Response of gibel carp (Carassius auratus gibelio) to increasing levels of dietary lysine in zero fish meal diets

To investigate the response of gibel carp to dietary lysine levels in zero fish meal (FM) diets, 8 experimental diets were designed. Among them, diet 1 and diet 2 with 150 g/kg FM, methionine, lysine and threonine were supplemented in diet 2. Diet 3 was without FM but matched the amino acid profile of diet 2 except for lysine. Diets 4-8 were based on diet 3, with graded levels of lysine. After 8-week feeding trail, final body weight, weight gain (WG), specific growth rate, feed conversion ratio (FCR) and feed intake were significantly improved by diets 1-2 and 6-8 compared with diet 3. Plasma glucose, triglyceride, immunoglobulin G and aspartate aminotransferase levels in diets 4-8 were higher than those in diet 1. Apparent digestibility coefficients (ADCs) of protein, energy, methionine, lysine, valine, histidine and phenylalanine in diet 1 were significantly lower than those in diet 6. The highest ADC of isoleucine was found in diet 3. TOR, IGF-1, JAK3, STAT4, STAT6 and PepT2 mRNA levels were significantly increased with increasing dietary lysine up to a certain level in zero FM diet and lower than diets 1-2. Based on WG and FCR, the optimal lysine requirement of juvenile gibel carp was determined to be 24.4 g/kg and 24.2 g/kg (65.9 g/kg and 65.4 g/kg of dietary protein) in zero FM diet, respectively, using quadratic regression analysis.