2002 Volume 68 Issue sup1 Pages 49-52
In masu salmon, Oncorhynchus masou, yearling immature fish migrate downstream into the sea after parr-smolt transformation in spring. However, precociously mature yearling males that had high plasma sex steroids levels show neither smolting nor downstream migratory behavior in this period. From spring through autumn, 2-year-old mature adults which returned to their home rivers and yearling precocious males migrate upstream to their spawning ground. In order to know the involvement of sex steroids in the regulation of migration, testosterone (T), estradiol-17β (E2), 11-ketotestosterone (11-KT), or 17, 20β-dihydroxy-4-pregnene-3-one was implanted into immature fish and castrated precocious males, and downstream and upstream migratory behavior, and spawning behavior were observed by use of an artificial raceway and stream channel. Sex steroids such as T, E2, and 11-KT inhibited downstream migratory behavior, and stimulated upstream migratory behavior and spawning behavior. These findings suggest that sex steroids are the factors stimulate mature masu salmon to migrated toward their spawning ground.
