CURRENTS. • 241 



Fisheries of Ceylon "), stated that '' there appeared to be no reason to beheve tliat the oysters were 

 subject to be covered with drifting sand." Any one who has endeavoured to remove a mussel or an 

 oyster from its attachment will understand that considerable force is required to effect this. A bottom 

 current of sufficient strength to sweep away and entirely annihilate whole beds of oysters must indeed 

 be strong. It would appear that the disappearance of beds of spat or adult oysters has been curiously 

 mixed up with surface currents, or rather with surface drift. 



Whilst we attribute the disappearance to other causes, it is worthy of note that the distinction 

 between surface current or drift and bottom current is most important. The oyster lives at the bottom, 

 and the surface current or drift is only important in so far as it affects tlie bottorn, or in that it is the 

 agency whereby spat is brought or carried away. It is of course possible on a small scale for a bottom 

 current to be independent of the surface current. Our investigations, however, even though limited in 

 extent, have so far tended to prove that the water on the bottom of the area in question is calm and 

 that currents do not exist. There are movements in the water, but these movements are vertical and 

 not horizontal ; they are gentle undulations, not currents ; and we have been unable so far to collect any 

 evidence to indicate that a bottom current exists either during the north-east or south-west monsoon. 

 It would appear likely that if bottom currents exist which are strong enough to remove oysters from 

 their anchorage, the evidence relating thereto would neither be scanty nor rare ; but the dress-diving 

 operations conducted by one of us over two seasons during the north-east monsoon has afforded no 

 evidence of any current. 



It is well known that from time to time beds of oysters or spat have entirely disappeared and left 

 no trace behind. This disappearance has in every case been attributed to bottom currents. Yet in no 

 single instance has any proof been adduced that such was really the case. The disappearance was merely 

 attributed to currents because no other cause seemed apparent. In Part IV. Report of this series it was 

 there sho^vn that the annihilation of a bed of spat on the Periya Paar Karai, numbering 400,000,000, 

 was due to predatory fish. The remains of the crushed shell on the area in question «'ere only obtained 

 when the dredge was lined with canvas, and the debris even escaped the notice of the divers. When these 

 catastrophes occur on a rocky bottom, the clue to the situation remains obvious for a long time ; but if it 

 happens on sand, the broken shell gradually sinks into the sand and disappears. This circumstance is, 

 however, not due to silting over by sand, but to the heavier shell debris gradually settUng. It thus becomes 

 clear how it may happen that a bed of oysters disappear and leave no trace behind them. 



Whilst we are not prepared to -state that every bed which has been lost has been devoured by 

 fish, it appears very probable from the analogous case just quoted that such was the case, and that 

 the apparently entire absence of any remains led to the conclusion that the bed had been swept away 

 by a current. 



It is impossible, however, with any degree of certainty to state why beds of oysters disappeared 

 in the past. In this paper we shall deal with data collected during our oa^ti experience, and although 

 our evidence is wholly negative, we recognize that bottom currents may have occurred spasmodically 

 in the past, and may do so again, but during the last five years it is certain that none have existed 

 or occurred. 



It might be argued that on a shallow water plateau, such as the one under lease, any degree of 

 surface current must of necessity produce a bottom current. This is most Ukely true, but, as we have 

 already pointed out, no surface current exists on the banks under lease. 



The almost regular appearance of spat on the Periya Paar, and its equally regular disappearance, 

 presents an important question, which up to the present has not been sufficiently investigated. This bank 

 is situated about 18 miles out at sea, towards the north of the plateau and due \^'est of Tuticorin. The 

 depth of water is from 8 to 10 fathoms, and within a cable's length there are tlie overfalls (200-300 

 fatlioms). The spat so regularly found thereon are undoubtedly brought from the Tuticorin side by the 

 oceanic current, whicli rmis east during the south-west monsoon. These spat usually disappear before 

 the end of the following Januaiy (or during the north-east monsoon). The oceanic surface current which at 

 this time runs west from Tallaivillu Point no doubt produces some bottom current on the southern shallow 

 water plateau over which it sweeps, but a reference to the chart will show that the Periya Paar is well 

 north (30 miles) of tlie point from which the current takes a westerly direction. The disappearance of 

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