Availability and utilisation of free lysine in rainbow trout (Oncorhynchusmykiss) - 1. Effect of dietary crude protein level

It was investigated whether the availability and utilisation of free lysine are affected by crude protein concentration in rainbow trout diets. Wheat gluten and free amino acids were used as sole sources of amino acids to for mulate two basal diets differing in crude protein (350 and 550 g/kg dry mat ter) but equal in lysine concentration (9 g/kg dry matter). Lysine was supp lemented at the expense of L-alanine to achieve further lysine levels of 13 and 17 g/kg dry matter. L-lysine . HCl was used as the supplemental lysine source at both crude protein levels. Diets were offered to three groups of 20 trout each over a period of 56 days. Amount of feed was restricted to a bout 16 g dry matter/kg BW daily in order to allow for an equal and complet e intake in all groups. Trout had an initial body weight of 50 g/fish, Dige stibility was determined using acid-insoluble ash as a marker. Faecal sampl es were collected via a sedimentation system attached to each tank. Retenti on was determined by comparative whole body analysis. Faecal lysine excretion was unaffected by lysine supplementation irrespecti ve of dietary crude protein level. Hence, L-lysine . HCl can be regarded 10 0% available to trout, In spite of comparable levels of DE intake, growth a nd gain/feed ratio increased with lysine supplementation. In gained body ma ss, protein concentration was increased and lipid concentration was decreas ed with increasing lysine supplementation. Amino acid pattern of gained bod y protein was changed by lysine supplementation as well. Lysine utilisation , determined as Delta retained/Delta digested X 100, was 55% on the low and 71% on the high dietary crude protein levels, respectively. This differenc e was close to significance, as judged by comparative regression analysis ( P = 0.11). Dose-response studies with a wider range in lysine concentration at different crude protein levels are necessary to identify the consequenc es regarding lysine allowances for practical type trout diets. (C) 2000 Els evier Science B.V. All rights reserved.