LAUSR

To study the effects of body weight on growth and physiological response mechanism of S.salar, fish of three average body weights were selected: 30.25 +/- 2.12g (SW1), 24.27 +/- 1.59g (SW2) and 18.05 +/- 2.46g (SW3). Condition factor, daily weight gain and feed conversion efficiency were significantly higher in the SW2 group compared with the other treatment groups (P<0.05). Food conversion ratio decreased with increasing body weight, and weight and specific growth rate of body length were highest in the SW2 group (P<0.05). Larger fish (SW1 and SW2) had higher haemoglobin concentration and number of red blood cells. The number of white blood cells, lymphocytes and neutrophils were greatest in the SW3 group (P<0.05), suggesting that these fish had strong resistance to disease. The content of total protein, albumin and urea were significantly higher in the SW2 group compared with other groups (P<0.05). The concentration of total cholesterol and triglyceride increased with increasing body size (P<0.05). Hormone (T3 and T4) levels first increased significantly and then decreased in fish of various sizes (P<0.05), and the concentration of cortisol rose, fell and then rose again in SW1 and SW2 groups. Light affected the endocrine activity of S.salar by influencing the growth performance of the fish, and smaller sized fish exhibited decreased smoltification rate and a reduced success rate of smoltification. Based on these findings, S.salar weighing about 24g were chosen to begin the process of smoltification.