(1) The yellowtail set net fishing of the “otoshi-ami" type is operated in many localities all along the Japanese coasts. At a fishing village where this fishing is conducted, economic prosperity of the community is profoundly influenced by the success or failure of the yellowtail catch. This fishery may be said to be one of such fisheries to which the prosperity and decline of a fishing village is closely linked.
(2) In spite of the importance of this fishery，little is known about the natural mechanism which causes the run of yellowtai1 to a particular fishing ground to vary from year to year. This mechanism has been speculated upon，bu t is left to be c1arified in many respects.
(3) In this study，attempt is made，o n the basis of an assumption，to supplya guiding principle for forecasting the run of the yellowtail into a coasta1 set net fishing ground.