WILLIAMS: MODELS OF MIGRATION OF YOUNG SKIPJACK 



(southern fishery group) were somehow retained 

 west of long 130°W from February to April, 

 when the NECC is stopped, then this would re- 

 duce the apparent distance westv/ard from the 

 coast that the entry times to the southern fish- 

 ery would otherwise imply. 



The existence of a surface SECC would cer- 

 tainly facilitate the recruitment of juvenile skip- 

 jack (of southern origin) into the southern fish- 

 ery. However, one would expect such a current 

 to be highly variable in space and time, and hence 

 such recruitment would be similarly affected. 



GENERAL COMMENTS AND RECENT 

 RESEARCH 



An active migration of juvenile skipjack from 

 the central to the eastern Pacific would be cate- 

 gorized as a "between habitat migration," using 

 the definition of Nakamura (1969). Similarly 

 with the gyral migration models, the active mi- 

 gration of juveniles from the gyres to the feed- 

 ing grounds would fit this definition. This type 

 of migration, Nakamura suggested, takes place 

 following a change in ecological state, for which 

 one might perhaps synonomize developmental 

 physiological state. Nakamura also hypothe- 

 sized that "between habitat migrations," which 

 entail changes in environment (current systems, 

 water masses) , occur principally at the equinox- 

 es, about March in the northern hemisphere and 

 September in the southern hemisphere, and that 

 the movements are rapid and on a large scale. 

 In the eastern Pacific the principal recruitment 

 into the offshore areas of the northern fishery 

 starts about March and lasts to about June; there 

 is also some recruitment about October (±1 

 month). However, recruitment to the offshore 

 areas of the southern fishery appears to be over 

 a considerably longer period, November to April, 

 and perhaps from as early as August (± 1 

 month) . 



The Bureau of Commercial Fisheries (now the 

 National Marine Fisheries Service), Honolulu, 

 undertook five quarterly cruises, from May 1969 

 to May 1970, between lat 12°N and 3°30'S along 

 the meridian of long 145°W, to consider the dis- 

 tribution and abundance of skipjack in the equa- 

 torial current systems. Preliminary data from 



these cruises (Walter M. Matsumoto, personal 

 communication) show longline skipjack catches 

 were generally low except in the area of the 

 NECC (lat 7°30'N), where they were relatively 

 high in the first and fourth quarters (February 

 and October-November). Trolled catches of 

 skipjack on the last three cruises were variable 

 but consistently high in the NECC. Catches by 

 both methods were lowest at the equator in the 

 vicinity of the EUC. Catches of juvenile skip- 

 jack made with a midwater trawl were rela- 

 tively high in the SEC at lat 3°30'S through most 

 of the year, while catches in the NECC peaked 

 in the second and third quarters (May and 

 June). Plankton hauls (1-m net) showed larg- 

 est catches of larvae were made in the SEC just 

 south of the NECC. 



These results suggest that skipjack north of 

 the equator spawn in the second and third quar- 

 ters, or even slightly earlier, and that the juve- 

 niles are concentrated in the NECC at this time. 

 The high year-round abundance of juveniles at 

 lat 3°30'S apparently indicates either sequential 

 seasonal spawning of the northern and southern 

 groups or continual equatorial spawning of a 

 single group. Richard A. Barkley and Richard 

 S. Shomura (personal communications) have 

 previously suggested that the shallow EUC could 

 be involved in the eastward transport of skip- 

 jack young stages. At least east of long 120°W 

 this seems unlikely as even in the upper regions 

 of the EUC temperatures (18°-20°C) are mar- 

 ginal for adult skipjack let alone larvae. Even 

 in the vicinity of the EUC at long 145°W, where 

 temperatures may be about 20 °C, the lack of ju- 

 veniles and adults would appear to confirm this 

 finding. 



Hida (1970) reported on an exploratory fish- 

 ing cruise for tuna made by the Bureau of Com- 

 mercial Fisheries, Honolulu, in October-Novem- 

 ber 1969 concentrated in the area, lat 6°N-8°S, 

 long 115°-125°W. Surface schools of tuna fish- 

 able by pole-and-line were found from lat 2°-5°N 

 (just south of the NECC) ; skipjack predomi- 

 nated in the catches, though some large schools 

 consisted of yellowfin, bigeye, or all three mixed. 

 The skipjack, caught by this method and trolling, 

 ranged from 45 to 79 cm FL (means 47-68 cm) 

 and gonads were maturing or mature (only one 



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