FISHERY BULLETIN, VOL. 76, NO. 3 



Figure 4. — Hypothetical minimum depth of the skipjack tuna habitat in the Pacific Ocean east of long. 180°, for fish weighing about 

 6.5 kg ( 14 lb) which are limited to water cooler than 24° C. Contours show the depth, in meters, of the 24 °C isotherm. 



station data were averaged within 2° areas of 

 latitude and longitude, for all months, to approxi- 

 mate annual mean conditions. 



Figure 3 shows the hypothetical "floor" of the 

 skipjack tuna habitat, i.e., maximum depths (in 

 meters) for this species. In Figure 3, the unhatched 

 areas off the Pacific coast of the Americas, and at 

 latitudes higher than about 40° in both hemi- 

 spheres, indicate water which, at all depths, is 

 colder on average than 18° C; presumably skipjack 

 tuna would not normally be present. 



Figure 4 shows the minimum habitat depth or 

 ceiling for 6.5-kg fish. Outside of the hatched area 

 annual mean surface temperatures are <24°C, 

 and 6.5-kg fish would normally have access to all 



of the water column above the habitat floor (Fig- 

 ure 3), up to and including the sea surface. 



Figure 5 shows the hypothesized habitat layer 

 thickness for 6.5-kg skipjack tuna. In some areas, 

 there is no water cooler than 24° C with more than 

 3.5 ml/1 dissolved oxygen, so in these areas there is 

 no habitat for 6.5-kg or larger fish; such areas are 

 double hatched on Figure 5. Extensive regions 

 around these areas have habitat layer thickness of 

 10 m or less, and large areas of the equatorial 

 Pacific Ocean have <25 m of habitat layer thick- 

 ness. This rather thin layer can lie beneath as 

 much as 1 50 m of water warmer than 24 ° C ( at lat. 

 4°N, long. 170°W, e.g.). North of the Hawaiian 

 Islands, the opposite situation is present: a 150-m 

 thick habitat layer lies under 25 to 50 m of water 



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