1973 年 23 巻 4 号 p. 147-159
In this part, the mode of breeding and some morphological variations of each population of the landlocked Ayu-fish, after the period of their sexual maturation, were investigated. The lake population of the Ayu of small size contained considerable numbers with nearly matured gonads at the end of summer, and they began to migrate to the lower reaches of the streams during the period from late-August to early-September, in crowds of high density. Spawning colonies with highly condensed aggregation of spawners comprising individuals of similar body size were formed at the spawning beds in the shallow parts at the lower reaches of the streams near the estuary. The stream population usually began to migrate to the spawning places in mid-September. Their spawning beds were around the lower reaches of the streams and with fairly larger limits compared with those of the lake population. The size composition, the time and the route of their spawning migrations were rather irregular, and they did not form such a definite spawning colony as stated above. The degree of the maturity of gonads and the mode of arrival to the spawning places suggested that the early ascending group would spawn a little later than the other. The spawners from the lake had higher dorsal fin ray counts than those from the upper streams, and the means differed significantly between both populations. The predorsal length, the numbers and the diameter of the ova, and the numbers of gill rakers of each population were measured or counted. From these results, it was revealed that these characters tended to be correlated with the body length. In other words, these differences between the populations conld be attributed to the differences in their rate of growth.