Abstract
To increase the production of “Ayu” Plecoglossus allivelis (T. & S.), it is necessary to catch the young in abundance near coasts or at estwaries of rivers and release them in rivers. The author studied on the relations between the number of the anadromous “ayu” and the water-temperature, weather, and season. The following results were obtained:
1) The anadromous period is from early April to early June, the water-temperature during the period ranging from 13.0 to 16.0°C.
2) The maximum number of anadromous young “Ayu” is seen at 9-10 A. M. or 1-3 P. M., then follows a decrease. In midnight they also enter rivers, but their number is very small compared with that of the midday.
3) The number of the anadromous “ayu” increases suddenly a few days before bad weather, rainfall or flood.
