SUMMER 



• • i • j» * • 



V r * 



• • \ y » • • • 



••J . . . y£ • « 



• • \ • m ^ * 



• # • • » 



20° 



10° 



20* 



WINTER 



30*E 



* • • • 1 / 



V I* • 



> * 



V ./• • • 



• • • • • 



10* 



20 • 



30° E 



Figure 4. — Distribution of 7. albidalT. audax around the 

 southern tip of Africa, summer and winter. Catch rates 

 (per 100 hooks) represented by number of dots in each 5° 

 square (one— <0.001, two— 0.001-0.004, three— 0.005- 

 0.009, four— 0.01-0.04, five— 0.04-0.1, six— >0.1). 



attempt any differentiation in the area, although 

 close to the Cape of Good Hope the statistics almost 

 certainly refer to spearflshes. In other areas the ma- 

 jority of fish close to land can be assumed to be 

 sailfish and those offshore to be spearfishes. As has 

 been noted above the sailfish has never been re- 

 corded from the Cape of Good Hope area. 



HYDROGRAPHY OF THE AREA 



Four distinct water masses can be discerned off 

 the southern African coast (see Fig. 10a): the up- 



welled component or the Benguela Current System 

 (9-16°C and 34.8-35.0%o); the Agulhas Bank mixing 

 water zone of varying composition (16-2TC and 

 35.2-35.5%o); the Agulhas Current water (22-27°C 

 and 35.4-35.5°/oo); and the South East Atlantic Sur- 

 face water (16-2TC and 35.5-35.8%o) (Shannon, 

 1966). 



The upwelled component of the Benguela Current 

 system is a clearly marked coastal low temperature 

 zone originating near the Cape of Good Hope and 

 separated from offshore oceanic water by a steeply 

 gradiented oceanic front (Shannon, 1966; Andrews 

 and Cram, 1969). The frontal system is most strongly 

 defined in summer, during the period of intense local 

 southeasterly winds. The continuous presence of the 

 front is clearly demonstrated as far north as lat. 22° 

 S, near Walvis Bay (Bang, 1971). The nutrient en- 

 richment of surface waters coastward of the front 

 produces rapid production and a high standing crop 

 of phytoplankton which supports the large pelagic 

 fish industry of South Africa. 



The Agulhas Bank mixing zone is characterized 

 by systems of eddies, and the structure is very vari- 

 able (Shannon, 1966). The Agulhas Bank water is the 

 product of mixing by South East Atlantic Surface 

 water and Agulhas Current water. Thus the temper- 

 ature of this region varies considerably with the sea- 

 sons between 16° and 21°C depending upon the ex- 

 tent of the contributions of its major sources. The 

 Agulhas Bank water frequently extends to the 

 northwest as a warm current extending around the 

 Cape of Good Hope intensifying the gradients with 

 the upwelled water. 



Bang (1970a, 1970b) found that the Agulhas Cur- 

 rent movements to the south of Cape Agulhas were 

 dominated by two systems, the Return Agulhas 

 Current and the Westward Extension of the Agulhas 

 Current into the southeast Atlantic (Fig. 5). At 

 about long. 22° E most of the Agulhas Current re- 

 curves as the Return Agulhas Current, but a portion 

 is unaffected by this deflection and continues west as 

 tongues of warmer water thrusting into the Atlantic. 

 Shannon (1966) deduced that the northward branch- 

 ing intrusion is likely to move northwards in isolated 

 patches as an anomalous part of the Benguela Cur- 

 rent system. Such patches have not been detected 

 north of lat. 32°S and it must be assumed that the 

 patches lose their dynamic integrity and are dissi- 

 pated by mixing. Darbyshire (1964) and Shannon 

 (1966) agree that the maximum flow of the Agulhas 

 Current is in April (late summer) and the minimum in 

 August (spring). Thus the maximum westward 



181 



