120 CEYLON PEARL OYSTER REPORT. 



available; and even when brood oysters on the latter are to be had, it may be 

 preferable to go to the Chilaw banks for the supply if the oysters thereon are older. 

 If year-old oysters can be had on the Chilaw beds in quantity, while those on the 

 Periya Paar are only three months old, then it is best to move the older, Chilaw, 

 oysters first, since they have already survived the critical first year of life, and are 

 probably worth three times their number of the younger brood. 



The transplantation system can be extended also to older oysters. We have shown 

 that even adults can throw off the old byssus and form a new attachment-cable 

 whenever necessary. Consequently, overcrowding or any other source of danger 

 should now be mitigated whenever possible by transplanting to unoccupied ground on 

 the more favourable paars. 



V. CAUSES OF DISASTER. 



The above-mentioned points raise the whole question of the causes of death in the 

 pearl oyster, the reasons of the intermittence in the history of the fisheries, and the 

 conditions which render some paars more reliable than others. These matters have 

 been discussed in various preceding sections of this Report (see especially Part I., 

 p. 120, Part IL, p. 35, and Part III., p. 25). 



The following gives a summary of our results : 



(I.) The most important agent in causing wide-spread death of pearl oysters both 

 young and old in the Gulf of Manaar is the shifting of sand due to the strong 

 currents prevalent during the south-west monsoon, and no doubt occasionally (but 

 rarely) to exceptional storms. We obtained a good deal of evidence as to the 

 manner in which the sand is carried about and piled up by the currents, and is 

 churned up in places by the swell of a strong south-west monsoon, and we made 

 observations as to the effect of burying oysters of different sizes in various amounts 

 of sand. The successive broods of young oysters which have appeared, and as 

 regularly disappeared, upon the Periya Paar during the last quarter century have, 

 there can be no doubt, been overwhelmed by the bottom currents caused by the south- 

 west monsoon upon that bank which lies furthest from land and faces the deep water 

 of the Indian Ocean. The destruction from this cause is enormous. In March, 1902, 

 we ran a line of observations along more than six miles of the length of the Periya 

 Paar, and estimated that the bank bore at that time not less than about a hundred 

 thousand millions of young oysters. When Mr. HORNELL returned the following 

 November, he searched the ground from end to end and found only a few dead shells. 

 In November, 1904, this paar was again found to be covered with millions of young 

 oysters, but a year later not a single survivor was left. On the Periya Paar this 

 colossal destruction is probably an annual occurrence. On certain other less exposed 

 paars it happens occasionally, and loss to a minor extent from overwashes of sand 

 limy occur almost anywhere under exceptional circumstances. For example, the 



