temperature, which sets limits of total range, and food supply, which determines 

 the patchy distribution within the range limits. They also showed that surface 

 chlorophyll is distributed like the tunas' supply off Baja California, and 

 could therefore be used to specify areas in which tunas would be expected to 

 occur (provided temperatures were suitable) . 



One might expect less close relationships between distributions of surface 

 chlorophyll and tuna food in other areas, where tunas are known to eat a greater 

 variety of species, many of which are not herbivores. Nevertheless I have data, 

 from cruises made at different seasons over a large area of the eastern tropical 

 Pacific on EASTROPAC expeditions, which show that surface chlorophyll may be a 

 good estimator of tuna forage, even in those situations. Figures 5 and 6, for 

 opposite seasons, show the distribution of the standing stock of animals that 

 skipjack tuna eat, in two different ways: as actually observed, and as esti- 

 mated from a regression on surface chlorophyll measured at the same time and 

 place. The agreement is at least fair: with more data and more understanding, 

 it could probably be much improved. 



These remarks give an idea of what it may be possible to accomplish with 

 a large and regular coverage of sea surface chlorophyll, such as might be 

 obtained from aircraft or a satellite — namely, the ability to specify at 

 short notice the areas of maximum concentration of food of tunas, and thus of 

 the tunas themselves, and possibly the same for other kinds of pelagic fish. 

 It may be more efficient to estimate the fish-food distributions from chlorophyll 

 distributions, than to measure them directly. In many situations, data on sur- 

 face temperature would be required as well. 



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