42 SPOLIA ZBYLAlSflCA. 



The divers say that a great inrush of fresh water kills the oysters. 

 It is impossible to say at present what truth there is in this state- 

 ment , since the inrush of fresh water synchronizes with the presence 

 of an abnormal amount of mud and gritty sand in suspension. 



The windowpane oyster fishery formerly gave employment to a 

 large number of divers, and added an annual sum of about Rs. 5,000 

 to the revenue of the Island. Since 1890, however, there has been no 

 successful fishery, and the divers of Kiniyai and Tamblegam have 

 liad to look elsewhere for a means of living. The object of the recent 

 work of Willey and Hornell has been to discover the reason of this 

 decline and, if possible, to suggest remedies. Their work, therefore, 

 has consisted of investigating the bionomics and life-history of 

 Placuna ; and the survey of Lake Tamblegam, which I have just 

 completed, bears on these points. 



The main object of the scientific work at Lake Tamblegam is to 

 restore the windowpane oyster fishery to its former prosperous con- 

 dition. As a starting point in such an investigation, it is necessary 

 to determine the distribution, rate of growth, period of maturity, and 

 frequency of spawning of Placuna placenta. At present we are in 

 ignorance of all these points, with the exception of the distribution. 

 Even our knowledge of this highly important item has rested mainly 

 on the statements of the local fishermen and divers. It is only too 

 true that they know exactly where oysters are to be found, but that 

 information is not definite enough for purposes of exact record. A 

 systematic survey of the lake should be made every year, as the 

 information thus gained is of the utmost value in the solution of all 

 the other problems. No such survey appears to have been made 

 hitherto. Hornell's survey of 1905 consisted of a series of irregular 

 lines of divings at various places in the lake, which, though of value, 

 were not detailed enough. My visit to Tamblegam in September, 

 1911, was for the purpose of making a thorough survey of the lake. 

 Two canoes were employed, one in charge of Mr. George Henry, my 

 Assistant, and the other occupied by myself. A series of parallel 

 lines of divings were made, which were sufficiently numerous to 

 ensure that no large area of the lake escaped examination. In all 

 161 stations were examined, and at each station two divers made at 

 least a dozen descents. The results of these operations are appended, 

 and a series of charts compiled from those results are given, showing 

 amongst other things the depth, nature of bottom, distribution of 

 living oysters, and distribution of dead oysters. 



Placuna placenta can live best on soft mud, in depths varying from 

 I fathom to about If fathom. From this information the potential 

 area of the lake has been determined, i.e., the area upon which the 

 oysters can grow. This potential area is more than half the area of 

 the lake, and includes practically the whole of the central part of 

 the lake, together with a small patch near the head of Nachchikuda 

 and scattered patches in the south-western corner. The large central 



