WILLIAMS: MODELS OF MIGRATION OF YOUNG SKIPJACK. 



(1964) show the area of water >29°C is much 

 less extensive than that >28''C, particularly 

 offshore to the westward, and also in duration, 

 e.g., in April-May 28°C water normally extends 

 out to long 118°-119°W, but 29°C water only to 

 long 102°-103°W. Even if the 28°C water was 

 accepted as a splitting mechanism, it is shown in 

 a subsequent section that not being fully devel- 

 oped in offshore extent (to long 120°W) until 

 April, it is too late to initiate the separation of 

 incoming recruits into the northern and southern 

 fishery groups. 



ENTRY OF RECRUITS INTO AREAS 

 OF FISHERY 



Fink and Bayliff (1970) discussed the migra- 

 tion of skipjack in the inshore areas of the east- 

 ern Pacific based on tagging experiments from 

 1952 to 1964 (Figure 2) . The data are substan- 

 tial for the immediate coastal areas of the fishery 



but much less so for the offshore island areas. 

 In the northern fishery (Figure 2A) the prin- 

 cipal entry point for small fish is the Revilla- 

 gigedo Islands (lat 19°N, long 111°W) in April, 

 and from there they move inshore from about 

 May to June. In the southern fishery (Figure 

 2B) the situation is more complicated, but the 

 entry of small fish only appears confirmed for 

 the northern Panama Bight in April (and also 

 into the Gulf of Guayaquil). 



In view of the indications of size-specific move- 

 ments through certain areas of the eastern Pa- 

 cific fishery (Rothschild, 1965), the skipjack 

 length-frequency data of the Inter-American 

 Tuna Commission (lATTC) for 1954-1967 were 

 reexamined, albeit subjectively, to obtain pos- 

 sible information on time of entry of young fish. 

 Size range in the eastern Pacific fishery is about 

 36-74 cm, but more usually 42-62 cm. [Eastern 

 Pacific length data from the U.S. fleet are 

 selective because of the California minimum 



FiGXJRE 2. — Inshore migration of skipjack of the (A) 

 northern fishery group (from Fink and Bayliff, 1970, 

 Figure 89) and (B) southern fishery group (from 

 Fink and Bayliff, 1970, Figure 90) based on tagging 

 data; numbers refer to months. 



743 



