Abstract
Observations on the spawning behavior of the fluvial dwarf form of masu salmon, Oncorhynchus masou (Brevoort), were made in the Ishido-dani, a tributary of Hitotsuse River in Miyazaki Pref., in the autumn of 1957 and 1969.During the spawning season from the last part of October to the first part of November, the water temperature was 13.5-15.5°C and the spawning act of a female and several males was observed usually in the pool-end.
In the male group the hierarchy of peck-right type is recognized according to their body sizes and the largest one mates with the female.The paired male immediately drives subordinate males away if they attempt to approach the female.The female defends her site from invaders such as males which are smaller than herself or another female which tries to dig the site of the paired female. The feeling and/or crouch (Fig.6 C and D) of the female is thought to be the signal posture that causes courting of males, and when the paired male is in the similar posture, homosexual courtship of another smaller male is occasionally observed.As the female repeats digging movement at the rate of twice a minute, a shallow depression is made in which the oviposition occurs.At the oviposition, 30-100 eggs are shed into the crevices of gravel remained in the center of the depression.In the postspawning stage only the female remains on her redd and she covers the eggs with sand and smaller gravel of the upstream part from the depression. Several hours after the first oviposition, the males appear again near the female which is making another depression at about 20 cm upstream place from the previous one, and they repeat the same acts as they did in the first prespawning stage.The next oviposition is usually observed 25-30 hours after the first one.Three or 4 days are necessary for 2 or 3 times of oviposition.
