The Effects of Brackish Water on Growth Hormone/Insulin-like Growth Factor-1 Gene Expression of the Caspian Trout, Salmo trutta caspius (Kessler, 1877), During the Early Stage of Smoltification

Caspian trout, Salmo trutta caspius, a subspecies of brown trout, S. trutta, is a commercially important but endangered fish species found in the Caspian Sea. To investigate the effect of changes in salinity during migration on the growth of Caspian trout, juvenile fish were challenged in brackish water, and the levels of growth hormone (GH) and insulin-like growth factor 1 (IGF-1) expression were quantified in the pituitary gland and the liver. Specifically, juvenile fish were reared in freshwater to a weight of 15-20g and subsequently transferred to brackish water for 28d. Fish were sampled at 1, 3, 5 and 28d post-transfer, and target gene mRNA was quantified using a real-time qPCR assay. A significant increase in the specific growth rate (SGR) as well as in the abundance of GH and IGF-1 mRNA was observed in fish that were reared in brackish water (P<0.05). Two transcripts of GH with 90% homology in the open reading frame and the complete transcript of IGF-1Ea4 were identified based on gene cloning. In conclusion, our results suggest that brackish water adaptation stimulates the SGR via the GH/IGF-1 axis and that GH and IGF-1 have been highly conserved during the evolution of the Caspian trout.