The migrationsp fishing groundSj, and fishing seasons of 

 the tunas and the skipjack in Japanese waters have in the past 

 been studied and actually explained mostly on t^e basis of 

 oceano graphical facto rs„ The policy and purport of such studies 

 rest on the idea that the various biological phenomena in the 

 seas are„ in the last analysis j, based on and directly or 

 indirectly controlled by the physical character of their waterso 

 There is no room for doubt as to the rationality of this viewo 

 Studies of this sort might be called the method of direct attacko 



Let us now, however, just consider as a factor in the 

 fishing season and fishing conditions of the black tufta^ the 

 water temperature „ which has hitherto been the factor most 

 emphasized and most widely utilized,. As far as is known at 

 presents the range of water temperatures habitable by this 

 species is very broad, extending from about 5° to 29OC0 This 

 is, of course, considering the whole range of the habitat of 

 the black tuna, and cannot be applied to such limited areas as 

 the waters off Tanegashima or off Hokkaido"c, In such limited 

 limited areas the extent of the fluctuations in oceanographic 

 conditions is not that great, and also the adaptability of the 

 black tuna is probably not such as to enable it to withstand 

 sudden temperature changes of such a magnitude They doubtless 

 do, however, possess a certain degree of adaptabilityj, and do 

 not respond to temperature changes with the exactness of physical 

 phenomeria, as when water freezes at 0° Co and boils at 100° Co 



As has already been stated, near the limits of the habitat 

 the water temperature is a verj' potent indicator of the fishing 

 conditions and the fishing seasons, but it cannot be said to be 

 infallibleo In the low latitudes ^ tdiere the water temperatures 

 are always within the habitable range, their significance and 

 value as an indicator of fishing seasons and fishing conditions 

 are markedly diminished. From the realistic point of view of 

 the operation of a fishery, it would be a very fine thing if 

 there were a more direct, effectivei, and simple indioator. 



There is no doubt that the direct cause of the distribution 

 and migrations of the tunas is related mainly to the distribution 

 of their natural foodo To cite again the example of the black 

 tuna, it is recognized that the schools which during the winter 

 are in the so-called Satsunan Sea Area begin a northward migra- 

 tion in the spring,, The explanation given hitherto for this 

 migration is that "as the water temperature rises in the Kuroshio, 

 the northward movement begins, and by midsummer the fish reach 

 the waters from Sanriku to Hokkaido'c" The author himself has 

 used similar expressionso Thorough notice should be taken, 

 however;, of the fact that this northward migration follows 

 nearly the same track as the migration of the sardine in Japanese 

 water So 



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