Endocrine growth regulation of adult Atlantic salmon in seawater: The effects of light regime on plasma growth hormone, insulin-like growth factor-I, and insulin levels

Plasma growth hormone (GH), insulin-like growth factor-I (IGF-I), and insulin were measured in two groups of Salmo salar L. during a one-year study. The fish were reared under either a simulated natural photoperiod (SNP) from January to December or a regime of continuous light from January to June, followed by SNP until December (LL/SNP). Plasma GH levels during spring were low, and lower in the LL/SNP fish (< 0.9 ng ml− 1) than in the SNP fish (> 1.9 ng ml− 1), although the LL/SNP grew better (0.8% per day) than the SNP fish (0.5% per day). Plasma IGF-I levels increased transiently from January (64.7 ng ml− 1) to maximum in late September in the LL/SNP (85.8 ng ml− 1) and in November in the SNP group (87.3 ng ml− 1). The ratio GH:IGF-I was lower in the LL/SNP group during spring when this group grew better than the SNP group.