Dietary lysine requirement of juvenile Pacific threadfin (Polydactylus sexfilis)

The objectives of this study were to develop an amino acid test diet for Pacific threadfin (Polydactylus sexfilis) and estimate its lysine (Lys) requirement based on growth performance. A basal test diet was formulated using Pacific threadfin muscle and a mixture of precoated crystalline amino acids as the protein sources to achieve the same amino acid profile as in Pacific threadfin muscle with the exception of Lys. An 8-week growth trial was conducted with juvenile fish (initial body weight, 3.0 ± 0.1 g) fed eight isonitrogenous (35% crude protein) and isoenergetic (19.1 MJ/kg diet) semi-purified diets containing 1.26% to 2.91% Lys. The fish were cultured in an indoor flow-through system with 31‰ seawater at 25 °C. There were three replicate groups per dietary treatment. No mortalities occurred during the growth trial. Fish fed diets containing 2.23–2.43% Lys had higher specific growth rate, feed efficiency, protein efficiency ratio and protein retention than those fed diets containing less than 1.69% Lys. Excess dietary Lys also decreased growth of fish significantly. The basal diet (1.26% of Lys) resulted in higher liver but lower carcass to body weight ratio. Fish fed the diet containing 1.26% Lys showed significantly higher whole-body lipid and energy levels but lower moisture and protein levels than those fish fed the other diets. Broken-line analysis of the growth data indicated that the Lys requirement was 1.79% of diet (5.1% protein) under current culture conditions.