Digestible choline requirement of juvenile yellowtail kingfish (Seriola lalandi)

The present study was conducted to determine the digestible choline requirement of juvenile yellowtail kingfish (Seriola lalandi; YTK) in the presence of 2-amino-2-methyl-1-propanol (AMP), an inhibitor of choline biosynthesis. The second aim of this study was to determine if choline supplementation of a practical diet made from common raw materials can improve the growth performance of YTK and if choline supplementation dosages are affected by water temperature. Two eight-week experiments were conducted. The first was a dose-response experiment in which juvenile YTK (156.3 ± 15.3 g) were reared at 16 °C and fed five isonitrogenous and isoenergetic semi-purified diets containing 0.42 (Diet 1), 1.10 (Diet 2), 1.37 (Diet 3), 2.96 (Diet 4) or 6.05 g digestible choline kg⁻¹ diet (Diet 5) by adding graded concentrations of choline chloride (CC). A sixth diet (Diet 6), comparable to Diet 4, was made without AMP to estimate the de novo synthesis of choline by YTK. The second was a factorial experiment in which juvenile YTK (157.3 ± 11.9 g) were reared at 16 °C and 24 °C and fed a practical diet supplemented with 0.0, 3.0 or 6.0 g of CC kg⁻¹ which equates to a digestible choline concentration of 1.77, 3.54 and 4.66 g kg⁻¹ diet, respectively. Results from experiment 1 indicated the breakpoint (broken-line regression model) in specific growth rate (SGR) and choline deposition rate occurred when digestible choline intake reached 26.1 mg kgBW⁻¹ d⁻¹ and 27.3 mg kgBW⁻¹ d⁻¹, respectively. On a dietary basis, the breakpoint in SGR and choline deposition rate occurred when diets provided 1.93 and 1.94 g digestible choline kg⁻¹ diet, respectively. Experiment 2 results indicated there were no significant interactions between dietary choline and water temperature with respect to production indices such as SGR, food conversion ratio (FCR), whole body composition and nutrient retention. Choline deposition rate, SGR and FCR tended to be better in fish fed the practical diet supplemented with 3.0 g CC kg⁻¹, but there was no additional advantage of raising the content to 6.0 g CC kg⁻¹ diet. The present study confirms the necessity of adding CC to practical diets for YTK given that practical diets with no CC supplementation contain lower digestible choline than the estimated requirement established in experiment 1.