Study of Thyroid Gland Activity in Caspian trout (Salmo trutta caspius) Juveniles in relation to various salinities

This study was carried out to determine thyroid gland activity during salinity changes in Caspian trout (Salmo trutta caspius Kessler, 1877) juveniles. Specimens were exposed in 4 weight groups of 5, 10, 15 and 20 g, in 3 salinity trials: Caspian water (11- 11.5 ppt), inshore water (7 ppt) and fresh water (control). Each trial was done in 3 replicates. Plasma osmolarity was measured by osmometer. The tissue fixations carried out from juveniles of control group (in fresh water) and different hours after exposure of fish in salinities. Histological indicators including thyroid gland morphometric parameters were assessed using classic preparation and optic microscope with digital camera. Histology of thyroid gland of juveniles acclimated to 7 ppt salinity and Caspian water showed that after the transfer of 10, 15 and 20 g weight groups to saline water, the gland became more active at early hours and gradually returned to initial condition after 10 days. The same activity was not observed for 5 g juveniles. Based on the results of osmoregulation variation, it can be confirmed that weight groups 10, 15 and 20 gr can do hypoosmoregulation of the salinity of the Caspian Sea, but the weight group of 5 gr do not have such a ability.