FISHERY BULLETIN: VOL. 75, NO. 4 



5°N 



- 5°S 



5°E 



20° 



FIGURE 7. — Depth (meters) to the 3.5 ml/1 iso-oxygen surface 

 from Undaunted data, February-March 1968. 



15° 



5°N 



0° 



5°S 



15° 



20° 



FIGURE 8.— Depth (meters) to the 3.5 ml/1 iso-oxygen surface 

 from Undaunted and Rocka way data, October-November 1968. 



stant throughout the limited area surveyed. The 

 sea-surface temperature ranged from 22 °C in the 

 south (lat. 18°S) to 29°C in the north (lat. 2°N). 



In order to portray the pycnocline topography 

 and minimize the differences in surface heating in 

 the two periods, an isopycnal surface found near 

 the bottom of the thermocline, the a, = 26.0 sur- 

 face, was chosen (Figures 9, 10). Comparison of the 

 vertical sections of density and oxygen from the 

 JISETA cruises (Cook et al. 1974) shows that the 

 26.0 iso-cr, surface parallels the oxycline and is 

 found in its lower levels. Therefore the topography 

 of the isopycnal surface also should reflect geo- 

 strophic circulation patterns in the lower oxycline. 



During the October-November 1968 period the 

 26.0 g/1 topography (Figure 10) deepened near- 

 shore north of lat. 10°S, but was shallow and ir- 

 regular south of there. The topography north of 

 lat. 10°S indicates a general southward flow in the 

 upper layer from about lat. 4° to 10°S, correspond- 

 ing with the southward Angola Current described 



by Moroshkin et al. (1970), but not extending as 

 far south as they portray it (Figure 1). 



RELATIONSHIP BETWEEN 



OXYCLINE DEPTH AND 

 SKIPJACK DISTRIBUTION 



Variations in the thickness of the habitable en- 

 vironment of skipjack tuna, bounded beneath by 

 the oxycline, should strongly inflence the distribu- 

 tion and availability of surface schooling tunas. To 

 test this contention, the positions of sightings of 

 skipjack schools during the October-November 

 1968 cruise period were plotted on a map of oxy- 

 cline (3.5 ml/1) topography (Figure 11). A cursory 

 study of this plot reveals that the fish were gener- 

 ally sighted where the oxycline was <50 m deep, 

 and over 809c of the schools were seen where it was 

 <30 m deep. 



An apparent relationship between school dis- 

 tribution or availability and oxycline depth can 



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