Effects of DL-methionine supplementation on the success of fish meal replacement by plant proteins in practical diets for juvenile gibel carp (Carassius auratus gibelio)

A ten-week feeding trail was conducted to investigate the effects of increasing DL-methionine (Met) supplementation on the success of fish meal (FM) replacement with plant proteins in practical diets for juvenile gibel carp, Carassius auratus gibelio. Twelve isoenergetic diets were formulated including two 150gkg(-1) FM diets (Diet 1positive control 1 reflecting a commercial diet and Diet 2positive control 2 reflecting a commercial diet but with balanced essential amino acid (EAA) profile) and ten 50gkg(-1) FM diets (negative controls) supplemented with graded levels (0-3.0gkg(-1)) of DL-Met (Diets 3-12). Each diet was fed to triplicate groups of gibel carp, near satiation four times daily for 10weeks. Diet 2 with balanced EAA profile produced better final weight, specific growth rate (SGR) and feed conversion ratio (FCR) than the negative control diet containing no supplemental Met (Diet 3), but did not significantly differ from Diet 1. However, DL-Met supplementation (0.5-3.0gkg(-1)) in the negative control diets (Diets 4-12) produced growth performances similar to those fed the positive control diets (Diets 1 and 2). Based on quadratic regression analysis, the optimal dietary Met level with 5.2gkg(-1) of dietary cysteine (Cys) was found to be 7.1gkg(-1) dry diet for SGR and FCR. The corresponding total sulphur amino acid requirements (Met+Cys) of this species were calculated to be 12.3gkg(-1) dry diet for SGR and FCR. DL-Met supplementation in 50gkg(-1) FM diets showed a decreasing trend in plasma cholesterol contents (p<.05). No significant differences were observed in whole-body composition, plasma protein, triglyceride and free EAA contents among dietary treatments, while plasma aspartate transaminase, albumin and ammonia contents were significantly influenced by dietary Met levels. Juvenile gibel carp grew equally well on 150gkg(-1) FM diet or 50gkg(-1) FM diets balanced for EAA profile with supplemental amino acids. The results of this study overall indicate that balancing dietary amino acid levels with DL-Met supplementation is a key strategy in successfully reducing FM levels in the diets of gibel carp.