The relationship between thyroxine surge and onset of downstream migration in chum salmon Oncorhynchus keta fry

Downstream migration in chum salmon fry was initiated soon after the gate of a hatchery raceway was opened. The relationship between the onset of downstream movement and thyroxine (T4) surge was investigated. In experiment 1, the gates of the raceways were opened at 0930 in March and April, and l–3 million fry were discharged to a creek pool through a drain channel. The fish were sampled in the raceway (initial), the pool (staying: preparing for migration), and a lower creek (migrating). Peaks of the number of the fry migrating from the pool to the creek and T4 surge appeared 1 h and 4.5–7 h, respectively after the gates were opened. In experiment 2, treatments with fall stress from l-m high buckets, turbid water (200 mg/l), and exogenous T4 (1 mg/g diet) were conducted in the raceway to examine the effects on downstream locomotion and plasma T4 concentrations. Fall stress did not induce an increase in either plasma T4 concentration or the number of migrants, whereas turbid-water treatment induced 10 ng/ml T4 increase and a greater downstream locomotion than the other treatments. T4 treatment induced a continuous high level in plasma T4 but had less effect on locomotion. These results suggest that although thyroid hormones play a pivotal role in the preparation for downstream migration, they are not the sole factors responsible for the onset of their downstream migration.