Lysine optimization of a commercial fishmeal-free diet for hybrid striped bass (Morone chrysops×M. saxatilis)

Substitution of fishmeal with alternate proteins in aquafeeds often results in dietary imbalances of first-limiting essential amino acids (EAA) and poorer fish performance. This growth trial was undertaken to test the hypothesis that ideal protein theory accurately predicts first-limiting amino acids and optimum lysine level for a fishmeal-free, commercial-grade diet formulated for hybrid striped bass (HSB). The ideal model for formulation was the amino acid pattern of hybrid striped bass muscle. A negative control diet based on soybean (45%), corn gluten (10%), and poultry by-product (13%) meals was formulated on an ideal basis to contain 18% lipid, 40% crude protein (CP), 36% digestible protein (DP) and 1.8% Lys from intact sources. Met and Thr were then added at levels equivalent to 40% protein from HSB muscle to form a basal diet that was fortified with seven levels of Lys (2.2 to 6.4g/g diet) that bracketed the predicted ideal Lys requirement (3.5g/g diet) for this formulation. Diets were extruded to achieve similar characteristics as a commercial-grade feed and fed to juvenile fish (118g BW) for 84 d. Response data were subjected to polynomial and exponential regression and the best model per response selected based on the lowest error (MSE) and p-values and the highest adjusted R2. Selected models were used to derive dietary Lys levels required to reach 95% (R95) or 99% (R99) of the minimum or maximum response. Final fish weights (328–369g) increased linearly with increasing dietary Lys. Weight gain, average daily feed intake (1.42–1.7%), and feed efficiencies at 4-, 8-, and 12-week intervals were modeled by cubic functions that yielded consistent R95 and R99 levels of about 2.4 and 4.3g Lys/g diet, respectively. These values evenly bracketed the predicted ideal Lys requirement. Slightly higher Lys requirement was found for optimum FE at 4weeks and smaller fish, as opposed to 8 or 12weeks and larger fish. Whole body composition was unresponsive to diet Lys level; however, higher dietary Lys was required to minimize liver size (4.8 to 5.2g Lys/g diet) than to minimize body fat (2.5 to 3.1g Lys/g diet) or maximize muscle ratio (1.8 to 3.9g Lys/g diet). Whole body protein, lipid, energy, and EAA retentions were also consistent with cubic functions that generally confirmed the predicted ideal Lys requirement with notable exceptions. Retention of Lys decreased exponentially with increasing dietary Lys, whereas, Met retention decreased in a linear fashion, corroborating that Met was first-limiting in this diet formula, as predicted. Retention of Thr and branched-chain amino acids were optimized at higher Lys levels (3.9–4.5g/g diet) than those required to maximize growth parameters. Protein accretion (g/fish/d) responded quadratically to Lys intake predicting maximum deposition at 0.10 to 0.16g Lys/fish/d. Histological assessment of intestines in fish fed these high-soybean meal test diets did not reveal any lesions associated with enteritis for any diet and overall normal intestinal morphology was observed in all fish sampled.