the Revillagigedo Islands region, between lat 

 18 and 20 N in the northwestern part of the 

 EASTROPAC area (Joseph and Calkins, 1969; 

 Williams, 1970). We are here concerned with 

 the possibilities of occurrence of abundant skip- 

 jack in other, more offshore areas where high 

 concentrations of potential skipjack forage reg- 

 ularly occur and where little or no fishing has 

 been done. These are located west of long 95 

 W just north and south of the equator, and 

 between lat 6 and 14 N; and in the north- 

 western part of the area between about lat 14 

 and 18 N. Since the levels of forage concen- 

 tration in these unexploited areas are compar- 

 able with the maximum levels in the areas that 

 are exploited, they may be equally attractive 

 to skipjack. 



There is a broad basis for the expected asso- 

 ciation of abundant adult skipjack with the 

 high offshore forage concentrations. Skipjack 

 are fairly heavy feeders, eating the equivalent 

 of 15% of their body weight per feeding day of 

 12 daylight hours when fed to satiation (Mag- 

 nuson, 1969); therefore, they would be ex- 

 pected to aggregate in productive areas of 

 abundant forage. Also, skipjack are known to 

 be highly mobile animals with maximum swim- 

 ming speeds in excess of 10 body lengths/sec 

 (Blaxter, 1969) and cruising speeds up to 6 

 body lengths/sec (Yuen, 1970), clearly able to 

 leave areas in which the forage is sparse in 

 search of richer areas elsewhere. When skip- 

 jack migrate between the central and eastern 

 Pacific, they possibly remain in biologically 

 productive latitudes (Williams'). 



The above-mentioned areas of offshore for- 

 age maxima would not be expected to contain 

 skipjack, in commercial concentrations, if 

 surface temperatures were unsuitable, i.e., 

 under 20 or over 29 C. Sea-surface-tempera- 

 ture information for the eastern tropical 

 Pacific (Wyrtki, 1964; La Violette and Seim, 

 1969; monthly charts issued by the National 

 Marine Fisheries Service, La Jolla, California) 

 indicates that the forage maximum in unfished 



■"■Williams, F. Meteorology, oceanography, and mi- 

 grations of recruit skipjack in the eastern Pacific 

 Ocean. Scripps Tuna Oceanography Research Pro- 

 gram, Institute of Marine Resources, Scripps Institu- 

 tion of Oceanography, La Jolla, Calif. 92037. Manu- 

 script. 



areas lying just north of the equator might 

 occasionally be too cold for skipjack at some 

 longitudes west of the Galapagos Islands in 

 the months July through November, although 

 longitudes west of 1 17 W would probably not 

 be affected. The same might apply to the maxi- 

 mum south of the equator. The maximum at 

 lat 6 -14 N might occasionally be too warm 

 for skipjack in some or all of the areas where 

 we have found it, especially in the months of 

 July through October. If fisheries develop in the 

 areas of these forage maxima, monitoring or 

 forecasting the occurrences of unusually high 

 or low surface temperatures might be useful 

 to fishermen. 



The zonal maximum just north of the equa- 

 tor has been observed before, both in the EAST- 

 ROPAC area and farther west in the central 

 Pacific, by King (1958) and King and Iversen 

 ( 1962). These authors also observed a second 

 zonal maximum just south of the equator (lat 

 2 S) on one occasion at long 120 W (but not 

 at 112 ), although they did not comment upon 

 it. King (1958) suggested an explanation of 

 the northern maximum based on the work of 

 Cromwell (1953). Northerly and southerly 

 components of water transport result from the 

 equatorial divergence, and because the prevail- 

 ing winds are from the southeast, the major 

 drift of newly upwelled water is towards the 

 north. As this drifting water becomes older it 

 gets warmer, and this together with its nutrient 

 richness enables it to support large populations 

 of organisms. The organisms at successively 

 higher trophic levels reach their biomass maxi- 

 ma at successively longer intervals of time, and 

 thus of space, from the original upwelling; 

 therefore, the forage maximum wiU occur some 

 distance north of the center of the equatorial 

 upwelling. We would amend this interpretation 

 to include the situation where the drift of the 

 upwelled water is to the south, whereby a forage 

 maximum may occur south of the equator, al- 

 though less pronounced than the northern one, 

 as we have observed in this paper. Similar ideas 

 have been expressed by Vinogradov and Voro- 

 nina (1965). 



The zonal maximum of skipjack forage be- 

 tween lat 6 and 14° N may represent a result 

 of high biological production along the ridge in 

 the strong thermocline that occurs in those lati- 



13 



