WILLIAMS : MODELS OF MIGRATION OF YOUNG SKIPJACK 



east of long 120°W. It is considered that from 

 February to April-May, when the surface NECC 

 is absent east of long 120°W, the juvenile skip- 

 jack are being recruited to the northern fishery 

 where they occur in the offshore area (the Revil- 

 lagigedo Islands) from about March to May- 

 June (arrival April- June onwards in Baja Cal- 

 ifornia) . From long 120°W (lat 0°-10°N) about 

 2-4 weeks would be required for the skipjack to 

 reach the Revillagigedo Islands, which agrees 

 closely with the actual occurrence of juvenile 

 skipjack at that location. 



It will be recalled that one of the possible split- 

 ting mechanisms proposed by Rothschild (1965) 

 was the >28°C surface water cell off Central 

 America, but which it has been shown is not well 

 developed out to long 120°W until April, Thus, 

 even if >28°C water was limiting for skipjack 

 (in quantity), and not >29°C water as shown 

 recently (Williams, 1970), then that cell could 

 not initiate the split as early as February. 



The occurrence of small size fish at the Revil- 

 lagigedo Islands in October (± 1 month) in some 

 years is not accounted for in this model. This 

 could occur if in midsummer some recruits des- 

 tined for the southern fishery and located close 

 to the northern boundary of the NECC (about 

 lat 10°N) were deflected northward from their 

 migration route by the >29°C water cell nor- 

 mally stretching southward from Central Amer- 

 ica at that time. 



It is proposed that a gating or shunting mech- 

 anism operates at about long 120°W across the 

 equatorial and northern migration routes of ju- 

 venile skipjack. When the NECC is continuous 

 east of long 120 °W, the gate is open and recruit- 

 ment is to the southern fishery (Figure 5A). 

 Then, with the annual southward movement of 

 the ITCZ, the surface NECC ceases east of long 

 120 °W, and the gate is closed from about Feb- 

 ruary to April-May. Recruitment to the south- 

 ern fishery ceases and instead the flow of recruits 

 is to the northeast and into the northern fishery 

 (Figure 5B). When the gate is closed there 

 is a loss of west-east orientation due to changes 

 consequent on the breakdown of the surface 

 NECC, such as the loss of the current boundary 

 conditions (with NEC and SEC), possible in- 

 terruption (total and partial) of the zonal "pro- 



ductivity" bands, loss of food concentrating 

 mechanisms (minor fronts), etc. The principal 

 question then is how the recruit skipjack be- 

 come oriented to the northeast, i.e., towards 

 the Revillagigedo Islands. There could well be 

 random dispersal of incoming recruits when the 

 gate closes at long 120°W, with only a small pro- 

 portion moving northeastwards into the (Cali- 

 fornia Current Extension and the majority even- 

 tually recycling to the central Pacific. Possibly 

 those fish moving northeast pick up a "food 

 bridge" linking the area around long 120°W in 

 the vicinity of the NECC with the Revillagigedo 

 Islands and Baja California. Such a "food 

 bridge" could be provided offshore by the pe- 

 lagic stages of the red crab (Pleuroncodes plan- 

 ipes) which are found in the California Current 

 Extension (Longhurst, 1967, 1968; Longhurst 

 and Seibert, 1971)', the gradient of abundance 

 of which increases towards the northeast (shore- 

 wards). It will be remembered that the Cal- 

 ifornia Current Extension is strongest and far- 

 thest to the southwest at this period. Red crab 

 are known to form a large part of the diet of 

 skipjack in Baja California waters during the 

 fishing season (Alverson, 1963). A "food 

 bridge" facilitating the northeast movement of 

 recruit skipjack, because of survival value, may 

 have become an inherited behavioral response 

 when west-east orientation is lost around long 

 120°W in the period February to April-May, 



One possible objection to the gating mecha- 

 nism could concern efforts by the juvenile skip- 

 jack to dive below its effects. This is considered 

 unlikely in view of adverse temperatures 

 (<20°C) at depths where subsurface eastward 

 flow is continuous and low oxygen concentra- 

 tions at relatively shallow depths, at least as far 

 south as lat 10 °N, Oxygen content as low as 

 3 ml/liter has been suggested as possibly lim- 

 iting for skipjack (Commercial Fisheries Re- 

 view, 1965), 



PASSIVE MIGRATION MODEL (Figure 6) 



In this model it is hypothesized that larval and 

 juvenile skipjack are passively carried eastward 

 in the equatorial countercurrent(s) from the 

 central Pacific spawning grounds to the offshore 

 areas of the eastern Pacific fisheries. 



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