?naen they migrate north in the summer these fish sometimes 

 enter the coastal waters, and they generally swim in the surface 

 layero When they are young they sometimes form mixed schools 

 with skipjack, and sometimes school separately by themselveso 



In the general viewj, the densest occurrence of the 

 yellowfin is in the area of the Equatorial Countercurrentj and 

 its north-south extension is comparatively narrowo In the 

 Indian Ocean area, tooj, its densest distribution is in the 

 vicinity of the Equatoro Among the fish of the open seas in 

 these low latitudes small individuals generally predominate^, 

 and it appears that those of around 30 =- 35 kgo are most numerouso 

 The fish which occur in subtropical sea areas and in enclosed 

 waters are generally larger in size^ with an appsirent tendency 

 for individuals of around 40 - 50 kgo to predominateo It is 

 impossible under present conditions to state definitely whether 

 or not such a phenomenon is conclusively establishedo If such 

 v;ere the case, it v^ould be very interesting and would have an 

 important significance ecologically^ 



In the case of the skipjack also, a roughly similar 

 phenomenon is seen in that those which migrate widely in the 

 open sea are said to be generally schools of young fish, while 

 among those that occur in subtropical waters and enclosed sea 

 areas mature fish are said to predominate for the most parto 



Figure 5 shows the distribution of yellowfin with respect 

 to latitude. In the figure the area from 140° Eo to 150° Bo is 

 divided every 2° and the catch rate is calculated for each 10° 

 of longitude and 2° of latitude on the basis of data from 

 fishing ground survey.So 



As the figure shows very clearly, the catch rates of the 

 yellowfin tuna have a south to north slope,. As it moves north- 

 ward from the Equator the catch rate drops rapidly and reaches 

 a low point in the vicinity of 10° - 12° No 



Around 20° No a low peak appears, and northward from this 

 point the catch rate gradually becomes lower o This figure shows 

 the range from 140° Eo to 150° Eo , but if the range of 120° - 

 160° Eo is divided every 10° and the same sort of catch rate 

 curve is drawn, the trend of the curves coincides perfectly, 

 although there is some difference in the catch rateSo In other 

 words, the distribution of yellowfin tuna in the western Pacific 

 does not show any great variation with longitude, that is in an 

 east-west directionp but may be said to be related mainly to 

 the latitude with north-south variationo 



As noted above^, the distribution of yellowfin is extremely 

 dense in the vicinity of the Equatoro However, as will be shown 

 in a later chapter, the distribution varies a good deal seasonally 

 and they are not distributed at all uniformly throughout the year, 

 (See figure 10)c 



20 



