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180° 170° 160° 150° 140° 130° 120° 

 WEST LONGITUDE 



JFM AM J J AS OND 

 SEASON 



Figure 5, Zooplankton Abundance by Lon.gitude and 

 Seasons . We think that the productivity at the Equator increases 

 from west to east. The diagram at the left confirms this at least 

 between 180° and 140° W. longitude. It appears that plankton pro- 

 duction levels off in the eastern sector of our survey area, but our 

 surveys east of 140° W, have not been comprehensive enough to be 

 sure of this. 



The diagram to the right shows that some parts of the 

 year are more productive than others. We expect that further 

 analyses will show that these changes in productivity are related to 

 the fluctuation in tuna abundance. 



While plankton is an important indicator of richness, 

 most plankton animals are too small for tuna food. Instead they 

 are fodder for the small fish and squid which in turn are eaten by 

 the tuna. 



