day of fishing from the Reports of Investigations of the 

 Jishing Situation by Special Reporting Vessels j and the 

 maximum number of elapsed days irom the records of recaptures 

 published in the same Reports!/ 5, and estimate the area of the 

 fishing grounds by counting the number of 1° squares in which 

 catches were made as shown on the charts of the fishing grounds 

 appended to the oceanographic charts published by the Central 

 Fisheries Experiment Station^ and infer the number of months 

 of the fishing season from the nvmiher of fish landed per 10-day 

 period as given in Kimura''s studys,/, and then attenpt to compare 

 by the method described above the number of fish that migrate 

 into the various fishing grounds (table 3) 5. it can be thought, 

 as Okamoto^/ imagined j, that the majority of the fish of age- 

 group III come directly to Kinan without passing through the 

 Satsunan area and from there move north to the Northeastern 

 Sea Area through the waters off Japan proper o 



Now if we assume that the above hypothesis is correct^ in 

 the age corapusilion on the southern fishing grounds the fish 

 of age-group III should show a gradual decrease in the late 

 spring and a gradual increase in the late autmmic. However, 

 according to Aikawa^ (table h) 3 the body-weight of fish of 

 age-group III is I06O =■ 3oU5 kg| if we try to bring in the 

 results of his measurements of the annuli in the vertebrae 

 (table 5)s since it appears that the annuli develop from winter 

 to summer, it can be considered that the body-weight of fish of 

 age-group III is 3o7 pounds in early spring and 7 oh pounds in 

 late autumn, while the weight of fish of age-group IV is around 

 8o27 pounds early in the springe For this reasonj in the 

 composition by large, medium, and small sizes y traces of a 

 recurrence of age-group III can be expected in late autumn but 

 not in early spring. On the Palau fishing grounds studied by 

 Kimur^s ^^ "the peak fishing seasons of spring and autumn, 

 there is clearly discernible in the age composition (table 6) 

 a gradual increase of the medium-sized fish of age-group III 

 in late auturano 



If we assume in this way that after the skipjack have 

 spent their Juvenile period in the region of the South Sea 

 Islands they make thoir- great migration north alorig the KuroshiOp 

 part of them as fish of age-groups II and IV but the majority 

 of them as age-group III^ and that thereafter they remain in 

 southern waters j, it goes without saying that we must go by the 

 composition on the South Sea fishing grounds in calculating the 

 survival rate, which is one of the important characteristics 



1/ Parts published in C5U)s (57) 5 (59), (61), and (63) < 



26 



