252 SCIENCE IN AFRICA 



scene of introductions made in the interest of the native popula- 

 tion. Here there were originally only one or two very small fish 

 of no value, and in 191 9 a small cat-fish, known locally as nsonzi, 

 was introduced and has given rise to an important native trade. 

 Later, the available food supply was found to be sufficient for 

 the further introduction of two kinds of Tilapia to this and the 

 neighbouring lakes of the Kigezi district. On the finding by Ga|> 

 tain Pitman that the temperature of certain tributaries of Lake 

 Bunyoni is cool enough for rainbow trout, these also have been 

 introduced and are likely to establish themselves in the lake. 



The Nile perch [Lates niloticus) is a predator which grows to an 

 enormous size and so might be used to feed on small valueless fish 

 and make them available in larger form. Its introduction from 

 Lake Albert to Lake Victoria has therefore been suggested, but this 

 step could not be taken without great risk to the existing well- 

 established fishery. 



The experimental introductions outlined above have been 

 criticized by zoological authorities in England, on the grounds that 

 the results from such interference with the balance of nature are 

 impossible to predict, and that the introduced fish may escape 

 from the original sites and invade other waters to the detriment 

 of the original fauna. The view is taken by these authorities that 

 where introductions are made with the purpose of increasing the 

 food supply or for other urgent economic reasons, it may sometimes 

 be necessary to take the risk of upsetting the natural balance; but 

 these should never be made simply in the interests of sport. The 

 Trustees of the British Museum, in particular, have expressed their 

 opinion that such introductions are rarely defensible and that the 

 introduction of the American black bass was much to be deplored. 



Such authoritative opinions emphasize the importance of full 

 control offish introductions, and of adequate biological surveys of 

 the waters for which the introductions are proposed. In the case 

 of trout there is no reason for alarm in tropical Africa since they 

 are barred from the lower waters by temperature. Experience has 

 shown that in some streams the trout have upset the balance of 

 nature by consuming their own food supply too rapidly, but that 

 eventually a new balance is struck. Black bass are also safe in Lake 

 Naivasha since there is no means of escape, but such a predator. 



