As to the sex ratio of the Ayu crowding together at the spawning ground, it has been known that in the early period of the spawning season males predominate, while, on the contrary, towards the end of it females predominate. But no attempt has been made to investigate thoroughly the matter.
The present study was undertaken to study this problem. For this purpose, observations were made, on the one hand, on the seasonal changes of the sex ratio in connection with those of the maturity, population size and quantity of eggs spawned at two well-known spawning grounds of Sagami River in Kanagawa Prefecture. On the other hand, the spawing behaviour was observed in detail at artificial spawning beds which were prepared in experimental ponds.
The results obtained were as follows:
1) The proportion of females to the population exhibited marked seasonal changes. The number of males increased gradually from the early period of the spawning season to the height of it, where reaching its maximum. Thenceforth it gradually diminished (Fig. 1).
2) Parallelisms and also high correlations were found to exist between the proportion of male and population size or maturity (Fig. 2, 3). Furthermore the amount of eggs spawned showed a tendency to be largest when the proportion reached the maximum (Table 1).
3) Males, as the gonads matured, crowded together densely at the spawning beds. By this prespawning behavior silts laid on the gravel of the beds were removed. A few attendant females joined the male shoals, thus depositing their eggs. From these observations it would be assumed that the great proportion of males in the height of spawning was ascribed to such a spawning behaviour.
4) During the actual spawning process, much gravel of the beds were moved by fish, thus a circular hollow being formed (Table 3). A high correlation between the amount of the movement of gravel, which seemed to indicate the intensity of the spawning activity, and the quantity of eggs spawned was observed to exist (Fig. 7).