MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



level, but the inclusion of billfishes is open to 

 question. 



On the subject of numbers of fish versus 

 weight, Appendix Figures 1 and 2 depict the 

 results of two section lines along long 150°W, 

 one representing good and the other poor fishing. 

 The members of each pair of panels give the 

 same impressions. In the first series abundance 

 is very high between lat 1° and 6°N and con- 

 siderably lower to the north and south. In the 

 second series the total abundance is much lower, 

 but the catches definitely peak between lat 2°S 

 and 4°N. The inclusion or omission of the bill- 

 fishes, thought to represent a slightly higher 

 trophic level than the tunas and sharks, has little 

 effect on the major trends. 



We may draw two important conclusions from 

 the plots of number and weight. Because the 

 trends are nearly the same, the average size of 

 each of the component species must change little, 

 if at all, with latitude. Even more important, it 

 is apparent that latitudinal variations in catch 

 are largely a function of variations in yellowfin 

 tuna abundance, particularly between lat 5°S and 

 5°N, the general area in which this study has 



been focused. Because the yellowfin tuna is the 

 most important species in this region, we lose 

 little biologically and gain much in ease of under- 

 standing by restricting comparisons of the fish 

 population and the environment to yellowfin 

 tuna. 



One flaw in these arguments for considering 

 yellowfin tuna only is our failure to take into 

 account the skipjack tuna, for it almost cer- 

 tainly belongs to the same trophic level as the 

 yellowfin tuna. Although small yellowfin tuna 

 are almost absent from longline catches in the 

 oceanic areas under consideration, skipjack tuna 

 are strongly represented. In fact the available 

 evidence suggests they are abundant to the north 

 of the concentration of yellowfin tuna (lat 5°- 

 10°N). We can offer no satisfactory solution, 

 for it is not yet possible to combine quantitatively 

 measures of abundance of deep-swimming fishes 

 with those of surface fishes. In our opinion, 

 however, if it were possible to add skipjack tuna 

 to the weight diagrams of Appendix Figure 2, 

 there would be little change in the relative dis- 

 tribution of the tuna-shark trophic level near 

 the equator. 



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