FISHERY BULLETIN, VOL. 76, NO. 3 



26° C). Fish weighing <4 kg should find some 

 thickness of habitable water, within and just 

 below the upper mixed layer, everywhere in the 

 eastern tropical Pacific. 



DISCUSSION 



Although we anticipate that temperature and 

 dissolved oxygen will prove to be primary deter- 

 minants of the habitat of skipjack tuna in all 

 oceans, it is possible that limiting values of these 

 variables may differ from one population or region 

 to another. The lowest temperature (ca. 15° C) at 

 which skipjack tuna are caught in Australian 

 waters (Robins 1952) is considerably lower than in 

 the eastern Pacific. The fish caught off Australia 

 may also differ in their ability to tolerate warm or 

 low-oxygen water. 



The gross features of the distribution of skipjack 

 tuna in the eastern tropical Pacific, where only 

 small skipjack tuna are found in large numbers 

 (Williams 1970), agi-ee with the hypothesis. Those 

 areas where large skipjack tuna do occur (Ma- 

 tsumoto 1975) are outside of the hatched area in 



Figure 6: the Revillagigedo Islands, e.g., are just 

 north of the hatched area, and Tahiti is well south 

 of it. 



The hypothetical habitat proposed here ex- 

 plains why skipjack tuna leave the northern 

 fishery of the eastern Pacific when they reach a 

 certain size. To find cooler, better oxygenated 

 water as they grow, these fish must move out of the 

 eastern tropical Pacific toward higher latitudes in 

 the central Pacific. Also, they must then spend less 

 time at or near the sea surface, since the thermo- 

 cline, where they live, is generally much deeper in 

 the central Pacific, and the water above the ther- 

 mocline is too warm to permit normal activity. 

 This size-specific movement in response to the en- 

 vironment is consonant with Rothschild's (1965) 

 migration model for the eastern Pacific skipjack 

 tuna population. It also suggests a mechanism for 

 the evolution of migratory processes, an important 

 topic in marine ecology. 



For several reasons, existing fishery data are 

 inadequate for making a refined judgment of our 

 skipjack tuna-habitat hypothesis: 1) Commercial 

 fishery data generally include neither information 



Figure 6. — Average water temperature in the eastern Pacific Ocean at those depths where the concentration of dissolved oxygen is 3.5 

 ml/1. Deeper water is cooler and lower in oxygen, shallower water is warmer and has more oxygen. See Figure lb for the relationship 

 between skipjack tuna size and upper temperature limits. 



660 



