CURRENTS. 231 



No. 20. 



CUREENTS ON THE CEYLON PEAEL BANKS, 

 SUPEEFICIAL AND DEEP. 



By T. SOUTHWELL, A.R.C.Sc. (Lond.), F.L.S., F.Z.S., and Lieut. J. C. KERKHAM, R.N.R. 



With three GJmrts. 



The interest attaching to currents on the pearl banks Ues almost wholly in the relation they bear 

 to the oyster, for it is commonly believed that beds of spat or adult oysters have from time to time been 

 entirely swept away by the agency of bottom currents. It is important, in the first place, to note that 

 the oyster is stationary, and almost defenceless against the vast majority of its natural enemies. Pish 

 of various species devour them omnivorously. Subtle and fatal diseases attack them. A variety of 

 other animals compete with them for food, and often grow on their shells, as if to purposely arrest and 

 accommodate such food as the gentle undulations of the bottom layer of water may bring near by. 



The outstanding character of the oyster (Margaritifera vulgaris) is its sedentary habits, and the 

 power it possesses of holding on to rock and other solid objects in such a way as to remain securely 

 attached. This condition is essential to the oyster's welfare, and so strong and powerful is this attach- 

 ment that it is only with difficulty that divers are able during fishery times to dislodge them from their 

 anchorage. The attachment is made by means of a beard or byssus, similar to that occurring in the 

 ordinaiy Enghsh mussel {Mytilus edulis). A very considerable percentage of oysters lose their byssus 

 in being pulled away from their attachment. This structure can, however, be replaced in a very short 

 time, and if necessary many times in succession. 



In deaUng with currents on the Ceylon pearl banks, it is essential, in the first place, to carefully 

 distinguish between surface and bottom currents, for they bear different relations to the oyster. There- 

 fore, in order to understand these relationships better, we will deal with each separately. 



I. — SUEFACE CHREENTS DUBING THE NOETH-EAST MoNSOON. 



Investigations of the surface currents have been carried on intermittently during the last four 

 years, but only during the north-east monsoon. The observations on the south-west monsoon have 

 only been carried out during two seasons, viz., 1908 and 1910. The results during the latter monsoon 

 were in every sense satisfactory. The percentage returns were high (51 "5 per cent.), and the results of 

 each year's work were similar. In 1910 alone 565 bottles were released at different stations during the 

 south-west monsoon, of which 291 were returned ; whilst in 1908 only 80 bottles were released, and 47 

 returned. The results from many bottles hberated during the north-east monsoons of the last four 

 years have had to be discarded, owing to the fact that the management of these drift bottles changed 

 hands, and the results of the first two years' work were useless, as the necessary data was not obtainable. 

 This paper, therefore, embodies the result of two years' work during each monsoon, viz., those of 1908 

 and 1909 during the north-east monsoon, and those of 1908 and 1910 during the south-west monsoon. 

 Full statistics are given at the end of the j)aper. 



Apart from the drift bottle expesiments, our knowledge and experience has been further extensively 

 supplemented by the observations made by commanders of coasting vessels, some of whom have had 

 many years' experience. 



