BARKLEY ET AL: SKIP.JACK TUNA HA15ITAT 



leave their primary habitat only for limited 

 periods of time without suffering thermal or oxy- 

 gen stress. Prolonged excursions to colder water 

 would require increased activity and thus more 

 dissolved oxygen and food. Skipjack tuna could 

 stay in the warm upper layers only if they would 

 reduce their physical activity, or tolerate over- 

 heating. Hypothetical consequences of these con- 

 ditions are illustrated in Figure 2 which shows the 

 hypothetical layers along long. 119°W for skip- 

 jack tuna of two sizes. The 4-kg fish are those most 

 abundant in catches by the eastern Pacific fishery; 

 9-kg fish are the largest normally found there, and 

 then only in certain areas such as the Revil- 

 lagigedo Islands (ca. lat. 17° N, long. 112°W). 



The deeper limit of the habitat should be the 

 same for skipjack tuna of all sizes. The upper limit 

 is deeper and more restrictive for larger fish, 

 which find essentially no habitable water between 

 lat. 5° and 12° N, a distance of more than 700 km 

 or 400 n.mi. Larger fish also have much less con- 

 tinuous access to the sea surface than those weigh- 

 ing 4 kg. Only skipjack tuna of the smallest size 

 commonly found in this area (<4 kg) could inhabit 

 all of the water above the lower limits in Figure 2. 



Figures 3 to 6 are maps of a hypothetical skip- 

 jack tuna habitat for the entire central and east- 

 ern Pacific Ocean, based on oceanographic station 

 data used in preparing the Oceanographic Atlas of 

 the Pacific Ocean (Barkley 19681. For these maps, 



Figure 3.— Hypothetical maximum depth (meters) of the skipjack tuna habitat in the eastern Pacific Ocean, as determined by the 

 depth of the 18°C isotherm (hatched area) or the 3.5 ml/1 (5 ppm) isopleth of dissolved oxygen (cross hatched area). Contour interval is 

 50 m except for a few areas near the coast, where a 25-m contour interval is used. 



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