protein sources supplemented with L-lysine·HCl in rainbow trout (Oncorhynchus mykiss) fry Tran Thi Nang Thu a,c , Charles Parkouda a , Sarah de Saeger b , Yvan Larondelle a , Xavier Rollin a, ⁎ a Laboratoire de Pisciculture M. Huet, Unité de Biochimie de la Nutrition, Université catholique de Louvain, Route de Blocry, 2, B-1348 Louvain-la-Neuve, Belgium b Laboratorium voor Bromatologie, Universiteit Gent, Harelbekestraat 72, 9000 Gent, Belgium c Faculty of Food Science and Technology, Hanoi Agricultural University, Trau Quy, Gia Lam, Hanoi, Vietnam Received 17 March 2007; received in revised form 21 June 2007; accepted 25 June 2007 Abstract The efficiency of lysine (Lys) utilization has recently received much attention in dietary formulation for fish especially for plant protein based diets. However, it remains unclear whether supplemental Lys utilization is similar in fish fed diets containing different plant protein sources equally limiting in Lys. In the present study, we compared the Lys retention efficiency in rainbow trout fry fed each of three plant protein based diets containing six graded levels of Lys (10, 12, 14, 16, 18, 20g/kg dry matter). Three groups of isonitrogenous (74.1 ± 0.9g N/kg dry matter) and isoenergetic (24.7 ± 0.2MJ gross energy) diets were formulated using L-Lys·HCL, crystalline amino acids and one of three plant protein sources: wheat gluten (WG), maize gluten (MG) or a mixture of sesame oil cake (39%) and wheat gluten (61%) (SW). Each diet was offered to one group of 70 rainbow trout fry with an initial mean body weight of 0.92 ± 0.01g over a period of 30days at 15 ± 0.6°C. Lys utilization efficiency in fish was assessed by comparative regression analysis. The results showed that Lys gain, protein deposition, liveweight gain and voluntary feed intake increased linearly with increasing dietary Lys content or Lys intake (P b 0.01). Voluntary feed intake was lower for fry fed the MG diets compared to fry fed the SW and WG diets at all levels of supplemental L-Lys·HCl. Lys retention efficiency above maintenance was higher for fish fed the WG diets with respect to trout fed the MG and SW based diets (0.75 vs. 0.63, P = 0.058). In the present study, we demonstrated for the first time that the utilization efficiency of supplemented Lys varies with the plant protein source that is supplemented. This variation should be taken into account in the formulation of plant protein based diets. © 2007 Published by Elsevier B.V. Keywords: Lysine; Utilization; Voluntary feed intake; Plant proteins; Rainbow trout fry 1. Introduction The sustainability of the growing aquaculture indus- try nowadays depends on the progressive reduction of wild fish inputs into aquafeeds (Naylor et al., 2000). Partial or even total replacement of dietary fish meal with plant protein sources has been the subject of numerous studies (Kaushik et al., 1995, 2004; Regost et al., 1999; Carter and Hauler, 2000; Espe et al., 2006). The use of plant protein often necessitates the addition of one or several crystalline amino acids in order to meet the Available online at www.sciencedirect.com Aquaculture 272 (2007) 477 – 488 www.elsevier.com/locate/aqua-online ⁎ Corresponding author. Tel.: +32 81 411570; fax: +32 81 411590. E-mail address: rollin@bnut.ucl.ac.be (X. Rollin). 0044-8486/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.aquaculture.2007.06.032