Even within the Equatorial Countercurrent the density of 

 distribution varies 'jvith the season as has already been stated 

 and as will be set forth in a later chaptero When the varia- 

 tions in the density of distribution in the northern and 

 southern hemispheres are compared by means of the catch rates, 

 there appears to be an increase in density in the summer in 

 both hemispheres, but unfortunately because of the paucity of 

 data from the southern hemisphere it is not possible to make 

 a thorough examination of the correlationc For the northern 

 hemisphere there are insufficient data on the situation in the 

 vicinity of the Equator during the sumirier and so the picture 

 is not very clear j, but on the curve shown in figure 5 the peak 

 which appears in the vicinity of 20° No is extraordinarily 

 clear and a tendency can be detected for it to move about 5° 

 to the northo 



Of course no conclusion can be drawn in any case because 

 of the insufficiency of the data, but if the above circumstances 

 are considered in conjunction with each other, it may be thought 

 that the schools seem to carry out a great north-south movement 

 centered around the Equator with the change of the seasonso If 

 this were established as a fact, it would probably give rise to 

 the conclusion that the yellowfin are a single stocko However, 

 before that conclusion can be drawn there is a need for detailed 

 and large-scale studies and surveys „ Considered within the 

 scope of the data actually available now, it is thought that 

 even assuming that the stock itself is single, there must be a 

 number of different age groups in different areas following 

 differing routes of migration^ 



Dr„ Kishinouye had the following to say of the scombroids: 



"Generally speaking of scombroid fishes, large and 

 old are caught at the beginning of the season, while at 

 the end of the season only young and small ones are caughto 



Generally the male fish come first, in the middle 

 of the fishing season the number of both sexes is nearly 

 equal, and at the end of the season the female fish pre- 

 dominate c" 



If these tTATo quotations are combined, they must mean that 

 the large, mature fish which are taken at the beginning of the 

 season are males, the medium-sized ones that are taken in the 

 middle of the season are equally divided between males and 

 females, and the small ones taken at the end of the season are 

 female So 



Such phenomena may have an extremely important significance 

 both to the study of the ecology of these fish and to the actual 

 operation of the fisheries o The author has investigated a 



22 



