Table 1. --Number of fish measured 



the smallest size participating in spawning. In 

 a rough approximation the following relation- 

 ship can be postulated for the size compositions 

 shown in a - d. 



a - b .- advanced age group .= c + d 



This is thought to mean that ( 1) on appearances , 

 at least, the distribution of medium and small 

 albacore i s limited to the area of the North 

 Pacific Current, or in other words to the north 

 side of the Subtropical Convergence (the bound- 

 ary between the North Pacific Current and the 

 North Equatorial Current). (2) The albacore 

 schools distributed south of the Subtropical 

 Convergence are all made up of mature 

 individuals. (Ueyanagi {1954b) considers the 

 albacore distributed in the North Equatorial 

 Current, at least, to be related to spawning. ) 

 (3) At the season of the changeover from a to h, 

 that is, the time when the albacore in the North 

 Pacific Current shift from a southward to a 

 northward movement, the advanced age group 

 is recruited from the North Pacific Current 



area into the waters south of the Subtropical 

 Convergence*. Consequently, we have on the 

 north side of the Convergence, that is, in the 

 North Pacific Current area, an immature group, 

 and on the south side of the Convergence, that 

 is, in the North Equatorial Current and the 

 Countercurrent, a mature group which has come 

 over from the north side, and this indicates a 

 remarkable ecological difference between the 

 albacore on either side of the Convergence. It 

 is thought that the discrepancy between the 

 length distributions shown in c and d may be 

 due to the marked difference in the sex ratios 

 of the albacore in the two current areas. 

 Figure 3 gives the length frequencies sepa- 

 rately by sexes for the albacore taken in the 



*The shift in fishing method from longline to 

 pole and line can also be thought of as a cause 

 of this, however, it is actually possible to 

 trace the group which crosses the Subtropical 

 Convergence to the southward. 



60 



70 



100 



no 



i2ocin. 



Fig. 2. --Average size composition in each current area. 

 Ordinate represents the permillage. 



a: North Pacific Current area (southward migrating season), 



b: North Pacific Current area (northward migrating season), 



c: North Equatorial Current area, 



d: Equatorial Countercurrent area. 



45 



