The effect of dietary choline and water temperature on the contribution of raw materials to the muscle tissue of juvenile yellowtail kingfish (Seriola lalandi): An investigation using a stable isotope mixing model

Two independent eight-week experiments were conducted to evaluate the effect of dietary choline content and water temperature on the contribution of major raw materials to the muscle of juvenile yellowtail kingfish (YTK; Seriola lalandi) using stable isotopes and a mixing model. Experiment 1 (dose-response design) focused on rearing fish (mean ± SD; initial weight = 156.3 ± 15.3 g) at 16 °C. Fish were fed six semi-purified research diets with increasing levels of feed-grade choline chloride (CC; 70% CC). The diets contained measured choline content ranging from 0.59 to 6.22 g/kg diet. Except for the control diet, all semi-purified diets contained 0.3% of 2-amino-2-methyl-1-propanol (AMP; de novo choline synthesis inhibitor). Three major raw materials assessed in the semi-purified diets were fishmeal 1, sodium caseinate, and gelatine. In experiment 2 (2 × 3 factorial design), fish (157.3 ± 11.9 g) were reared at 16 °C and 24 °C, and fed three practical diets supplemented with 0.0, 3.0, or 6.0 g CC/kg diet. The six major raw materials assessed in the practical diets were fishmeal 2, poultry meal, meat meal, blood meal, lupin, and wheat flour. Stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) were used as natural tracers and the stable isotope mixing model (SIMM) was based on a Bayesian framework (R package ‘MixSIAR’). In general, the calculated raw material contributions were within the 95% credible intervals range of the SIMM estimates. Based on the SIMM estimate, experiment 1 showed that increasing dietary choline content elevated the mean proportional contribution of sodium caseinate (56.8–81.1%) to the muscle of juvenile YTK. In comparison, fishmeal 1 and gelatine contribution decreased with increasing dietary choline. Unlike experiment 1, experiment 2 showed that choline supplementation and water temperature had little effect on raw material contribution to the muscle of YTK when fed practical diets. In addition, the SIMM estimated contribution of fishmeal 2 (20.7–45.9%) was lower than that of the calculated (47.6%). The study indicated that raw material contributions to muscle of YTK was more profoundly affected by severe choline-deficiency than temperature, at 16 °C and 24 °C. Furthermore, the study indicated that fishmeal inclusion in YTK diet could be reduced. The present study demonstrated the novel use of naturally-occurring stable isotopes and SIMM to evaluate aquafeed down to ingredient level. The findings can guide feed formulation for YTK and other species by demonstrating the potential micronutrient-raw material-environment interactions.