96 THE FISHING INDUSTRY 



tiles or frames for the spat to fall on. When the young 

 have become affixed to these " stools " they are fre- 

 quently carried away to develop in a different locality. 

 The oysters are finally fattened in sea ponds or inlets 

 that contain a large diatom population. At Marennes, 

 on the west coast of France, the water in which the 

 oysters are grown contains a particular blue diatom. 

 After feeding upon these diatoms, the beard of the 

 oyster becomes stained a bluish-green colour the 

 well-known " Marennes vertes " oysters. 



A natural oyster bed is formed on stony ground free 

 from mud and sand, so that the oyster, after becoming 

 attached to a stone, is completely surrounded by clear 

 sea-water. Oysters do not flourish in water containing 

 less salt than ordinary sea- water. Thus, there are no 

 oysters in the Baltic Sea. 



The chief enemies of the oyster are the dog-whelk 

 that bores through the shell, and the starfish that pulls 

 the valves apart and attacks the oyster inside. 



The oyster is widely distributed in tropical and 

 temperate seas all over the world. The approximate 

 value of the annual oyster crop of the world is 4,000,000, 

 representing a crop of 10 billion oysters. 



In Europe up to 75 per cent of the oysters are reared 

 from spat in artificial beds not more than 7 per cent 

 being " native." In the United States, however, over 

 40 per cent are still obtained from natural beds. 



The simplest form of oyster culture is the preservation 

 of the natural bed. These beds are easily destroyed or 

 made unproductive by over-dredging. Colonies are 

 broken up. Other animals are admitted. Breeding 

 oysters are covered up by stones and shells, and 

 suffocated. Ridges suitable for the development of the 

 spat are broken down. 



After the beds have been properly protected and 



