The material on which the present papar is based was taken from the daily records made by the Tokyo Fisheries Experimental Station during the fishing season of the 3 years (1938-40.
The water-and air-temperatures relate positively with the catch of the young “Ayu” in the period from the middle part of March to late April, but they relate negatively from late April to May. In these two periods, the water temperature runs 7-15°C and 13-22°C respectively. Accordingly it seems that the optimum water temperature for the anadromous young “Ayu” ranges 14-16°C.
Further the catch of the young “Ayu” seems to be influenced by rainy weather, as well as direction and intensity of wind. Unsual catches more than 100% against the standard haul obtained by three-yearly average occur more frequently in the case of the breezy south wind, and those less than 50% from the standard on take place more frequently in rriny weather. The raeson of the former appears to be that the southern winds are favorable to g ather the shoals of the young “Ayu” of the sea to the much of the river in which the down-stream runs straightly southwards. But that of thc latter is unknown.