Migratory Behaviors in Masu Salmon (Oncorhynchus masou) and the Influence of Endocrinological Factors

© 2012 TERRAPUB, Tokyo. All rights reserved. doi:10.5047/absm.2012.00502.0029 and Kobayashi 2010). Most of the non-migratory forms will live continuously in their natal rivers throughout their lives (Fig. 1). Regardless of these life history types, most salmonids will spawn in freshwater environments, mainly in their natal rivers (Fig. 1) (Quinn 2005). From these phenomena, salmonids are considered to be of freshwater (fluvial) fish origin and their migratory behaviors by and large start from the rivers (Fig. 1). Among the four genera of salmonids, two genera Hucho and Salvelinus are considered evolutionally ancient groups, based on the phylogenic analyses (Norden 1961; Murata et al. 1993). Genus Hucho inhabit only the northern Eurasia continent and genus Salvelinus inhabit northern Eurasia and the American continent (Quin 2005). On the other hand, genus Salmo (i.e., Atlantic salmon) and Oncorhynchus (i.e., Pacific salmon), which are considered evolutionally new 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. Migratory Behaviors in Masu Salmon (Oncorhynchus masou) and the Influence of Endocrinological Factors