THE DENSITY OF ANIMALS ON THE OCEAN FLOOR 201 



tellus, and as many as 1,910 of a small crustacean (a cumacean). This 

 gives no less than 12,500 animals and 330 grams to the square metre; 

 in short, a density of the same order as in Danish waters — and partly 

 species of similar t)pe. The small razor-shell, for instance, could easily 

 be mistaken for the one common at similar depths in Danish waters, 

 where it is one of the principal food animals of the plaice. The samples 

 taken off South-west Africa thus reveal a very abundant fauna, fully in 

 keeping with the biological investigations of production. 



Off Walvis Bay, nearly every year, there is what is known as the 

 "fish-death". It takes place when the sun is in zenith ■ — about Christmas. 

 Fish die in their thousands, drifting ashore and forming in many places 

 almost a sea-wall of their dead. Various explanations of this phenomenon 

 have been suggested. One is that sulphuretted hydrogen developed on the 

 bottom produces an azoic zone off Walvis Bay; another that there is a 

 prevalence of toxic substances secreted by micro-organisms, which here 

 are particularly abundant owing to exceptionally favourable growth con- 

 ditions. Our visit on Christmas Eve 1950 confirmed the latter theory. 

 The water in the bay was then coloured brown by a small micro-organism, 

 whereas, as indicated above, the bottom samples from the alleged azoic 

 zone were extremely rich in live animals. It follows that the poison ema- 

 nates not from the bottom, but from the surface waters which produce 

 the large growth of micro-organisms. 



This is only one example of the work carried out with the Petersen 

 grab in shallow water. By employing this gear and demonstrating its use- 

 fulness not only in the deep ocean but also in coastal waters in various 

 parts of the world, the Galathea Expedition successfully achieved this 

 particular objective. 



