Role of the cortisol on the onset of downstream migration in hatchery reared chum salmon Oncorhynchus keta fry

Abstract

The relationship between downstream migration and plasma cortisol (F) level in chum salmon fry was investigated. The releases of chum fry (age 0, BW: 0.8.1.8 g) were carried out twice either in a day and a night. Approximately a million individuals of chum fry were released from the hatchery raceways to a hatchery pool, and the fry schools spontaneously moved down from the pool to the hatchery creek. The fish were sampled in the raceways (initial: before the migration), in the pool (staying: preparing period for the onset of migration), and in the creek (migrating: immediately after onset of the migration). The plasma concentrations of the migrating fry were significantly higher than those of the staying fry at 1 h after the releases in both the day- and the night-release. In addition, the F levels of the staying and migrating fry in the night-released were significantly higher than those of the day-released. Results can be interpreted that the F secretion ability may active with higher stress sensitivity in night. Chum fry show the downstream migration in nighttime in the early migratory season. Thus, the cortisol can relate to the onset of the downstream migration in the chum salmon fry