low latitudes the distribution of the tunas is by no means 

 everywhere equalo Unless one has as much knowledge as can 

 possibly be obtained concerning the regional variations in 

 fishing seasons and fishing grounds, as set forth above, 

 venturing forth to fish in distant waters is fraught with the 

 danger of unforeseeable lossesc 



In connection with the fact that in Japan the main season 

 for tuna longlining is the winter, Kimrura'^^^ has put forward 

 the explanation that "the fishing season is the season of con- 

 vection currents in the ocean, that is, the winter, when 

 convection currents are set up in the sea water and the tempera- 

 tures at the 50-100 meter level become roughly the same as the 

 surface temperatureSo'n24 J 



itich of the data cited above, however, show rather that 

 the density of distribution increases in the summer, and the 

 season of migration appears to be different for each specieso 

 It follows that there is great doubt as to whether or not a 

 sweeping explanation like the foregoing can be applied generally 

 without modifications. The fact that the fishing season tends 

 to be in the winter may probably, more simply considered, be 

 ascribed to its relationship to the skipjack fisheryo 



iio Ocean currents and fishing grounds 



The ocean currents each have their different physical 

 characteristics, and they have the greatest effect on changes 

 in the oceanographic conditions in the sea areas which belong 

 to their systemso They therefore naturally bear a very important 

 relationship to the fishing seasons and fishing grounds of the 

 tuna fishery^ 



The tuna grounds of Japanese waters are for the most part 

 under the direct control of the Kuroshioo Furthermore, the grounds 

 north of the Bo'so" Peninsula are directly affected by the Oyashio 

 in its reciprocal relationship with the KuroshiOo This has 

 already been set forth in outline in the section on '^Distribution 

 and Migration,," 



Even in cases where there impinge upon each other two 

 currents which, unlike those thatp like the OyashiOp have com- 

 pletely negative characteristics as regards the distribution of 



■JJ-, — 



'' ' Kimura, Kinosuke, 1942o Tuna and spearfish fishing 



(24) 



conditions. Lectures on Fisheries Technology,, 



The albacore and the big-eyed tuna are the main species 

 whose catch increases in the winter,, 



96 



