WILLIAMS: MODELS OF MIGRATION OF YOUNG SKIPJACK 



ity" bands in the eastern tropical Pacific. More 

 detailed statistical treatment (for significance 

 of interactions) of EASTROPAC primary pro- 

 ductivity, phytoplankton, zooplankton, and mi- 

 cronekton data (Owen and Zeitzschel, 1970; 

 Blackburn et al., 1970) from the western EAS- 

 TROPAC area (lat 16°N-3°20'S, long-100°30'- 

 121°30'W) show clearly the equatorial zonal 

 band (s) . However, the band at the NECC/NEC 

 boundary is not clear at trophic levels other than 

 zooplankton, and possible reasons for such ano- 

 malies are discussed in some detail. In addition, 

 Owen and Zeitzschel (1970) pointed out that 

 latitudinal eflfects may have been eliminated due 

 to selection of geographic zones which do not 

 coincide with natural (zonal) current systems. 



Blackburn and Laurs (1972) have discussed 

 the distribution of that part of the EASTROPAC 

 micronekton catches which can be classified as 

 skipjack forage. They not only confirm the ex- 

 istence of equatorial bands of high forage con- 

 centrations just north of the equator, and occa- 

 sionally south of it, but also of a band at the 

 NECC/NEC boundary, particularly in dajrtime 

 catches. 



There is some biological evidence from EAS- 

 TROPAC data of increased "productivity"-zoo- 

 plankton, micronekton, occurrence of birds 

 (Love, 1970, and in preparation; R. M. Laurs, 

 personal communication) — at certain times be- 



tween lat 5° and 15°S, particularly lat 12°-14°S, 

 the general region where the surface SECC 

 might be expected. Such increased "produc- 

 tivity" could occur if a divergence existed in 

 these latitudes, say at the southern SECC/SEC 

 boundary. 



Generally in the EASTROPAC area standing 

 stocks decreased from east to west (Blackburn 

 et al., 1970) , that is from inshore to offshore, as 

 did zooplankton in the charts of Reid (1962). 



PROPOSED MIGRATION MODELS 

 ACTIVE MIGRATION MODEL (Figure 5) 



Skipjack larvae are rare east of long 130°W 

 (Matsumoto, 1966; Ueyanagi, 1969), and this 

 generally appears to be borne out by the results 

 of the EASTROPAC expedition (Love, 1970, 

 1971, and in preparation) . It would seem, there- 

 fore, that adult skipjack spawn in the surface 

 waters to the west of long 130°W, which in some 

 way must be ecologically suitable for optimum 

 survival and development of the larvae. In this 

 model it is assumed that these larvae, and early 

 juveniles, are maintained within the central Pa- 

 cific by some passive migration system, perhaps 

 related to the equatorial zonal current systems, 

 eddies associated with island wakes, as well as 

 diel vertical migrations. 



Figures. — Active 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 Counter- 

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



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