1. Grounds North of the Subtropical Convergence 

 (1) North Pacific Ground 



The most notable point is that the fishing grounds, which until 

 March were nnoving southward, in April come to be distributed in two 

 narrow zones lying separate but parallel in an east-west direction, with 

 albacore fishing active only on the northern side. Signs of this develop- 

 ment can already be seen in March, but in April it becomes extremely 

 clear. For convenience in explanation, these will be referred to as the 

 northern zone and the southern zone. The northern zone runs east and 

 west, centered on 30 N. latitude, while the southern zone, also running 

 east and west lies between 23 N. and 26 N. The position of the Sub- 

 tropical Convergence in April is at 23 N. to 24 N. between 140 E. and 

 170 E. , while farther to the eastward it runs in the vicinity of 25 N. 

 Consequently, the northern zone is far to the north of the Subtropical 

 Convergence, while the southern zone, on the other hcUid, is thought to 

 consist of fishing ground which have preserved a close relationship with 

 the Convergence. (The fishing grounds which are noted in the vicinity of 

 22 N. to 23 N. or farther south are thought to reflect the effects of the 

 restrictions on fishing areas which obtained from 1945 to 1950.) 

 The albacore fishing conditions and the size composition of the catch in 

 the southern zone show great differences from the northern zone and the 

 North Pacific fishing grounds above it, indicating that the northern and 

 southern zones are of a connpletely different character. The principal 

 catch in the southern zone is striped marlin, and in April rather dense 

 concentrations of this species are seen between 20 N. and 25 N. ; this 

 point indicates that the southern zone corresponds exactly to the northern 

 edge of the range of the striped marlin. 



In the northern zone the positions at which fishing vessels are 

 operating are in the sea area centered at 30 N. and thus are somewhat 

 north of their position in March. In April the fishing season is already 

 near its close and the number of vessels operating has fallen off, so data 

 are few and the values shown for unit areas are not reliable, but from a 

 comparison with the catch rates in March (see table 1 for February) it 

 appears that between 28 N. and 30 N. the catch rates are lower in April. 

 At 30 N. to 32 N. , on the contrary, the April rates tend to be higher. 

 Considering points of this sort, it is thought that the albacore, which con- 

 tinued moving southward until February and stopped in March, have in 

 April begun a simultaneous northward migration both in the east and in 

 the west. Catch rates in the northern zone are still high in April, with 

 many cases of values over 5.0. Many areas with high catch rates appear 

 north of 30 N. As for the intermediate zone, it is not very clear because 

 of the scarcity of data, but it appears that with the passage of time it 



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