This study was undertaken to assess the efficacy of utilizing insulin-like growth factor 1 (IGF-I) and insulin-like growth factor binding protein 1 (IGFBP-1) as endocrine biomarkers to provide a rapid indication of yellowtail kingfish growth using real-time, reverse-transcriptase quantitative PCR (RT-qPCR). Two experiments were conducted separately (5 weeks each) at an optimal water temperature (22°C) using fish weighing 95.6 ± 0.1 g (mean ± standard deviation, n = 3) and at a sub-optimal water temperature (18°C) using similar-sized fish (101 ± 0.1 g, n = 3). Four experimental diets were formulated to contain poultry oil (PO) or canola oil (CO) at 50% or 100% inclusion as a substitute for fish oil (FO). A diet containing 100% FO was used as a control. Irrespective of diet, fish grew faster at 22°C than at 18°C. The 100% replacement of FO by CO resulted in the poorest fish growth at both temperatures. IGF-I expression correlated with growth rate at 22°C, but the same trend was not seen at 18°C. Hepatic IGF-I and IGFBP-1 mRNA expression were also positively correlated. This study suggests that hepatic IGF-I mRNA assessed by RT-qPCR with additional methodological development may be used to accurately assess the influence of diet on growth of some fish species used in aquaculture.