FISHERY BULLETIN: VOL. 70. NO. 3 



Figure 6. — Passive migration model: (A) routes of young skipjack into the southern fishery and (B) routes 

 of young skipjack into the northern fishery. NEC = North Equatorial Current; NECC := North Equatorial 

 Current; SEC = South Equatorial Current; SECC = South Equatorial Countercurrent. 



Many of the larval and early juvenile skipjack 

 in the central Pacific would originate in, or be 

 involuntarily transported into, the NECC and 

 while developing in this current (few larvae are 

 found east of long 130° W) would be transported 

 eastward with it. Although generally increas- 

 ing in a west-east direction the productivity, in 

 terms of zooplankton, of the NECC appears rel- 

 atively low (Reid, 1962), though the incidence 

 of small fronts in the NECC area (Murphy and 

 Shomura, 1972) would be likely locations for ag- 

 gregations of food organisms. The rapidly 

 growing and increasingly active skipjack juve- 

 niles would need therefore to forage extensively 

 for food and, in view of the large numbers in- 

 volved, this may also necessitate foraging near 

 the edges of the "productivity" bands at or close 

 to the NECC boundaries (the net transport of 

 the fish would necessarily be eastwards) . 



The concept of a passive migration eastwards 

 with the NECC eliminates the otherwise difficult 

 question of orientation in the young fish. The 

 dispersion of juveniles from the NECC, at its 

 nearshore terminus, to the southern fishery 

 would be relatively simple and similar to that 

 in the active migration model (Figure 6A) . For 

 recruitment to the northern fishery the problem 

 is more complex, depending on whether the fish 

 destined for that area are transported when the 



NECC flows through to the coast or when it is 

 intermittent or stopped east of long 120 °W, 



The surface current system along the central 

 American coast (Wyrtki, 1965), at least from 

 May- June to December, is such that juveniles 

 could be passively transported from the near- 

 shore terminus of the NECC to the Revillagigedo 

 Islands area (Figure 6B) . However, from about 

 April to September warm water cells (>29°C) 

 of varying sizes off Central America would pre- 

 vent transport of the juveniles close to the coast 

 and with increasing offshore distance the risk 

 would increase of passive transport westwards 

 into the NEC and not to the Revillagigedo Is- 

 lands. With the principal influx of fish into the 

 offshore areas of the northern fishery being from 

 March to June, approach to these areas would 

 need to be taking place in the first months of the 

 year, and this is in fact the time when the coastal 

 current system appears least likely to support 

 such a passive migration. Thus, there are some 

 real problems of timing with the passive mi- 

 gration of young skipjack into the northern fish- 

 ery in the NECC and Central American coastal 

 currents. 



If recruitment to the northern fishery takes 

 place after the cessation of the NECC east of 

 long 120°W, then it would require an active mi- 

 gration from the terminus of the NECC to the 



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