L\*iS CEYLON MARINE BIOLOGICAL REPORTS. 



but never or seldom reached the soutlicni paars. The statement is only partly true, but our investigations 

 ^^•oro at that time in their infancy, and the results of further investigations have enabled us to understand 

 the situation better, even though the factors at work are still somewhat complicated. 



It is to be remembered that pearl banks exist in the vicinity of Tuticorin, and it has been for a 

 long time believed that sj)at from the oysters there drifted at times on to our own banks, i.e., those under 

 lease. This circumstance is by no means impossible, or even at times unlikely. 



Vk'o must further note that oysters usually .sj)at in July and August, and that their microscopic 

 larv£e float about on the surface of the sea for about the first five to seven days. The exact period is 

 somewhat uncertain. The larvae are, therefore, liberated in one or other of the phases of the surface current 

 during the south-west monsoon, and the M-hole interest attaching to surface currents hes in the distribution 

 of these larvae, which is effected by the surface current at the time of their liberation. The entire plateau 

 on the West Ceylon coast is potential paar ground, and other and less productive tracts occur as noted 

 near Tuticorin. It is evidently important to know what becomes of the spat shed by oysters living on 

 the several areas. If the spat is retained on the area where it was liberated, the bank within limits 

 continues to be productive. If the spat drifts away, the oysters tend to eventually become extinct. 

 On the other hand, if wcll-defuied currents exist, there is always the possibihty of banks on one area 

 being replenished with, spat from other areas, which latter must of necessity be remotely distant. 

 This phenomenon has been repeatedly -natnessed on the pearl banks under lease. There have been periods 

 when on this area scarcely a single oyster was to be found. Yet the banks recuperated and became 

 stocked wth countless millions of oysters. This rejuvenescence was certainly not due to their owir 

 recuperative powers. At present the banks are barren all along the coast, and no oysters are to be found. 

 This circumstance has often been repeated during historical times. From where do the exotic spat 

 come, and what brings them ? 



With reference to the Tuticorin banks, it is easy to understand that should spat be liberated there 

 when the south-west monsoon is light or moderate (when the current rul^s north), it is almost certain 

 that the larvae would be carried away through the Paumben Pass into Palk's Strait, or further north along 

 the Indian coast. 



If hberated during a strong monsoon, the possibiUties are that the westerly wind would drift 

 them out into the gulf, where they would become involved in the oceanic current and be carried 

 towards Ceylon. Moreover, we have seen that during a strong monsoon the oceanic current actually 

 sweeps the Tuticorin beds. 



It follows from the nature of the currents tliat the Tuticorin banks receive no exotic spat, for, as 

 far as we are aware, no other oyster beds exist round Cape Comorin. Moreover, their own resources 

 are being continually drained, and it is not to be wondered at that they are improfitable. 



On the Ceylon side, and on the areas still mider Government control, it is well Imown that they 

 arc markedly unproductive. Yet the bottom is veiy suitable, and a few scattered oysters have been 

 found thereon from time to time. No Hving oysters occurred there when we inspected the area in March, 

 1909 (Part III., " Ceylon Marine Biological Reports "), and only three fisheries have been held south of 

 Tallai\allu Point siiice 1800. They were aU held on the Chilaw Paar, and took place in 1803, 1815, and 

 1884. We beheve tliis sterility due also to the prevailing currents, which during the south-west monsoon 

 either run south when the monsoon is strong, or north with the wind when moderate or weak. Local 

 spat, whenever they occur, are swept away and lost. Moreover, the chances of this area being supphed 

 with exotic spat are very remote indeed. It is only during an intensely strong and continued monsoon 

 that the current penetrates far enough north into the gulf to sweep the Company's banks, and thus 

 convey spat therefrom to the banks still mider Government control, and even then it is highly probable 

 that such spat would be driven ashore. Their chances of receiving spat from South Lidia, whenever 

 spat occurs there, is equally remote, for as we have seen, when the monsoon is only weak the currents on 

 the Indian side run north and through the Paumben Pass. Even if a moderate monsoon persists, the 

 easterly oceanic current which eventually reaches the banks still under Government control almost 

 certainly does not sweep over the Tuticorin banks, or penetrates far enough north to allow of spat 

 being drifted into it by the prevailing westerly wind. 



