Salmonid reproductive migration and effects on sexual maturation

Abstract

Salmon has amazing abilities to migrate long distances from the ocean to their natal streams for reproduction. Reproductive homing migration was investigated by two different research approaches from behavioural biology to endocrinology using chum salmon in the north Pacific Ocean as well as two landlocked salmon species. These species are lacustrine sockeye and masu salmon in Lake Toya and Lake Shikotsu, Hokkaido, Japan where the lakes serve as a model “ocean”. Physiological biotelemetry techniques were applied to examine the homing behaviours of adult chum salmon from the Bering Sea to Hokkaido as well as lacustrine sockeye and masu salmon in Lake Toya, and revealed that salmon can navigate in open water using different sensory systems. The hormone profiles in the brain-pituitary-gonadal (BPG) axis were investigated in chum salmon and lacustrine sockeye salmon during their homing migration, and clarified that salmon gonadotropin-releasing hormone (sGnRH) plays leading roles on homing migration. The interaction between metabolism and sexual maturation under exercise, as experienced during the salmonid anorexic reproductive migration, has been studied by swimming the facultative migrant rainbow trout in a large swim flume. RNA sequencing of the red and white muscle transcriptome and microarray analysis of the ovary has been performed to identify hormonal and metabolic actors that are important in switching from the growth phase to the migration phenotype. Olfaction of spawning ground specific smell is suggested as switch from the navigating migration to the sexual maturation phenotype.