As the foregoing table shows g 



Eo The range of water temperatures within which 

 fish were taken was 18° -- 26^0 



bo The most favorable water temperature changes with 

 the season (during the fishing season of June - 

 November) from a lower to a higher and then back again 

 to a lower temperature o 



Iherefore in the choice of fishing grounds it is essential 

 to take the 20° isotherm as the center off the Boso Peninsula 

 (Chiba Prefecture) u^ to May =■ June^ and the 22° isotherm off 

 Tokiwa and Sanriku (northeastern Japan) from July to September, 

 or in other words to seek the most favorable temperature for 

 the season^, as given in the preceding tableo Ihis should be 

 clearly apparent from the fact that over kO percent of the total 

 catch was taken from fishing grounds with these temperatures o 

 (In the middle of summer there are also sometimes fishing 

 grounds on which 2ii° is the most favorable temperature o These 

 will be discussed latero) Of couxsej, in making this choice it 

 should be within awann-current area where the water color is 

 2 ■= 3, the transparency about 20 meters, and the specific 

 gravity about 1,025° Particular attention should always be 

 paid to the pattern of the northward movement of the 20° 

 isothermo 



(2) Lines of disdontinuity and the distribution of 

 skipjack fishing grounds within the areas of 

 favorable water temperatures in the Northeastern 

 Sea Area 



Even in such areas of favorable water temperatiiresj the 

 actual distribution of the skipjack fishing grounds is by no 

 mears uniform, and there is a tendency for them to be concen- 

 trated unevenly here and there. Furthermore j there are great 

 differences in the density of the schools even though the water 

 temperatiores may be the sameo Tflfhat sort of oceanographic 

 factors may give rise to this phenomenon? It is believed that 

 in this connection the following factors cannot be overlooked? 



ao As stated earlier, the main fishing grounds are always 

 distributed within the area of favorable water tempera- 

 tures having surface water of 20° = 2ii°, with their 

 center at the 22° isotherm^, but in view of the fact 

 that the schools of fish assemble where there are many 

 irregularities in the isotherms and where the isotherms 



