MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



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Figure 28. — Relation between temperature and zooplank- 

 ton abundance between lat 1° and 5°N. 



In attempting to specify the expected relation 

 between yellowfin tuna and water maturity 

 (temperature) , it is helpful to start with the tuna 

 and work back through the trophic levels. The 

 tuna are highly motile and are capable of con- 

 centrating where food is most abundant. That 

 is, they are not restricted to development in situ 

 in a mass of water. To a lesser extent some of 

 their food organisms (the larger squid and 

 fishes) are undoubtedly able to concentrate in 

 favorable areas that they discover either by fol- 

 lowing gradients or by random movements. It 

 seems extremely unlikely, on the other hand, that 

 the members of the next lower trophic level (or- 



ganisms that feed on the small zooplankton) are 

 capable of long migrations. 



If this intermediate link is incapable of mi- 

 gration, it must develop in situ; it must be, at 

 least indirectly, responsible for grazing down the 

 "net zooplankton" which we measure, and be- 

 cause of the time needed for growth, it must 

 reach a climax at some time after the zooplank- 

 ton. Thus, we expect the forage for tuna to keep 

 improving for some time after the zooplankton 

 has reached its maximum development. 



Since the water is warming during this period, 

 there should be a positive relation between tem- 

 perature and yellowfin tuna abundance until the 

 water reaches senescence, at which point further 

 increases in temperature should be accompanied 

 by a decline in yellowfin tuna abundance. It ap- 

 pears, however, that actual senescence is never 

 attained, at least in the portion of the system 

 under study (long 120°-170°W) . Instead, water 

 probably is withdrawn from the system through 

 convergence and possibly by the Countercurrent 

 before reaching senescence, or even possibly be- 

 fore reaching maturity. 



When fish catches are plotted against temper- 

 ature (Figure 29), a striking relation is shown 

 between maturity of the water (as measured by 

 temperature) and abundance of yellowfin tuna. 

 The measure of abundance of the tuna requires 

 some explanation. As shown by Murphy and 

 Elliott (1954), the catch at a single longline sta- 

 tion is subject to very wide sampling errors — so 

 wide in fact that any relations between catch and 

 variables in the environment are likely to be 

 poorly defined. For this reason the several 

 catches in the zone of lat 1° to 5°N, during a 

 single crossing, are considered as a unit. This 

 treatment, in addition to suppressing sampling 

 error, also has the advantage of directing atten- 

 tion to a zonal situation rather than to a point 

 in the ocean, thereby eliminating the possible ef- 

 fect of other regressions along a meridional sec- 

 tion. The temperature index parallels the tuna 

 index in that it is also the average surface tem- 

 perature over the lat 1° to 5°N zone. 



Returning to the temperature-yellowfin tuna 

 regressions, the steep slopes on long 120° and 

 130 °W suggest a positive relation between ma- 

 turity and catch. At long 148° to 155°W the 



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