FISHERY BULLETIN: VOL. 70. NO. 3 



in the eastern Pacific (east of long 130°W), by- 

 few fish with fully developed gonads (Schaefer 

 and Orange, 1956; Orange, 1961) and few larvae 

 (Matsumoto, 1958; Klawe, 1963), and the oc- 

 currence of some long distance tag returns indi- 

 cating movements from Baja California to the 

 central Pacific, including the Hawaiian Islands 

 (Schaefer, 1963; Inter- American Tropical Tuna 

 Commission, 1964). (Subsequent data on skip- 

 jack larval distribution and tagging have sup- 

 ported the inference for a central Pacific origin.) 

 Schaefer's and Rothschild's opinions differed, 

 however, as to whether skipjack from south of 

 the equator entered into this common genetic 

 pool. Fujino (1970) indicated there was no ge- 

 netic evidence to suggest South Pacific skipjack 

 are not part of the central-east Pacific subpop- 

 ulation(s) and also stated that present evidence 

 for any differences between skipjack from vary- 

 ing parts of the central-east Pacific (Hawaiian 

 Islands, Line Islands, Ecuador, Baja California) 

 is very slight. 



Rothschild (1965) generated a hypothesis on 

 the movements of skipjack within the central- 

 east Pacific ( Figure 1 ) . He postulated that skip- 

 jack juveniles in the central Pacific are contin- 

 ually dispersing from that region and that a 

 large component of them move eastward into the 

 eastern Pacific, where one contingent enters the 



MEXICAN 

 FISHERY 



CENTRAL e 

 SOUTH 



AMERICAN 

 FISHERY 



Saaaonol 

 a Olffuaioi 

 Movamanla 



Figure 1. — Diagram showing the flow of skipjack be- 

 tween the central Pacific and the Mexican and South 

 American fisheries of the eastern Pacific Ocean (from 

 Rothschild, 1965, Figure 2). 



Baja California fishery and the other the Cen- 

 tral-South American fishery. Recruitment to 

 these fisheries commences at about 35-40 cm 

 (Rothschild, 1965; Joseph and Calkins, 1969). 

 The fish remain in the eastern Pacific until they 

 are maturing or mature at 40-65 cm, which 

 probably is for 12 months or less, but only rarely 

 longer than this. Thereafter, the fish leave the 

 region and return to the presumed spawning 

 grounds in the central Pacific and do not usually 

 return to the eastern Pacific surface fishery. 



The boundary between the two eastern Pacific 

 fishery groups appears to be centered on the 

 Gulf of Tehauntepec, with the northern fishery 

 mainly off Baja California and the southern one 

 mainly off Central America and Ecuador. There 

 appears to be little mixing between these fishery 

 groups as seen by lack of intergroup returns 

 (Joseph and Calkins, 1969; Fink and Bayliff, 

 1970) and morphometric analysis (Hennemuth, 

 1959) even though genetic data (Barrett and 

 Tsuyuki, 1967; Fujino, 1970) show no gross in- 

 tergroup differences. The degree of geographic 

 separation of the groups varies considerably 

 from year to year (Williams, 1970). 



According to Rothschild (1965) the mecha- 

 nism causing the split into northern and south- 

 ern fishery groups could be many and not fixed 

 in time or space. The extent to which they split 

 the skipjack into the northern and southern 

 groups could be a function of the north-south 

 and temporal distribution of the incoming east- 

 ward-moving recruits. One possible splitting 

 mechanism was considered to be the warmwater 

 cell (surface temperatures>28°C) in the vicin- 

 ity of lat 15°N off the Central American coast 

 (see monthly average temperature conditions, 

 Wyrtki, 1964). Blackburn (1962) suggested 

 that this same warmwater cell impeded north- 

 south movement and intermingling of the groups, 

 the extent of which varied with surface temper- 

 ature from' year to year. Williams (1970) has 

 shown that in the eastern Pacific, skipjack occur 

 at all temperatures >17°C with the majority 

 from 20° to 30°C, though apparent abundance 

 was only high up to 29°C. This increase of 1°C, 

 from 28° to 29°C, in the limiting temperature 

 for skipjack in quantity is important. The 

 monthly average temperature charts of Wyrtki 



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