Figure 2 shows the same area at a later stage, such as August in a warKi 

 year, when the tongues of upwelled water have become much warmer although they 

 are still richer in chlorophyll and herhivores than any other waters. Tunas 

 are now found in the middle of the tongues as well as along the edges, exploiting 

 the richest areas of forage without any restriction imposed by unsuitable sur- 

 face temperature. They are not found in the equally warm areas where biologi- 

 cal material is scarce, however. 



Later still, as in September (Figure 3) , tongues of upwelled water can no 

 longer be recognized by their surface temperature, but they can still be recognized 

 by their relatively high content of surface chlorophyll and herbivores. Tunas 

 are foimd in these food-rich areas, and not elsewhere, although surface tem- 

 peratures are suitable (> 20° C.) everywhere. 



Figure 4. shows the situation late in the tuna season (November) , when all 

 signs of upwelling have disappeared; thermal and biological conditions are 

 rather uniform and suitable for tunas over large areas, and tuna occurrences 

 are scattered through these areas. Later still, temperatures become unfavor- 

 ably low throughout the whole area, and the tunas retreat to the tropics. 



In Figures 1 and 2 the areas of low temperature (< 20° C.) and high 

 chlorophyll were broadly congruent, so that distribution of tuna food and tiinas 

 could perhaps have been specified from data on temperature only; but in Figures 

 3 and 4 these distributions could not have been specified from temperature data, 

 although they could have been specified from chlorophyll data. 



These studies supported a hypothesis, for which there was previous 

 evidence as well, that two main ocean properties determine the distribution of 

 tuna (and possibly other pelagic fish) at any particular time: namely 



5-5 



