Influence of water temperature on thyroid hormones and on the movement behavior of juvenile rainbow trout ( Oncorhynchus mykiss ) in water flow

We studied the influence of long-term (3 months) and short-term (3 days) water temperatures of 13 °C and 18 °C on movement behavior of juvenile rainbow trout ( Oncorhynchus mykiss ) in water flow. Levels of free (free T 3 ) and total triiodothyronine (T 3 ), and thyroxine (T 4 ) in the blood of fish under different water temperatures were estimated. During the long-term exposure of fish at 13 °C, they tended to maintain their initial location. During the long-term exposure at 18 °C, fish tended to leave their location moving upstream. The concentrations of free T 3 , total T 3 , and the rate of deiodination (conversion of T 4 to T 3 ) were lower in fish under long-term exposure at 13 °C than at 18 °C. The short-term effect of water temperature on fish was estimated by transferring some fish from 18 to 13 °C and vice versa. Fish under long-term and short-term influence of water temperature of 13 °C had similar behavioral and hormonal parameters. Juveniles under long-term and short-term influence of water temperature of 18 °C had similar movement behavior and a similar rate of deiodination. Thus, synchronous changes in movement behavior and the levels of thyroid hormones in juvenile rainbow trout with a change of water temperature were observed.