INGHAM ET AL: OXYCLINE AND SKIPJACK TUNA DISTRIBUTION 



region extending from long. 8°-10°E to the shelf, 

 and from lat. 7°-9°S to 17°-18°S, and 3) the region 

 of the quasi-stationary cyclonic gyre to the west of 

 long. 6°E. 



In the shelf region south of lat. 17 °S, water with 

 extremely low-oxygen content ( <1 ml/1) forms in 

 the near-bottom layer (80-150 m) and spreads 

 northward and westward beneath the warmer, 

 less dense surface water by advection and turbu- 

 lent diffusion. This water forms the shallow 

 minimum layer that is characteristic of this re- 

 gion, extending westward to about long. 0° where 

 it loses its identity due to mixing. 



10° S 



10° w 



FIGURE 1. — Diagram of geostrophic water circulation in the to 

 100 m layer. 1) South equatorial countercurrent; 2) Angola Cur- 

 rent; 3) west (main) branch of Benguela Current; 4, 5, 6) north 

 branches of Benguela Current; 7) eddies in inner region of cy- 

 clonic gyre; 8) anticyclonic curl; 9) Benguela divergence; 10) 

 merging zone of Angola Current and north littoral branch of 

 Benguela Current. From Moroshkin et al. (1970). 



The "eastern coastal region" and the region of 

 the cyclonic gyre are areas where low-oxygen 

 water Kl ml/1) forms at "intermediate" depths 

 (Bubnov 1972). These waters apparently are ad- 

 vected and mixed to the west, forming the primary 

 oxygen minimum in the eastern South Atlantic. 

 These observations provide further evidence to 

 support the hypothesis that the coastal waters off 

 Southwestern Africa are a source for much of the 

 low-oxygen water that forms the oxygen minima 

 in the South Atlantic. 



RESULTS OF JISETA CRUISES 



In 1968 the National Marine Fisheries Service, 

 then Bureau of Commercial Fisheries, joined with 

 the U.S. Coast Guard and the Missao de Estudos 

 Bioceanologicos e de Pescas de Angola in the Joint 

 Investigation of the Southeastern Tropical Atlan- 

 tic (JISETA); an oceanographic and biological in- 

 vestigation in the coastal waters of southwestern 

 Africa. Distribution of tunas and oceanographic 

 conditions from the Equator to lat. 18°S were in- 

 vestigated on cooperative cruises of the RV Un- 

 daunted, the USCGC Rockaway, and the RV Goa 

 during February through April and September 

 through December 1968. 



Low-Oxygen Layer 



Vertical sections of dissolved oxygen concentra- 

 tion developed from the JISETA data (Cook et al. 

 1974) characteristically showed a layer of low 

 oxygen concentration, including a minimum 

 which frequently was<1.0 ml/1 and occasionally 

 <0.5 ml/1 in concentration. The minimum values 

 were not well defined because of the means of 

 sampling employed: 1 cast of 10 Niskin bottles 

 spaced throughout the upper 1,000 m of the water 

 column at each station. However, the samples 

 were spaced well enough to portray the layer of 

 low concentration and the sharp oxycline which 

 formed its upper boundary. 



The transects obtained in March 1968 (Figures 

 2, 3) showed a layer of oxygen concentration-^. 

 ml/1 of variable thickness (50-450 m) extending 

 from lat. 15° to 18°S in the upper 500 m of the 

 water column. In the southern portion of the area 

 the layer was thicker and nearer the sea surface. 



In the October-November transects (Figures 4, 

 5) a thick layer of water containing<1.0 ml/1 dis- 

 solved oxygen was found to extend from lat. 17° to 

 7°S in the upper 600 m of the water column. Once 

 again the layer was thicker (up to 550 m) and 

 nearer the surface in the southern portion of the 

 area. In the northern portion it thinned to <100 m 

 and was detected at about 300-400 m depth at the 

 outer stations, about n.mi. (180 km) offshore. 



The form of the layer of very low oxygen con- 

 centration (<1.0 ml/1) observed in October- 

 November 1 968 ( Figure 6 ) appears to be consistent 

 with Bubnov's (1972) contention that the source of 

 the layer is located in coastal waters between lat. 

 18° and 23 °S, from which it is advected northward 

 by the northern branches of the Benguela Cur- 



859 



