Dietary crude protein requirement of golden rabbitfish Siganus guttatus fed low fishmeal diets with varying soybean meal: influence on growth, plasma biochemistry, amino acid profiles, and IGF-1 mRNA expression

A 4-month feeding trial was conducted in 1.5 m3 floating sea cages to determine the optimum dietary protein requirement for cultured golden rabbitfish, Siganus guttatus fed low fishmeal-based diets. Four test diets were formulated to contain different levels of crude protein (CP) at 25, 30, 35, and 40% designated as P25, P30, P35, and P40, respectively. Golden rabbitfish (n = 20) with an initial body weight of 41.9 ± 0.7 g were randomly stocked into 1 × 1 × 1.5 m3 triplicate cages per dietary treatment. Fish were hand-fed three times per day to satiation. The optimum level of dietary CP to produce maximum growth of golden rabbitfish was 35.2% determined by second-order polynomial regression analysis (r = 0.751) for three parameters (specific growth rate, protein retention, and intestinal IGF-1 mRNA expression). Protein retention significantly increased as the protein increased up to 30% and then further decreased as dietary protein increased from 35 to 40% with the maximum value obtained at CP of 33.2% (r = 0.896). Fish fed the P35 diet had the highest IGF-1 mRNA expression level in the intestine, 3.6-fold from the P40 diet and 5.1-fold from the P25 diet with optimal CP was 35.1% to obtain the maximum value (r = 0.776). Based on techniques implemented for the three variables as mentioned above, of SGR (35.2%), PR (33.2%) and IGF-1 (35.1%), the golden rabbitfish required an optimum level of dietary CP at 34.5% when fed SBM based diets and low fishmeal.