Effects of growth hormone and cortisol on the downstream migratory behavior in masu salmon, Oncorhynchus masou

The effects of ovine growth hormone (oGH) and cortisol on downstream migratory behavior in yearling (1+) smolts and underyearling (0+) parr of masu salmon, Oncorhynchus masou, were examined during the downstream migratory period in spring using artificial raceways. In May, each of 22 1+ smolts and 0+ parr were implanted with cholesterol pellets containing 250 μg of oGH and/or 2 mg of cortisol. Their downstream migratory behavior was subsequently observed in artificial raceways, along with control groups 4–23 days after implantation. In 1+ smolts, the frequency of downstream migratory behavior was 23%, 18%, 72%, and 82% in the control, oGH, cortisol, and oGH + cortisol-treated groups, respectively. The frequency was significantly higher in the cortisol and oGH + cortisol-treated groups than in the control and oGH-treated groups. In 0+ parr, the frequency of downstream migratory behavior in the cortisol (82%) and cortisol + oGH-treated (90%) groups was significantly higher than in the control (18%) and oGH-treated (0%) groups. These results indicate that cortisol is an important endocrine factor inducing downstream migratory behavior in both 1+ smolt and 0+ parr of masu salmon.