The results obtained at Clarion Island agreed 

 with those previously obtained in denying 

 zooplankton the role of a tuna-attracting prop- 

 erty. It is possible that tuna are attracted by 

 micronekton which subsist on phytoplankton 

 or plant detritus. Pelagic crabs (Pleuroncodes 

 planipes), an important item of tuna food in 

 Baja California waters, were very abundant 

 in the micronekton at one of the offshore 

 Clarion Island stations on SCOT Expedition, 

 and might enter the area of the fishery. 

 Figure 12 shows the high standing crops of 

 micronekton that were encountered locally 

 near Clarion and Cocos Islands, although not 

 at Clipperton Island. 



It would therefore be premature to conclude 

 that oceanic island aggregations of tuna do 

 not depend on the presence of some kind of 

 food. 



SUMMARY AND EVALUATION 



The material presented above may be ap- 

 praised in various ways, of which the most 

 rigorous is to list the scientific conclusions 

 most clearly pertinent to the problem of 

 understanding and forecasting abundance and 

 distribution of tuna in the eastern tropical 

 Pacific. These conclusions (1-9), together 

 with brief explanatory comments, are as 

 follows: 



1. Yellowfin and skipjack tuna at the north- 

 ern end of their range are generally scarce 

 in waters where the temperature in the upper 

 10 m. is less than 20° or 21° C. According to 

 other workers the southern end of the range 

 of each species is likewise associated with 

 certain temperatures, although perhaps not 

 with the san-ie temperatures as at the northern 

 end. Skipjack are scarce in waters where 

 temperatures in the upper 10 m. exceed 28° C. 

 The effect of these apparent temperature- 

 limitations in a year of high temperature 

 in the entire eastern Pacific, such as 1958, 

 is to pern-lit each species to extend its dis- 

 tribution a few degrees of latitude polewards 

 in each hemisphere (e.g., Baja California to 

 California, Peru to Chile), while causing 

 skipjack to vacate the thermal-equatorial re- 

 gion off southern Mexico and Central America. 

 In a year of low temperatures such as 1955, 

 the distribution of each species is more or 

 less continuous over a narrower band of 

 latitude along the coast of the American 

 continent, with the thermal equator near its 

 center. 



2. The correspondence between the north- 

 ern limits of tuna and of 21° C. surface 

 water is always so close that the effect of 

 the temperature on the tuna may be assumed 

 to be almost immediate, and direct. Because 

 the nnain effect in a nornnal year is to permit 



tuna to enter the Pacific coast waters of 

 Baja California in sumnner only, it was con- 

 sidered that the tuna might enter those waters 

 in a current of warm water flowing from 

 the south at that season. However, a study 

 of the average annual heat budget of those 

 waters virtually removed this possibility, by 

 showing that little advection of warm waters 

 occurred in the upper 200 m. at any season. 



3. The possibility of predicting 30-day or 

 I 0-day changes of mean surface temperature, 

 from previous averages of surface tempera- 

 ture and wind velocity in the same area, was 

 investigated statistically both in the eastern 

 tropical Pacific (Gulf of Tehuantepec) and 

 adjacent to it (northeastern Pacific, where 

 longer series of data were available). This 

 empirical approach was not highly successful; 

 the best combination of variables for predic- 

 tion accounted for about 40 percent of the 

 variance of the next month's temperature. It 

 was thus important to identify and understand 

 more adequately the relationships between 

 wind and surface temperature in different 

 areas (4, 5, and 9 below). 



4. Within-year chauiges in relationships be- 

 tween wind and sea temperature were investi- 

 gated for the California Current and Peru 

 Current regions as part of a comparative study 

 of eastern boundary currents of the world. 

 Seasonal values of Ekman transport of sur- 

 face water away from the coast, computed 

 for different latitudes from values of average 

 wind stress and orientation of coastlines, 

 agreed qualitatively with known seasonal and 

 geographical variations in upwelling on four 

 out of the five eastern boundary coasts (west 

 coasts of North America, South America, 

 North Africa, and South Africa, but not Aus- 

 tralia). 



5. Major changes in surface temperature 

 regime between years are also considered 

 to be wind-connected, as a result of investi- 

 gations of oceanic disturbances called Ninos 

 in Peru (and the more detailed studies by 

 others on similar but better-measured dis- 

 turbances in the northeast Pacific). Ninos 

 result in warming of the sea surface such as 

 occurred between 1957 and 1959 (1 above). 

 A hypothesis has been developed which defines 

 the Nino as the set of conditions developing 

 off any upwelling coast when reduction of the 

 wind stress causing upwelling, during an 

 extended period of time, leads to weakening 

 or cessation of vertical mixing. 



6. Abundance of yellowfin tuna, from place 

 to place within the temperature-controlled 

 limits of its distribution, is probably deter- 

 mined mainly by the abundance of zooplank- 

 tonic or nnicronektonic organisms in its food 

 chain, with surface tennperature playing a 



42 



