Atlantic water occurs within 20 nautical miles of the 

 coast at the Cape, "compressing" the Agulhas Bank 

 water against the upwelled water. In this particular 

 summer season, therefore, one would expect all 

 species of billfishes to occur within the survey area 

 in reasonable numbers in the well-defined interwo- 

 ven oceanic areas. 



SUMMARY OF POTENTIAL BILLFISH 

 MOVEMENT 



East-west movement is possible by two methods. 

 Firstly, billfishes could be present in the Agulhas 

 Extension which curves northeastward into the 

 South East Trade Wind drift, west of the Benguela 

 Current system. This extension could become iso- 

 lated and move farther north as an eddy until its 

 identity is lost through mixing. Secondly, billfishes 

 could become involved with the Agulhas Bank 

 mixed water when its temperature is suitable and 

 move westward in the nearshore current around the 

 Cape of Good Hope, to seawards of the front be- 

 tween the ocean and the upwelling area. East-west 

 movements would be assisted in late summer by the 

 maximal westward penetration of the Agulhas Cur- 

 rent (Fig. 2 suggests that this may occur), and in- 

 hibited in winter when the Agulhas penetration is at a 

 minimum. 



Movement from west to east could also be en- 

 couraged in two ways: firstly, with the assistance of 



the eastward intrusion of South East Atlantic Sur- 

 face water extending onto or near the Agulhas Bank ; 

 secondly, by the close inshore movements of water 

 in a southerly or easterly direction round the Cape of 

 Good Hope and along the south coast. Both these 

 water movements are considerably enhanced during 

 winter and correspondingly diminished or absent 

 during summer. In winter, however, billfishes ap- 

 pear to be rare in the southeast Atlantic. 



Thus two patterns emerge: the possibility of a 

 long-term or a short-term residence in alien water. 

 The long-term residence could be caused by a west- 

 ward movement in the Agulhas Extension or inshore 

 current during summer followed by a period of resi- 

 dence in the southeast Atlantic, possibly feeding on 

 pelagic fish at the edge of the upwelling area. Later, 

 in winter, an eastward movement would commence 

 in the South East Atlantic Surface water as it pushes 

 towards the Agulhas Bank. The short-term resi- 

 dence is possible by a similar mechanism, but ac- 

 cepts no delay before the fish take advantage of the 

 common South East Atlantic Surface water intru- 

 sion to return eastwards. Naturally, the inverse ap- 

 plies to Atlantic billfishes extending into the Agulhas 

 region, but appears unlikely to take place; the wider 

 coverage of the Japanese fishery suggests that the 

 blue marlin and the longbill spearfish, T. pfliiegeri, 

 caught off the Cape at this time are attracted by the 

 rich feeding and will not move further east. 



This much can be deduced from available data. 



li E 15° 16° 17° 



I ■ 



19 20 21° 22° 



■ E 15° 16° 17° 16° 



I.' ■ ■ ■ ■ I 



20 21 22 



Figure 10.— Surface temperatures and salinities off South Africa, January 1962. A. Temperature. B. Salinity. 



186 



