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Figure 29. --Yellowfin tuna fishin 

 by months (24 to 30 

 to 150° E.) 



area 

 nnove 

 these 



eaat of the Mariana Islamds, where 



ments and migrations of the school 



schools by nneana of these curves 



As we proceed farther 

 north the yellowfin catch becomes 

 extrennely small and the seasonal 

 distribution of the data becomes 

 extrenmely uneven, for which reason 

 we cannot discuss the distribution 

 of the fish as shown by the catch 

 rates. It appears, however, that 

 the highest catch rates of the year 

 occur in Septennber and October, 

 with only one peak in the curve 

 during the year. This trend be- 

 comes more marked the farther 

 north we go, smd a tendency can 

 also be detected for the peak to 

 appeair earlier. These facts are 

 believed to indicate that we have 

 reached or are approaching the 

 limits of migration and distribu- 

 tion of this species. 



Conaparing the catch 

 rate curves for the various sea 

 areas described above, one can 

 see considerable regional differ- 

 ences in the seasons at which the 

 peaiks and low points appear. There 

 are cases like that of the area east 

 of the Ogasawara Islands and the 

 such differences can be thought to indicate directly the 

 8, but it is difficult to grasp the direction of motion of 



conditions 



N. . 140° 



A peak appears in May through the whole area from 25 N. , south to the Equator 

 (the situation in the South China Sea is unknown because of the lack of data), and it is impossible 

 to detect any trend which would indicate local movennents of the schools from one region to 

 another. In the waters to the west of the Izu, Ogasawara, Mariana, and Caroline archipelagoes 

 a conspicuous peak appears everywhere in October. In addition to this commonly-shjired peak, 

 each area's curve shows a second peak, but the season in which the secondary peaks apjjeaj 

 seems to vary greatly between areas. 



In the case of the bigeye tuna it was stated that some agreement in the trend of the 

 curves as a whole could be detected, even though there were seasonal differences in the position 

 of the peaks jind low points. The same sort of relationship can be detected in the case of the 

 yellowfin tuna. The fact that the trends of the curves show resemblances between areas not only 

 for the yellowfin but for a different species, the bigeye tuna, should have an extremely important 

 significance in studying the distribution and migration of these fishes and the presence or absence 

 of sub-species. 



It has already been mentioned several times that the density of distribution of yellow- 

 fin tuna is high in the vicinity of the Equator and drops off as one goes into the high latitudes. 

 The following graph shows Ccitch rates for yellowfin in blocks of 10 of longitude ajid 2 of latitude 

 E. longitude. Material from the Sulu Sea, the Celebes Sea, and the South 



between 120 and 160 



China Sea, where environmental conditions are different, has been left out of the data for the 



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area between 120 and 130 E. longitude. 



153 



