Growth performance and endocrine response of fingerling rainbow trout (Oncorhynchus mykiss) in adaptation to different environmental salinities.

To determine the changes of plasma levels of cortisol،T3 (triiodothyronine) and T4 (Thyroxine) and growth factors of rainbow trout in reponse to a wide range of salinities، The fingerlings of raivbow trout Iinitial weight of (12. 22±0. 11 g) were introduced to different environmental salinities. A compelately randomized desine with 5 Treatment and each treatment contained 3 replicate was achived. One hundreds of fingerlings were gradually acclimatized to freshwater (control)، 10،، 20، 30 and 40 ppt for 10 days. The experiment was then run for 50 days using a commercial trout feed. The obtained results showed that final weight was different from 37. 32±0. 29 (g) in freshwater (control)، and other water salinities include 34. 92±0. 31 (g) in 10 ppt and 30. 98±0. 07 (g) in 20 ppt. with increasing in water salinity other growth factors of rainbow trout including (daily growth rate، specific growth rate and feed conversion efficiency) decreased (P<0. 05). At salinity above 20 ppt significant mortality occurred compared to control group (P<0. 05). Also plasma level of cortisol was increased from 5. 97±0. 78 to 5. 95±0. 14 in freshwater (control)، 12. 50±0. 25 in 10 ppt and 13. 92±0. 12 (mOsmol/kg) in 20 ppt at the end of culture period. with increasing in salinity، the Hormones including، tri- iodo thyronin (T3)، and thyroxine [Tetra – iodo – thyronin (T4)] increased (P<0. 05). Based on the results obtained rainbow trout is a semi euryhaline species and it is possible to grow the rainbow trout fingerlings of about 12 g at the salinity up to 20 ppt، although the growth rate of the fish is less than that in the fresh water.