FISHERY BULLETIN: VOL. 70, NO. 3 



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 Migralron routes 



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Migration routes 



Possible addilionol rout«s 



Figure 7. — Schematic representation of gyral mi- 

 gration model: (A) routes of young skipjack into 

 the northern fishery, (B) routes of adult skipjack 

 of northern fishery, and (C) routes of young skip- 

 jack into the southern fishery. NECC = North 

 Equatorial Countercurrent; SECC = South Equa- 

 torial Countercurrent. 



major ocean currents. Such movements appear 

 to be counterclockwise in the southern hemis- 

 phere and clockwise in the northern hemisphere. 

 The exception is in the eastern North Pacific 

 where movement seems to be counterclockwise 

 and corresponds to the narrow gyre of the north- 

 ern equatorial water mass. 



The above findings have suggested consider- 

 ation of another type of model for skipjack, the 

 gyral migration model, which involves all life 

 history stages and both passive and active mi- 

 grations. [Generalized gyral migration patterns 

 are briefly discussed in Harden Jones (1968).] 



For the northern fishery group it is proposed 

 that the fish are moving counterclockwise around 

 a zonally narrow equatorial gyre consisting of 

 the NECC and the NEC, with the western limit 



some considerable distance west of long 130 °W 

 (possibly as far as long 170°W). In this model 

 it is considered that mature skipjack spawn pri- 

 marily in the northern spring-summer (May to 

 October) in equatorial waters west of long 

 130°W, a large proportion of the subsequent 

 larvae and early juveniles entering the NECC 

 and with development then taking place in that 

 current as proposed in the passive migration 

 model (Figure 7A). The majority of the ju- 

 veniles on reaching the terminus of the NECC 

 are carried northwestwards with the coastal cur- 

 rent off Central America, outside the warm- 

 water cells >29°C. Movements of the adults 

 would generally parallel the young stages, but 

 they are in no way restricted to the NECC and 

 may be found in a wide band covering the NECC 



754 



