FISHERY BULLETIN: VOL. 70, NO. 3 



(2°-3° of latitude) South Equatorial Counter- 

 current (SECC), and shows that there appears 

 to be a subsurface component at about lat 5°S. 

 The surface SECC is ill-defined in the eastern 

 Pacific (east of long 140°W) and M. Tsuchiya 

 (personal communication) states that so far 

 there is no physical oceanographic evidence from 

 EASTROPAC data to confirm its existence. 



FACTORS INFLUENCING SKIPJACK 

 DISTRIBUTION 



Blackburn (1965) considered that simple oce- 

 anic properties, such as temperature, directly 

 determine overall limits of distribution of tunas, 

 but that oceanic features and processes, among 

 them surface currents, determine temporal and 

 spatial differences in abundance within these 

 limits. Though tuna distributions sometimes 

 follow currents, he thought such relationships 

 were often indirect, i.e., through property dis- 

 tributions associated with the currents. Black- 

 burn was also of the opinion that the case for 

 causal relationship between distribution of water 

 masses and that of tuna species was inconclusive. 

 However, Nakamura (1969) hypothesized that 

 (i) tunas, according to species and life history 

 stage, have their centers of distribution in dis- 

 tinct current systems or water masses, which 

 provide specific habitats for them, and (ii) mi- 

 grations of tunas are of two types, within a ha- 

 bitat and between habitats. 



The consideration is now of the oceanographic 

 factors influencing possible routes taken by skip- 

 jack when migrating from the central to the east- 

 ern Pacific. Throughout the tropical central-east 

 Pacific near-surface temperatures are optimal 

 for skipjack, except along the equator west of 

 the Galapagos Islands out to long 100°W in cer- 

 tain months, and, of course, in the cold waters of 

 the Peru Current. Given optimal temperatures, 

 then the next factor governing distribution is 

 probably the supply and distribution of food 

 (Blackburn, 1965, 1969a, b) . Adult skipjack are 

 carnivorous on macrozooplankton and micronek- 

 ton, and there is no reason to believe this does 

 not hold for juvenile and adolescent skipjack, 

 although the size range of the diet is probably 

 smaller, that is more zooplankton. Even in adult 



fish Yuen (1959) for the Hawaiian Islands, Al- 

 verson (1963) for the eastern tropical Pacific, 

 and Nakamura (1965) for the Marquesas and 

 Tuamotu Islands presented evidence of smaller 

 proportions of crustaceans and/or molluscs in 

 large skipjack (over 60 cm) than in small ones. 

 Contradictory evidence is found in the results of 

 Waldron and King (1963) , which showed no sig- 

 nificant differences in the principal components 

 of stomach contents of central Pacific skipjack. 



Previous work in the equatorial central Pacific 

 (Sette, 1955, 1956, and references therein; King 

 and Hida, 1957; King, 1958; King and Iversen, 

 1962; Murphy and Shomura, 1972) indicates 

 that there occur zonal "productivity" bands, rep- 

 resenting various stages from nutrient enrich- 

 ment to trophic levels, such as those represented 

 by zooplankton and micronekton, which may be 

 correlated with fish distribution. The principal 

 such band normally exists between the southern 

 edge of the NECC and a few degrees either side 

 of the equator. Within this range there are lat- 

 itudinal displacements of the "productivity" 

 band (and its components) with time, probably 

 as a function of the occurrence of the prevailing 

 wind systems, the southeast and northeast trades 

 (see Murphy and Shomura, 1972, for detailed 

 discussion). A second, less intense' and more 

 transient (or poorly documented) "productivity" 

 band appears to exist close to the pycnocline at 

 the NECC/NEC boundary. Its intermittent na- 

 ture may be due to the nature of the mixing pro- 

 cesses, such as ridging (Cromwell, 1958). Reid 

 (1962) presented charts of average zooplankton 

 volume (for upper 150 m) that indicated the ex- 

 istence of these two zonal bands in the central- 

 east Pacific. In addition, he indicated a narrow 

 zonal "productivity" band from lat 15 °S in the 

 east to about lat 10°S in the central Pacific. Close 

 to the shore the zonal bands merged with fea- 

 tures related to coastal distribution. 



EASTROPAC zooplankton and micronekton 

 data published in Atlas form (Love, 1970, and 

 in preparation) in general confirm the existence 

 of the northern and equatorial zonal "productiv- 



^ May be due to biased sampling procedures (Black- 

 burn and Laurs, 1972; Maurice Blackburn, personal 

 communication) . 



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