So stated it appears quite simple and clear that if we investi- 

 gate oceanographic conditions p it will naturally become possible to 

 foreknow the migrations of the tunas o The actual problem cannot j, 

 however^ be solved so simply. One reason is thti oceanographioal 

 observations on the large scale necessary in order to gain a tho= 

 rough grasp of oceanographic changes in the sea areas inhabited by 

 the tunas cannot be so easily carried outo Even supposing that such 

 observations could be made, there are in addition to sucn physical 

 factors as temperature and salinity of the sea water other com- 

 plexly intertwined and as yet unknown factors involved in the mutual 

 relationships of the tunas with other living organisms j> so tha^ it 

 can hardly be expected that we can easily gain a foreknowledge of 

 the causes and paths of their migrationsc At any rate^, within the 

 scope of our present knowledge it is not^ unfortunately, possible 

 to find a method by which these migrations can be accurately fore- 

 tcldo 



io Fishing grounds of the low latitudes and of the high 

 latitude So 



Because the tunas are in general so-called warm water 

 species i they do not like very low temperatures. The species 

 differ in this respect^ but each has limits on the temperature 

 range within which it can exist and the fish are continually on 

 the move seeking favorable waterso In sea areas which are 

 bounded by zones of limiting temperatures as for example in the 

 fishing grounds of the Hokkaido area or at the advance point of 

 the gradual northward migration of the schools which begins in 

 the spring, the schools are continually being restrained by 

 limiting temperatures and their progress is a series of alter= 

 nate advances and retreatSo In fishing grounds where such 

 factors operate i the relationship between oceanographic condi-' 

 tions and fishing conditions is very apparent and observations 

 of the water temperature contribute immediately to the fishery 

 as an effective indicator for the location of fishing grounds,, 

 Also in mary cases the fishing season is clearly defined^ and 

 when the season unsuitable for the habitation of tuna comes 

 around, they disappear and the fishery likewise comes to a com-> 

 plete endo 



In the warm seas of the low latitudes the water temperature, 

 salinity, and so forth always offer favorable conditions for 

 the habitation of the tunas^ and, in fact, it is knoiwn that most 

 of the species are at all times precent there,. It follows that 

 the fishing grounds and fishing seasons are exceedingly general- 

 izedo However, even in the low latitudes the density of dis- 

 tribution is not always the same, but shows rather marked 

 changes both as between different seasons and as between dif- 

 ferent areaso In this area the year is divided roughly into 

 two seasons, that of prevailing southwesterly winds from April 

 to September and that in which northeasterly winds prevail from 



87 



