|Vol. 5 (No. 2), pp. 29-65, 2012||doi:10.5047/absm.2012.00502.0029|
Department of Biology, Miyagi University of Education, Aoba-ku, Sendai, Miyagi 980-0845, Japan
(Received on April 1, 2011; Accepted on September 22, 2011; Online published on November 20, 2012)
Abstract: In the freshwater phase of their lifecycle, masu salmon (Oncorhynchus masou) comprise two different phenotypes. A portion of the juveniles (migratory form) exhibit downstream migratory behavior after smoltification. However, some masu salmon (non-migratory form) such as precociously mature males live continuously in their natal rivers throughout their lifetime. The coexistence of migratory and non-migratory forms within the species indicates that this salmon can be effectively used as a model fish to illuminate both inhibitory and stimulatory physiological control mechanisms of migratory behaviors. In masu salmon, it was found that sex steroid hormones inhibit the occurrence of downstream swimming behavior, the initial step in seaward migration. Moreover, after the commencement of downstream migration, sex steroid hormones induced the upstream swimming and subsequent spawning behaviors. These findings indicate that sex steroid hormones influence the occurrence of the downstream and upstream swimming behavior in the resulting rheotaxis fashion (negative and positive, respectively). In contrast to sex steroid hormones, it was also found that cortisol, which is involved substantially in smoltification, stimulates the downstream swimming behavior. These findings indicate that the occurrence of seaward migration is controlled competitively by sex steroid hormones (sexual maturation) and smolt-inducing factors such as cortisol, in masu salmon and potentially other Pacific salmon.
Keywords: cortisol, downstream migration, masu salmon, Oncorhynchus masou, Pacific salmon, sex steroid hormone, spawning, testosterone, upstream migration
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