EFFECTIVE USE OF CARBOHYDRATES AS A DIETARY ENERGY SOURCE IN CALIFORNIA YELLOWTAIL Seriola lalandi
Farmed California yellowtail Seriola lalandi are typically fed high levels of dietary crude lipid (CL) ranging 18-25% of the diet with 40-45% crude protein (CP). Excessive dietary energy may impede nutrient utilization and growth performances by placing an unnecessary metabolic burden on the fish. Indeed, previous findings showed good growth and nutrient utilization with diets formulated with 11% dietary lipid, thereby suggesting that current diets can be optimized for energy content. To this end, two series of 44% CP diets were formulated: five diets with varying CL (6.19-21.21%), starch (0.87-15.87%) and energy (4331-5138 kcal digestible energy /kg) contents, and five isoenergetic (4495 kcal/kg) diets varying in CL (4.80-14.50%) and starch (0-24%) in trial 1 and 2, respectively. In trial 1, growth (TGC) and protein retention efficiency (PRE) increased up to 4780.5 kcal/kg of the diet (TGC=0.294±0.002, PRE=38.0±1.4%, means ±SD, Fig. 1). However, fish fed 5138.1 kcal/kg, 21.21% CL showed poorer TGC and PRE. Additionally, whole body protein linearly decreased as dietary lipid increased. In trial 2, little response was observed in response to the increasing dietary ratio of carbohydrates (as nitrogen-free-extract, NFE) to CL. Only fish fed the diet containing 0.57 NFE:CL showed slightly reduced performances (Fig. 2).
These results indicate that growth and nutrient utilization of S. lalandi can be improved by reducing the dietary lipid level to 12-15%. Additionally, this species is able to efficiently utilize complex carbohydrates as a source of dietary energy. These results allow for tailored for California yellowtail, promoting growth and optimizing nutrient utilization, while also reducing inclusion of expensive and limited ingredients such as fish oil.