are close togetherj or in other words at marked 

 current boundaries (lines of' discontinuity) j it 

 may be thought that these current boundaries are 

 the greatest factor in causing the unequal concen- 

 tration of the schools o Especially in the case of 

 the main fishing grounds of midsummer, there is a 

 tendency for them to be located in the area of the 

 most conspicuous boundaries between the cold current 

 system and the northern-most extension of Uie 22° 

 isotherm (where the cold and warm current systems 

 approach each other and the gradient of the water 

 temperature is the steepest) j within which area they 

 tend to appear at the tips of the warm water masses 

 or on their west sides (the east sides of the cold 

 water masses) o (See the map of the distribution of 

 the main skipjack fishing grounds and the surface 

 water tempera tureso) 



Notes Ihis' corresponds with the rule concerning ttie 

 fishing situation insisted upon by the late MTo Kitaharap 

 "The schools of fish are numerous along the line where two 

 currents impinge on each other o" 



Sometimes in August a second major fishing ground}, distinct 

 from those described above, is found in an area of high tempera- 

 tures and conspicuous current boundaries centering on the 2U° 

 surface isotherm, and in such' years the catch is heavyo 



bo Next we can cite the factor of a rich and plentiful 

 supply of plankton along these current boundaries 

 accompanied by a concentration of skipjack in search 

 of foodo Howevers it is believed that the main factor 

 causing the concentration of schools at the current 

 boundaries must be chance gatherings of schools which 

 cannot escape from the areas in which they find them- 

 selves befcause of vortical movements where warm water 

 penetrates the cold watero Ihus it can be inferred 

 that the more marked the impact of the currents along 

 the boundary^ the denser will be the concentration of 

 the schools o (See the cross section of water tempera- 

 tures off Kinkazan and the chart of the positions of 

 the skipjack fishing groundso) 



Notes (a) The "current boundaries" mentioned here are 

 the boundaries where different water systems (such as the waters 

 of the two great systems of the Kuroshio and the Oyashio) meet 

 and mingle (zones of convergence) o 



