FISHERY BULLETIN: VOL. 70, NO. 3 



related to the waters of the Equatorial Front, 

 especially those of Peru Current origin. 



The adult skipjack at this period would nor- 

 mally remain offshore in the eastern Pacific. 

 When the adolescents leave the feeding grounds 

 they, and the adults, are maturing and would 

 move westwards in the SEC (north and south 

 of the equator) , and possibly even in the NEC, 

 to the central Pacific spawning grounds. There 

 is some indication from the data of Miyake 

 (1968) that maximum occurrence of longline 

 caught skipjack was at about lat 10°-15°S east 

 of long 105° W, at lat 5°-10°S from long 105° 

 to 130°-140°W, and at lat 0°-5°S west of long 

 130°-140°W, that is tending towards the equa- 

 tor away from the coast. This could be related 

 to equatorial low temperature areas west of the 

 Galapagos Islands. 



A principal problem with a gyral model for the 

 southern fishery group concerns the entry of 

 larvae and early juveniles into eastbound cur- 

 rents. For those originating in the equatorial 

 areas of the NEC, or the SEC north of the equa- 

 tor, access to the NECC through known circula- 

 tory mechanisms is relatively simple (similar to 

 that in the passive or northern gyral migration 

 models) . For larvae and early juveniles of this 

 group south of the equator the problem is more 

 complex. Here the SEC has a southerly compo- 

 nent in the westward transport tending for pas- 

 sively drifting animals to be carried away from 

 the equator. For those between lat 0° and 5°S 

 a passive movement northwards and across the 

 equator, and hence into the NEC, could occur 

 under the stress of unusually strong southeast or 

 south winds, the equatorial divergence then be- 

 ing farther south than usual. 



This problem related to skipjack young stages 

 south of the equator raises the question of the 

 role of the SECC in the southern gyral migra- 

 tion model. Migration in the subsurface SECC 

 is improbable as mean temperatures are about 

 14°C (M. Tsuchiya, personal communication). 

 However, the existence of a surface SECC, even 

 if as narrow as 120 miles, could be of importance 

 in completing the gyre south of the equator (see 

 Figure 7C) . Larvae south of the equator in the 

 central Pacific would tend to be carried towards 

 the SEC/SECC boundary, and then eastwards 



with it. As mentioned earlier EASTROPAC and 

 other data showed increased "productivity" in 

 the general region where a surface SECC might 

 be expected (lat 10°-15°S), and this would ob- 

 viously be of importance in development of the 

 juveniles. Subsequent movement of juveniles 

 from the surface SECC terminus would presum- 

 ably be in the form of an active migration. There 

 could also be some type of link with the Peru 

 Countercurrent. In the southern summer ado- 

 lescent skipjack often occur off northern Chile 

 and the north-south migration route of these fish 

 must be offshore in view of the low coastal tem- 

 peratures off Peru. Fish that migrated onshore 

 in the SECC would move, after feeding off South 

 America, westwards in the SEC and return to 

 the central Pacific. 



Obviously timing in the two gyral migration 

 models would be different, even though they 

 share a common eastbound current, the NECC. 

 When this current is flowing through to the 

 coast, it would carry recruit skipjack of two dif- 

 ferent sizes representing the two groups. South- 

 ern fishery group recruits would be considerably 

 larger (4-5 months older) than those of the 

 northern group. Separation of the juveniles of 

 the two groups at the NECC terminus would be 

 little problem as only those of southern group 

 origin would be in the developmental physiologi- 

 cal state requisite to active migration. Those 

 juveniles belonging to the northern group would 

 continue their passive migration. Intermingling 

 of groups could occur if some northern group 

 juveniles were passively carried into southern 

 fishery areas especially off Central America. 



As in the passive migration model (see page 

 753), to account for the size differences in in- 

 coming recruits, one would have to propose that: 

 the southern group spawning grounds are far- 

 ther west than those of the northern group, and/ 

 or the transfer mechanisms involved in the 

 movement of young stages into eastbound cur- 

 rents are complex and take longer (certainly true 

 if no SECC). Either or both of these proposals 

 could account for southern group juveniles, 

 spawned November to April, not reaching the 

 area of long 130°W at the time when the NECC 

 has stopped east of that meridian. Alternatively, 

 if juveniles from the southern spawning group 



756 



