BARKI.EY KT AI.: SKIPJACK TUNA HABITAT 



Figure 5.— Thickness of the hypothetical habitat layer (meters), for 6.5-kg skipjack tuna in the eastern and central Pacific Ocean. 

 Contours were obtained by subtracting depths of the upper habitat limit (Figure 4) from the lower one (Figure 3). In the crosshatched 

 areas off Mexico and Peru, there should be no habitat suitable for fish of this or larger sizes. In the hatched area, water warmer than 

 24°C is present above the habitat layer. Immediately beyond this area the habitat (dashed depth contours) extends to the sea surface. 

 Outside of the 18° C surface isotherm, the water is probably too cold for skipjack tuna. 



warmer than 24 C. Clearly, the South Pacific 

 Ocean offers the roomiest habitat to large skipjack 

 tuna, and in fact some of the largest known indi- 

 viduals of this species were caught slightly south 

 of Tahiti (lat. 17"S. long. 150"W), according to 

 catch records of longline fishing boats (R. A. 

 Skillman, Southwest Fish. Cent. Honolulu Lab., 

 Natl. Mar. Fish. Serv., NOAA. Honolulu, HI 

 96812, 1977, pers. commun.). 



An interesting feature of the habitat map for 

 6.5-kg fish I Figure 5) is a channel of relatively cool 

 and adequately oxygenated water some 200 km off 

 the coast of Mexico. This channel should allow 

 skipjack tuna as large as 6.5 kg to pass from the 



Baja California fishery to the equatorial fishery, or 

 vice versa, when fish as small as 4 kg would find 

 stressful conditions for hundreds of kilometers on 

 either side of that channel. Seasonal and year-to- 

 year closure or shifting of this channel could readi- 

 ly explain puzzling variations in the distribution 

 of skipjack tuna catches in the eastern Pacific. 



Figure 6 shows areas of presumably stressful 

 environment (zero habitat thickness) in the east- 

 ern tropical Pacific for fish of various sizes and 

 therefore temperature limitations. Skipjack tuna 

 >11 kg should find no habitat at all within the 

 shaded area. Fish 4 kg in size would find no habitat 

 within the smallest contour (temperatures above 



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