276 DISCOVERY REPORTS 



And the peach and the nectarine, the apple, the pear, 



Attain such a size that the gardeners stare, 



And cry, 'Well! I never saw fruit like that 'ere!' 



One cabbage thus reared, as a paper maintains 



Weighed twenty-one stone, thirteen pounds and six grains, 



So no wonder Guano celebrity gains. 



Attention was first drawn to the value of the deposits on Ichabo by Morrell (1852) 1 , an American 

 sealer. His earlier reports had been rapidly followed up, and by 1845 most of the guano had been re- 

 moved from Ichabo and ships disappointed in their search there turned to the other islands and coastal 

 deposits. Now, the utilization of the recent deposits is controlled, and for this purpose the islands 

 are divided administratively and geographically into two groups : the ' northern group ' comprising the 

 islands to the north of Orange river mouth, and the ' colonial group ' extending from 3 1 ° S. to the Cape. 



Hutchinson (1950), in his monograph on vertebrate excretion, has dealt at some length with the 

 South African deposits, and his work presents us with a very useful summary of what is known of them. 



In both groups of islands there are three species of birds which make the major contribution to the 

 deposits. These are as follows: 



The Cape Gannet [Morns capensis Lichtenstein), (2) the Cape Penguin (Spheniscus demersus 

 Linnaeus), and (3) the Trek Duiker (Phalacrocorax capensis Sparrman). 



In addition to these three species it is probable that the other cormorants make minor contributions. 

 These birds fulfil the principal biological attributes necessary for deposition of guano, in that they are 

 colonial in nesting, and excrete at their nesting-sites. The best guano is formed where there is a 

 minimum of extraneous matter such as sand and feather, etc. introduced into the deposit. 



The guano, once deposited, is subjected to weathering, and it is through this process that the 

 characteristic types of guano are developed. Rain and spray both wash the guano, and this results 

 in the leaching of nitrogenous compounds from the deposit, producing the rather inferior phosphatic 

 guano, with a low nitrogen content. 



There is some evidence that the guano of the ' northern islands ', which resembles the modern 

 Peruvian guano, is relatively rich in nitrogen compared with the guano of the ' colonial islands '. The 

 difference may be related to the higher rainfall in the 'colonial group' islands (e.g. Dassen island, 

 see Fig. 3). 



It appears that relatively flat islands, which are sufficiently high to allow deposition of guano well 

 clear of the splash zone, form the most favourable substrate for successful accretion. The island of 

 Ichabo in the ' northern group ' is the richest producer, and some 2000 tons of guano are deposited there 

 annually. From all of the west South African deposits about 1 0,000 metric tons are harvested annually, 

 which can be taken as roughly equivalent to the annual deposition. Assuming an average content of 

 phosphate (P 2 5 ) of 10%, thiswould indicate an annual removal of about 440 metric tons of phosphorus 

 from the sea in this region — an amount which would be equivalent to the complete removal of 

 phosphorus from about 5^ km. 3 of upwelled water. As the total volume of water in the Benguela 

 current is probably of the order of io 4 km. 3 , the phosphate removed and deposited as guano must 

 represent a very small part of the available phosphate in the upwelled water. 



There is a certain amount of fluctuation in the total annual yield of guano and to some extent this 

 may be related to the availability of food for the birds. In years of catastrophic mortalities of fish 

 (see p. 199) there is some evidence of a fall in guano production, but such variations are by no means so 

 pronounced as on the Peruvian coast. Perhaps this may to some extent be explained by the fact that 

 the region most affected by these catastrophic phenomena is situated to the north of the islands, in 



1 The publication date of his narrative; his voyages took place much earlier. 



