Assessment of lysine requirement for maximal protein accretion in Atlantic salmon using plant protein diets

Two trials (I and II) were conducted to finally determine the Lys requirement in the Atlantic salmon during the fast growing period in seawater. First three test diets with well balanced AA's as well as a IAA:DAA ratio close to 1, but differing in amount and source of an attractant, were developed allowing for both a reduction of dietary Lys and a reasonable performance when supplementing Lys. The diets were based on our previous study in which all fish meal was replaced by plant proteins, of which resulted in both inferior body weight gain and lipid deposition ([Espe, M., Lemme, A., Petri, A., El-Mowafi, A. 2006. Can Atlantic salmon grow on diets devoid of fish meal? Aquaculture 255, 255–262]). Basically, the amino acid profiles of all test diets did not differ (p > 0.05) from the control fish meal based diet although 90% of the fish meal was replaced by plant proteins. The diets were fed to Atlantic salmon of BW 327 g for a period of 85 days. The test diet supporting both growth and deposition of protein and lipid not differing (p > 0.05) from the fish meal control finally consisted of plant proteins supplemented with 5% fish meal, 5% fish soluble and 3% squid hydrolysate. This diet then was used in a dose response experiment aiming to determine Lys requirement in the fast growing salmon. Atlantic salmon with BW of 642 g were fed graded amounts of Lys (2.85 to 9.19 g Lys/16 g N) for a period of 85 days. Dietary Lys did not affect growth (p > 0.05), but the protein accretion suggested an optimum dietary Lys supply of 5.04 g /16 g N (corresponding to 0.12 g Lys/fish/day). This response was accompanied with reduced fat accretion. To obtain weight gain to the level present in the fish meal control diet 3.17 g Lys/16 g N was sufficient. Thus lean growth rather than body weight gain should be the response parameter to determine Lys requirement in salmon.