Figure 1 5. --Vertical temperature sectional 164 -165 E. longitude. 

 Adapted from Robinson (1952). Isotherms are in Fahrenheit. 



Even if the difference between pre-war survey catch rates and post-war connnnercial 

 fishing catch rates is not a reflection of a change in abundance, there remains a considerable 

 difference between the catches of commercial operations in 1940-41 and 1950-51 (roughly 6 fish 

 per 100 hooks versus 3 fish per 100 hooks). The cause of this apparent decline in abundance is 

 presently obscure. It should, however, be pointed out that the decline occurred during a period 

 (World War II) characterized by little or no fishing for tuna in the western Pacific, indicating 

 that either there were natural changes in the population level or there were changes in the en- 

 vironment in the mothership area that reduced its suitability for yellowfin tuna. 



29 



