Dietary lipid and protein levels influence the growth and feed utilisation of large yellowtail kingfish (Seriola lalandi) at summer water temperatures

Abstract

Yellowtail Kingfish (Seriola lalandi; YTK) have a fast growth rate, and as a result, a high energy demand that could be satisfied by dietary lipids. The aim of this experiment was to determine the optimum dietary protein and lipid levels for large YTK maintained during a summer growing season. In this 84-day experiment (2 × 3 factorial design), the growth, feed utilisation and health of YTK (initial weight 2.13 ± 0.02 kg, mean ± SE) fed two nominal lipid levels 25% crude lipid (CL) (20 MJ kg−1 energy) and 30% CL (21 MJ kg−1 energy) and three nominal crude protein (CP) levels (40, 44 and 48%) were investigated. The specific growth rate of fish was significantly affected by the interaction between dietary CP and CL level. The results indicate that fish fed the 25% CL diets grew better than those fed the 30% CL diets. The interaction effect was primarily driven by the significant increase in growth for fish fed diet 1 (40% CP × 25% CL) and diet 2 (44% CP × 25% CL) compared to their respective 30% CL diets, diet 4 (40% CP × 30% CL) and diet 5 (44% CP × 30% CL). The growth of fish fed diet 3 (48% CP × 25% CL) and diet 6 (48% CP× 30% CL) was not significantly different. Feed conversion ratio (FCR) was not significantly influenced by diet however, the fish fed the 25% CL diet series and 40% protein series consumed the most feed. Fish fed the 25% CL series increased their feed intake with decreasing dietary CP level, to potentially compensate for reduced dietary protein. In contrast, fish fed the 30% CL series did not. Crude protein intake was higher when fish were fed the 48% CP diets and was reduced when fed the 40% and 44% CP diets. This response was more pronounced in the 30% CL diet series. There were no negative health effects noted during the study for various analyses including blood biochemistry and haematology, and gastrointestinal tract morphology. Numerically, the highest final weight, biomass gain, SGR, and FCR were consistently observed in the fish fed diet 2, however these were not significantly different to diet 1. Based on the combined results for 2.1–3.5 kg YTK raised during summer, the diet should contain at least 42.9% CP and 25.3% CL (equivalent to diet 2) corresponding to 19.9 MJ kg−1 GE diet with and a CP: GE ratio of 21.6 g MJ−1. In terms of actual protein and energy intake required this corresponds to 5.2 g CP−1 kg BW−1 d−1 (4.5 g DP−1 kg BW−1 d−1) and 242 KJ−1 GE kg BW−1 d−1 (207 KJ−1 DE kg BW−1 d−1) which appears to be optimal. We do not recommend diet lipid levels of approximately 30% over summer periods, as demonstrated in this experiment.