MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



high in zooplankton volume is only about 50% 

 greater than the low, whereas for yellowfin tuna 

 this difference is about 300% . These facts could 

 indicate the inadequacy of some of the implicit 

 assumptions, such as the standing crop of zoo- 

 plankton is a measure of the standing crop of 

 tuna forage, and the latter is a measure of the 

 abundance of tuna; the longline catches are a 

 measure of the abundance of yellowfin tuna; the 

 abundance of yellowfin tuna is representative 

 of its trophic level; and the system is equally 

 efficient at all levels. Nevertheless, the general 

 correspondence of the peak of zooplankton and 

 yellowfin tuna appears more than casual (King 

 and Hida, 1957). 



o 28 



IT 



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 Ijj 



a. 



z 



UJ 



u 



12 



IT 



3 



o 

 o 



K 

 UJ 



o. 



o 

 o 



to 



o 

 o 



X 



8 



c 



UJ 



Q. 



X 



o 



S LATITUDE N 



Figure 24. — Mean distribution of deep-swimming tuna. 

 (The zero figures in the panels represent fishing effort 

 expended, but no catches made; all stations 96.5 km or 

 more miles from land.) 



50 



o 

 o 

 o 





40 



30 



20 



10- 



4 O 



^ O 



X 



o 

 o 



a. 



- I 



E 180° 170' 



ISO" 

 LONGITUDE 



110' W 



S 8° 6° 4" 2" 0* 2° 4° 6» S° I0» 12° 14° N 

 LATITUDE 



Figure 23. — A. Mean surface temperature from the 

 equator to lat 5°N (U.S. Weather Bureau, 1938). B. 

 Mean zooplankton from lat 5°N to lat 5°S (King and 

 Hida, 1957) . C. Mean yellowfin tuna catch from lat 5°N 

 to lat 5°S. 



Figure 25. — Relative north-south displacement of zoo- 

 plankton and yellowfin tuna. (Adapted from King and 

 Hida, 1957.) 



895 



