28o DISCOVERY REPORTS 



We believe that these events contributed, at least in part, to a small mortality of fish which occurred 

 at Walvis Bay in March 1950. At times such mortalities assume catastrophic proportions 

 (Brongersma-Sanders, 1948) and may seriously affect the fish stocks in this area. 



Brongersma-Sanders (1948) has preferred to consider that the mortalities are due to poison 

 originating from the discoloured water which is so frequently coincidental with mortalities. Some of 

 the organisms causing such discolorations are known to be toxic to higher animals. The toxicity, of 

 course, depends on the organism causing the bloom, and in the Walvis Bay region Dr Brongersma- 

 Sanders suggests that a species of Gymnodinum may be the origin of such poisons. One of us (T.J.H.) 

 has examined several discolorations or blooms in South African waters, and none of these were 

 caused by that particular genus. Peridinium triquetrum, known to have predominated in some of them, 

 has once been recorded in quantity coincidentally with a fish mortality in the Baltic (Lindemann, 

 1924), but it is a common dominant of polluted inshore waters so that its occurrence near Walvis Bay 

 after a minor mortality might be the result rather than the cause of that phenomenon. This does not, 

 of course, rule out the possibility that Brongersma-Sanders' hypothesis may be proved correct by 

 further work, but from the existing evidence it seems to us more probable that the mortalities are 

 caused by the coincidence of several unfavourable factors, one of which may be toxicity of bloom- 

 forming phytoplankton organisms. 



Not much more need be said about the animal populations. The collections of zooplankton have as 

 yet only been analysed from survey I. They show a certain individuality in the general fauna of the 

 current, but, as might be expected, the planktonic animals do not appear to show such a close cor- 

 relation with the water masses as do the phytoplankton. Apart from this the ' William Scoresby ' 

 collections have provided some interesting material in connexion with the wider questions of the 

 distribution of planktonic animals, a subject which lies outside the scope of this paper. 



The abundance of the higher animals — the large populations of pelagic fish, seals and birds — and 

 the presence of rich guano islands, are all characteristic features of upwelling regions and bear 

 testimony to their productivity. The apparent scarcity of whales is seemingly anomalous but is 

 perhaps an understandable exception (p. 275). 



The discovery of a rich breeding-ground of the pilchard (p. 272) is yet another facet of this 

 interesting region and it emphasizes the fact that many important features may yet be revealed by a 

 more detailed examination of the region. 



COMPARISON OF THE BENGUELA CURRENT WITH OTHER 



UPWELLING REGIONS 



The features which we have described in the Benguela current have their parallel in other upwelling 

 regions in the world, and in comparing these one is struck by the surprising similarity of the process 

 and its attendant phenomena, in widely separate geographical localities. 



Upwelling is characterized by a divergence of subsurface water masses towards the surface, and as 

 such it is a widespread phenomenon throughout the oceans. By virtue of the effect of the earth's 

 rotation on a current in the ocean the more dense water is found on its left-hand side (looking in the 

 direction of flow) in the northern hemisphere, and on its right-hand side in the southern hemisphere. 

 If the velocity of the current is sufficiently great, more dense (and consequently deeper) water will be 

 lifted to the surface. This may occur along the edge of any suitable current, and may consequently 

 occur either at a solid geographical boundary or at an oceanic boundary with another current. In the 

 latter case we have examples in the divergences in the equatorial regions of the ocean, and in the 

 former the divergences on coastal boundaries of fast-moving currents, for example the Agulhas 



