changes osmolarity, cortisol and thyroid hormones (t3, t4) of rainbow trout (oncorhynchus mykiss) in response to different salinity levels

abstarct at the present study effects of salinity were studied on growth and feeding performance, survival rate and blood biochemical parameters of pre on-growing rainbow trout (oncorhynchus mykiss) with initial weight 93.22±4.11 g for 60 days. after adaptation for 10 days to water salinity, 450 fish were distributed in 5 treatments and 3 replicates according to completely randomized design. before adaptation to water salinity and 1, 10, 25 and 50 days after introduction to different salinity (10, 20, 30 and 40 ppt), growth and feeding parameters and some blood biochemical parameters were investigated. according to the obtained results from this study, salinities up to and including 20 ppt significantly decreased growth and feeding parameters including daily growth rate, specific growth rate, feed conversion efficiency between salinity groups and control group (p<0/05) and higher salinities (30 and 40 ppt), caused gregarious mortality. during the experiment, blood biochemistry parameters including osmolality, chlorine, sugar, cortisol, tri-iodo thyronin(t3), and tetra–iodo–thyronin (t4)  significantly increased with increasing in water salinity (p<0/05). overall, the obtained results indicated that rainbow trout with initial weighing 90 g could be cultured successfully in water with salinity up to 20 ppt, although fish growth rate is lower than that in the fresh water.