Temperature and ration effects on components of the IGF system and growth performance of rainbow trout (Oncorhynchus mykiss) during the transition from late stage embryos to early stage juveniles

The study investigated the effects of incubation temperature, and the size of ration fed to the transitional embryo/juvenile stage of rainbow trout (Oncorhynchus mykiss) on growth, liver and gastrointestinal (GI) tract IGF-1 content, and the expression of insulin-like growth factor-related genes (IGF-1, IGF-2, IGF-RIa, and IGF-RIb) by the liver and GI tract. Embryos were reared from zygote to “swim-up” at either 8.5 °C (E8.5) or 6.0 °C (E6.0); at “swim-up” (51-days post-fertilization [dpf] and 72-dpf for the E8.5 and E6.0 groups, respectively), the embryos were transferred to grow-up tanks supplied with water at 8.5 °C. Late stage embryos (LSEs) at the same developmental stage from the two temperature treatment groups (64-dpf and 86-dpf for the E8.5 and E6.0 groups, respectively) were fed with salmonid starter diet at levels of 5.0%, 2.0%, and 0.5% of live body mass per day. Embryos were sampled just prior to first feeding (PFEs), and before complete absorption of the yolk [late stage embryos (LSEs)], and early stage juveniles (ESJs) were sampled after yolk sac absorption when they were fully reliant on exogenous sources of food. The early incubation temperature and ration levels had significant affects on mortality (with lower mortalities in the E6.0 group) and growth performance of the fish; dry body mass values for fish fed the 5.0% ration were significantly lower in the E6.0 group of LSEs and ESJs compared with the respective treatment in the E8.5 group; a similar pattern was seen for total body length, although this was only significant for the LSEs. Whole embryo IGF-1 content was significantly lower in the E6.0 group compared with the E8.5 group of PFEs, and hepatic IGF-1 content was significantly lower in the E6.0 group fed the maintenance ration (0.5%) compared with the E8.5 fed a similar ration; restricted ration significantly elevated hepatic IGF-1 content in the LSE stage for both temperature treatment groups. GI tract IGF-1 levels were considerably lower than in liver tissue, and there were no differences among treatment groups. Ration size-related differences were found for the expression of genes encoding for hepatic IGF-1, IGF-2, and IGF-RIb, and GI tract IGF-1, and IGF-2. Rearing temperature-related differences were also found for genes encoding for GI tract IGF-1, IGF-RIa, and IGF-RIb. The results of the study showed that the early rearing temperature of the embryos affected subsequent growth, and hepatic and GI tract gene expression by the LSEs and ESJs. As was the case for tissue IGF-1 content, with some exceptions, a restricted ration significantly elevated the expression of the targeted genes indicative of an important metabolic-regulating role for the IGF system during this transitional developmental phase. In addition, the higher abundance of IGF-2 mRNA compared with IGF-1 mRNA, and the higher abundance of IGF-RIa, relative to IGF-RIb, suggests that these two genes may also play a regulatory role during this transitional developmental phase.