small, mediuin, and large iishj, ws gex, table 2o Table 3 shoT.s 

 the months of maximviin catch for each sea area, for each size 

 of fish, taken from the data given in table 2o A few observa- 

 tions based on these two tables concerning the skipjack's 

 migrations are presented belGw, Small skipjack appser in the 

 Satsunan .and Nankaido sea areao in April and reach their 

 raaximijmi abundance in May, b'at in the Z'.inan Sea Area tJrie peak 

 is in June and July, In the Hokkai-Sanriku. Sea Area the 

 somewhat obscure second r!i.axlmuffi which can be seen in Septem- 

 ber may perhaps be tiiought to be due to the northward ruO%'a- 

 ment of schools of small skipjack of the same strain„ Hcvravers 

 in order to explain the appearance of the first peak in May - 

 June in the Hckkai-Sanriku Sea Area, it is probably necessary 

 to imagine the northward movement of a second group of small 

 skipjack whose origin centers around the Zunan islands and 

 reefs at roughly the same time that the small skipjack of the 

 Satsunan area appear-. Next, with regax-d to the medium-sized 

 skipjack, it is thought that the schools which originate in the 

 Satsunan Sea Area around March and April shift the center of 

 their group of schools to the Nankaido ,3ea Area in April and to 

 the Zunan and Kokkai-Sanriku sea areas around J^alyo Another 

 peak in Septsmbsr in the Satsunan Sea Area is thought to be due 

 to the reappearance of do-irnboiind skipjack from the Hokkai- 

 Sanriku. Sea Area. Furthermore, it may be wondered lAihether the 

 peak which can be seen, though somewhat obsc^orely, in April and 

 May in the Zunan and Hokkai -Sanriku sea sreas may not be due 

 to a second group of raedjj^c skipjack originating in the Zunan 

 Sea Area and moving north from therSo As for the large skipjack^ 

 the first maxiriuiTi is the Satsunan Sea Area in Mavo That moves 

 north to the Zur;.an Sea Area in Jul;;- and is thought to form the 

 peak which shows in the Hokkai -Sanriku Seaiii'ea in July and 

 August, but a second peak in the Satsunan oes. Area in May sug- 

 gests a second origin of schools of large skipjack in this sea 

 areao Of course, it is presumed that the number of small skip- 

 jack vfhich grow into medium skipjack, and the n-uE:ber of mediun 

 skipjack that grow into large skipjack during the migration must 

 be great, and iiiis must be particularly taken into consideration 

 in waters where the supply of natural foods is ab'ondant, but 

 because of the paucity of data loith regard to this point, it 

 has been onatted from the foregoing discussion. The point that 

 should be noted in the migration theory propounded above is 

 that it en'vlsicns two strains for large« mediu'i:,, and small 

 skipjack alike, one originating in the Satsunan Sea Area, and 

 the other in the Z'^nan Sea Area, Of course, it has been taken 

 into consideration that the schools of skipjack that move 

 north to the Hokkai -Sanriku Sea Area in the summer retreat 

 southward again in the fallo It is probably correct to 

 consider that in the Hokkai -Sanrikii Sea Ar-ea there is no year- 

 round occurrence of skipjack but a seasonal migration, because 

 the high temperature water masses of 20"^ Co and above that are 

 present on the fishing grounds off Sanriku in the summer change 



57 



