The choline requirement of juvenile yellowtail kingfish (Seriola lalandi) and the interactive effects of choline and water temperature on nutrient assimilation

Abstract

Yellowtail kingfish (Seriola lalandi, Valenciennes 1833; YTK) farming is considered the greatest opportunity for aquaculture expansion and diversification in Australia. Consequently, the demand for pelleted feed is expected to increase significantly for this species. Nutritional research on YTK has focused on macronutrient requirements and testing alternative protein and lipid sources to fishmeal and fish oil. However, there is a lack of information on micronutrient requirements and their effects on the growth and health of YTK. Therefore, it is important to acquire more knowledge on this topic to optimise the feed formulation for YTK. Choline is an essential nutrient typically supplemented in aquafeed and is abundant in raw materials such as fishmeal and soybean meal. Substituting raw materials for alternative food sources without knowing the choline requirement or its role in YTK performance might hinder YTK production. Accordingly, the main aim of this thesis was to improve our understanding of dietary choline on the growth performance and liver health of juvenile YTK at sub-optimal water temperatures. Liver was selected as a target organ because it is one of the major sites that facilitates various metabolic processes, including choline metabolism, and is susceptible to choline-deficiency induced liver disease. This thesis further explored the efficacy of using naturally-occurring stable isotopes to better understand the interactive effects of dietary choline and water temperature on nutrient assimilation of YTK. The aim was achieved and addressed by conducting two independent experiments with a series of interlinked studies. The research first determined the digestible choline requirement of YTK using a dose-response approach. The choline supplementation of a practical diet formulation at two temperatures (16 °Cand 24 °C) that bracket the culturing water temperatures commonly experienced by farmed YTK in Australia was also conducted. The digestible choline requirement of juvenile YTK was estimated to be 27.3 mg kgBW−1 d−1 or 1.94 g kg−1 diet (choline chloride supplementation: 0.0−10.0 g kg−1 diet), which is higher than for most finfish species. Choline and lipid apparent digestibility improved with increasing dietary choline. In contrast to many animal models, higher liver lipid deposition (in the form of free fatty acids and triacylglycerol) was associated with increasing dietary choline in YTK. However, this species did not exhibit signs of fatty liver disease related lesions which indicates that, if not beneficial, this amount of lipid is not detrimental to juvenile YTK. Upon further examination, dietary choline does not significantly alter the liver lipid composition but might protect liver health of YTK. This research confirmed that supplementing choline in fishmeal-based practical diets was necessary for juvenile YTK reared between 16 °C and 24 °C. Water temperature was largely responsible for the change in liver lipid composition and morphology; however, there was no indication of disease in fish fed practical diets. Importantly, this research demonstrated that the current industry practice of supplementing 3.0 g CC kg−1 diet to a fishmeal-based formula would ensure that the production performance and liver health of juvenile YTK are not compromised when cultured between 16 °C and 24 °C. Furthermore, using naturally-occurring stable isotopes, this research showed that fish had an affinity for assimilating certain raw materials such as sodium caseinate, poultry meal, meat meal, lupin, and wheat flour, irrespective of their inclusion levels. However, dietary choline content and water temperature did not significantly alter the assimilation of raw materials in the practical diet. The results indicate that higher inclusion of poultry meal, meat meal, lupin, and wheat flour in YTK feed is possible and might offer complementary information to digestibility studies. This thesis is the first to determine the choline requirement of YTK and established a scientific knowledge platform for further research on choline metabolism and its interaction with metabolically-linked nutrients. The research also provides scientifically-validated information that will help guide aquafeed formulation to reduce production costs and improve the growth performance and quality of YTK.