WILLIAMS: MODELS OF MIGRATION OF YOUNG SKIPJACK 



and adjacent areas of the NEC and the section 

 of the SEC north of the equator (Figure 7B). 

 This implies a navigational system in the adults. 

 Thus, spent and spent-recovering fish would be 

 expected in the areas immediately to the east of 

 the spawning grounds. When the juveniles are 

 off Central America and Mexico, the adults have 

 also turned north and are located farther off- 

 shore and/or in deeper water. [It should be re- 

 called that tag returns show little interchange 

 of fish of the size caught in the eastern Pacific 

 between the northern and southern fisheries 

 (Fink and BayliflF, 1970).] 



As the gyral group approaches the southern 

 part of the northern fishery area, south and 

 southeast of the Revillagigedo Islands, the ju- 

 venile skipjack actively migrate out of the gyre 

 into the waters around the Revillagigedo Islands 

 and subsequently into the feeding grounds off 

 Baja California. The cue for the start of the 

 active migration of the juveniles out of the gyre 

 could involve a summation of external stimuli, 

 such as those received on encountering Califor- 

 nia Current Extension (CCE) water, coupled 

 with internal endocrinal stimuli, such as in- 

 creased thyroidal and gonadal activity. Some 

 adults migrate with the juveniles but the ma- 

 jority probably remain offshore. With the 

 southerly advance of cold water (18°-21°C iso- 

 therms) along the west coast of Baja California 

 in late fall, the adolescent skipjack leave the area 

 of the northern fishery (Blackburn, 1969a; Fink 

 and Bayliff, 1970; Williams, 1970). This they 

 do via the CCE and rejoin the gyre, although no 

 longer dependent on it for movement, the gen- 

 eral transport of which is in a west-southwest- 

 erly direction (NEC) . The adolescent skipjack, 

 now maturing for the first time, and the adults 

 thus return to the spawning grounds in the cent- 

 ral Pacific in time for the next spawning season. 



With regard to timing of movements in the 

 gyre, the only real fact is that the principal re- 

 cruitment to the offshore areas of the northern 

 fishery, the Revillagigedo Islands, lasts from 

 about March to June, with movement to Baja 

 California waters normally from May onwards. 

 A continuous passive migration of young stages 

 from long 130°W to 85°W in the NECC and then 

 to the Revillagigedo Islands in coastal currents 



might take about 9-10 months (see page 753). 

 Backtracking from the entry time at the Revil- 

 lagigedo Islands would suggest movement past 

 the meridian of 130 °W from about May- June to 

 August-September. This is close to the suggest- 

 ed principal spawning season. May to October, 

 west of that meridian for the northern spawning 

 group. The adolescent skipjack leave offshore 

 Baja California waters principally from Novem- 

 ber to January and, even if only moving at the 

 speed of the current (CCE, then NEC), there is 

 adequate time to reach the central Pacific west 

 of long 130°W by spawning time. 



The time between the proposed spawning pe- 

 riod and first entry into the ofl'shore areas of the 

 northern fishery is about 10-12 months and might 

 suggest that the higher of the growth rates men- 

 tioned earlier (see page 754) is the more likely 

 in the first year, i.e., >40 cm. With a spawning 

 period of May to October the proposed passive 

 transport of juveniles east of long 130° W coin- 

 cides with the period when the NECC flows 

 through to the Central American coast. When 

 the gyre is interrupted by the breakdown of the 

 NECC east of long 120°W from February to 

 April, there are probably no northern group 

 young stages to be transported eastwards. 



It is proposed that the skipjack forming the 

 southern fishery group (from which the catch 

 is usually much greater than that in the north- 

 ern fishery) spawn principally in the southern 

 spring and summer, November to April, in the 

 central equatorial Pacific. A large proportion 

 of the larvae and juveniles eventually enter the 

 NECC where development ensues during the pas- 

 sive migration eastwards towards the American 

 coast (Figure 7C) . As with the northern group, 

 movements of the southern group adults would 

 parallel the juveniles in the NECC but would be 

 primarily in the SEC. On reaching the terminus 

 of the NECC, the juveniles migrate actively into 

 the offshore areas, and subsequently the feeding 

 areas, of the southern fishery off Central and 

 South America (mainly off Ecuador) . It is dif- 

 ficult to suggest a specific environmental cue 

 which may act as the external stimulus involved 

 in triggering this change from passive to active 

 migration. Perhaps in the southern part of the 

 area of the NECC terminus the cue could be 



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