Springtime Hepatosomatic Indices, Plasma Cortisol Levels, and Hepatic 5'-Monodeiodinase Activities in One-Year-Old Atlantic Salmon from Two Growth Modes

Abstract

Abstract Changes in hepatosomatic indices (HSI), plasma cortisol levels, and hepatic 5'-monodeiodinase (5'-D) activities associated with bimodal development and smoltification were examined in Atlantic salmon (Salmo salar). Atlantic salmon originally graded into the lower modal group (LMG) were graded a second time so that they separated into a secondary LMG (2°-LMG) of parr and a secondary upper modal group (2°-UMG) of smolts. Between March and May the mean HSI decreased from 1.11 to 0.88 in 2°-UMG smolts and from 1.25 to 1.05 in 2°-LMG pan. Fish from the 2°-UMG also had a significantly lower mean HSI than did fish from the 2°-LMG in April, in May, and overall (March–May). Mean cortisol levels did not change over time in either modal group, but they were significantly higher in 2°-UMG fish than in 2°-LMG fish in April (23.2 versus 3.9 ng/mL) and overall (19.4 versus 6.0 ng/mL). Mean 5'-D activity ranged from 0.15 to 0.77 pmol 3,5,3'-triiodothyronine/(mg protein·h), but it did not change over time in eith...