In this study the suitability of canola oil (CO) and poultry fat (PO) as alternatives to fish oil (FO) was assessed. Furthermore, a real-time RT-qPCR assay to detect hepatic insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-1 (IGFBP-1) was developed. Four isoproteic (450 g kg-1), isolipidic (250 g kg-1) dry extruded diets were formulated to contain PO or CO at 50 or 100% inclusion, as a substitute for FO. Yellowtail Kingfish growth and growth factor gene expression was compared with fish fed a diet containing 100% FO.

Two experiments were conducted separately at optimal (22 oC; 33 d) and sub-optimal (18 oC; 34 d) temperatures with mean initial fish weights of 95.6 ± 0.1 g and 101 ± 0.1 g respectively. At 22 oC Yellowtail Kingfish fed the 50% PO diet grew significantly larger (281.2 ± 1.2 g) than individuals fed the 100% FO diet (266.9 ± 5.9 g). Inclusion of CO at 50% produced equivalent weight gain to the 100% FO diet, however, 100% inclusion of CO produced poor performance in both experiments. Trends in gene expression reflected the response in weight gain to alternative lipids. A positive relationship between hepatic IGF-I mRNA levels and weight gain and also hepatic IGFBP-1 mRNA and weight gain was found. A positive correlation between hepatic IGF-I and IGFBP-1 mRNA levels was also found. Irrespective of diet, fish grew larger at 22 oC than at 18 oC. The findings of this research indicate that improved diet formulations for Yellowtail Kingfish may be achieved through the inclusion of alternative lipid sources and that IGF-I mRNA may be used as a rapid indicator of growth in this species.