THE PEARL FISHERY OF 1904. 25 



knowledge of their origin will warrant. This circumstance may account for some of 

 the differences of opinion as to the ages of young oysters in the past (see, for example, 

 the oysters attached to the Karativo buoy in 186G, descrihed by Mr. Holdsworth 

 and discussed by Sir W. C. Twyn am Report, &c, Colombo, 1900). 



Causes of Decrease among the Ftshable Oysters since March, 1902. 



The factors which tend to reduce the number of oysters, both old and young, have 

 been treated of at some length in former parts of this Report, and it is only necessary 

 here to discuss the causes which have led to great decrease since 1902 in the old 

 oysters that were fished this year on the Western Cheval and the disappearance of 

 the same generation from the Muttuvaratu, the Dutch Modragam and the Karativo 

 paars. The decrease appears to have arisen from two chief causes, overcrowding and 

 sand disturbance, the ill-effects of which have been intensified by several other 

 influences of subsidiary importance. The former affected the beds in the North-west 

 and Mid-west of the Cheval sections, with the Muttuvaratu and Karativo paars; the 

 latter those of the South-west Cheval. 



Sand Disturbance did the greatest harm upon the South-west Cheval, as this area 

 has but few stretches of exposed rock surface, while "cultch" is not sufficiently 

 abundant. When first seen by Captain Doxnan in 1900, the whole area was thickly 

 spread with oysters on the sand as well as on the rock and on the cultch-strewn areas. 

 With every recurrent period of stormy weather since that time these oysters have 

 suffered thinning by encroachment and overwash of sand, till eventually they were 

 cleared from off the sandy uncultched areas and only those were left that lay on rocky 

 ground and on sufficiently cultch-covered bottom. 



A subsidiary cause of destruction was the ravages of the boring sponge, Cliona 

 marga/ritif&rce, which appeared specially active on this section, riddling the valves 

 and apparently sapping the vitality of the oyster by the drain it caused upon nacre- 

 secretion. Out of 400 individuals examined 310 were affected by Cliona less than 

 23 per cent, were free from the infection. 



Apart from this the oysters of the South-west Cheval were the healthiest, the 

 bodies being plump and well developed, whereas a considerable proportion of those 

 from the Mid- west and North-west sections were thin and of poor appearance. 



The Effects of Overcrowding. Sand disturbance produced comparatively little 

 numerical reduction upon the North-west and Mid-west sections, these regions 

 possessing more extensive stretches of rocky bottom than any others in this neighbour- 

 hood. The reduction here was due primarily to the effects of overcrowding, not, 

 perhaps, so much because of their own numbers, great as these were originally, but 

 rather because of deposits of spat which fell in July and December of 1901 upon 

 the older oysters along the eastern part of these sections. For the first year the 

 effects were little marked, but as the myriads of new-comers flourished and increased 



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