Owing to the continuous failure of the fishery for ten years from 1891, the Government determined to call in the aid of experts. In the spring of 1901 Professor Herdman of Liverpool was asked by the Colonial Office, then under the direction of Mr. Chamberlain, to visit Ceylon and to report upon the state of the fishery. He reached Colombo early in 1902. He was fortunate in taking out an exceptionally well qualified assistant in Mr. J. Hornell. After a thorough examination of the fishing-grounds, Professor Herdman reported to the Government of Ceylon as follows:
‘The oysters we met with seemed, on the whole, to be very healthy. There is no evidence of any epidemic or of much disease of any kind. A considerable number of parasites, both external and internal, both protozoan and vermean, were met with; but that is not unusual in molluscs, and we do not regard it as affecting seriously the oyster population.
‘Many of the larger oysters were reproducing actively. We found large quantities of minute “spat” in several places. We also found enormous quantities of young oysters a few months old on many of the paars. On the Periya paar the number of these probably amounted to over a hundred thousand million.
‘A very large number of these young oysters never arrive at maturity. There are several causes for this. They have many natural enemies, some of which we have determined. Some are smothered in sand. Some grounds are much more suitable than others for feeding the young oysters, and so conducing to life and growth. Probably the majority are killed by overcrowding.
‘They should therefore be thinned out and transplanted. This can be easily and speedily done, on a large scale, by dredging from a steamer at the proper time of the year, when the young oysters are at the best age for transplanting.
‘Finally, there is no reason for any despondency in regard to the future of the pearl-oyster fisheries if they are treated scientifically. The adult oysters are plentiful on some of the paars, and seem for the most part healthy and vigorous; while young oysters in their first year, and masses of minute spat just deposited, are very abundant in many places.’
The chief causes of the failure of the fisheries, at any rate the chief causes which can be dealt with by man, are overcrowding and over-fishing. It might be supposed that these factors would counteract each other; but it must be remembered that they become effective at the two opposite poles of the oyster’s existence, which is thought to cover five, six, or seven years. The overcrowding takes place when the oyster is quite young and hardly fixed on the submerged reefs, whilst the over-fishing takes place when the animal is fully matured and perhaps growing old. The fact that Professor Herdman and Mr. Hornell conveyed the young oysters from Manaar in the north of the island by boat to Colombo and then on by train to Galle in the south, and there succeeded in rearing them, shows that there would be little difficulty in artificially rearing oysters in convenient localities and then transplanting them to such fishing-grounds as show danger of depletion. With regard to over-fishing, if the grounds are under the charge of a trained zoologist there is no reason why this should go on.
When Professor Herdman was called in to advise the Government, he saw at once that it was the oyster that had failed in the last ten years, not the pearls within the oysters. Microscopic examination of thin sections made through decalcified pearls showed that they are almost in all cases deposited around a minute larval cestode or tapeworm. These larvæ make their way into the oyster, and the irritation they set up induces the formation of the pearl, just as was the case with the cercaria-formed pearls of the mussel. Where do these larvæ come from? We cannot say with absolute certainty. Older specimens of tapeworms belonging to the new species, Tetrarhynchus unionifactor, also live in the oyster; and it may be that, were a larva to escape entombment in a pearl, it would grow up into one of these. But even these never become mature in the oyster; to attain sexual maturity they must be swallowed by a second host. What is the second host of the pearl-forming cestode? This question we are only recently able to answer, and here, again, without absolute certainty. I have recently described the adult form of T. unionifactor from a large ray, Rhinoptera javanica. In this fish, which feeds largely on oysters, the cestodes exist in swarms in the stomach, and the eggs make their way from the fish into the oysters, and there some of them grow up, but most of them perish in their pearly casket. If, as I believe, this is the history of the pearl-forming organism, we must regard the Rhinoptera as a friend to the industry, and not, as hitherto, an enemy which helps to destroy the oyster-beds.
The discovery of the cestode larva as a real cause of pearl-formation received an interesting confirmation shortly after it had made it. M. G. Seurat, working independently at Rikitea on the island of Mangareva, in the Gambier group, discovered a very similar larva in the local pearl-oyster around which pearls are formed; this larva, if we may judge from pictures, is almost certainly the same as the one from Ceylon. Professor Giard regards it as belonging to a tapeworm of the genus Acrobothrium; and, if he be right, then Professor Herdman’s larva is an Acrobothrium too. We have so little knowledge of the early forms of cestodes that we cannot accept this attribution as final. We may, however, hope for further information, for a French zoologist, M. Boutan, started some little time ago for the East to work at the problem; Mr. Hornell is still at work in Ceylon; and Mr. C. Crossland, who has had much experience in marine work in the tropics, has been appointed, at the request of the Soudan Government, to investigate the pearl-oyster beds of the Red Sea. Finally Dr. Willey, of the Colombo Museum, has recently described similar larvæ in the pearls of the ‘window-pane’ oyster, Placuna placenta, from the eastern shores of Ceylon.
In 1904 it was again found possible to hold a fishery in Ceylon. It was held at a place called Marichikaddi, also on the north-west coast. In the course of thirty-eight days over 41,000,000 oysters were taken. The trade was very brisk; the prices paid were unprecedented. The 1905 fishery, which began on February 18, promised to beat all records. On February 22 the catch was nearly 4,500,000 oysters; and the Government’s share for that day was £9,000. Since this date each year has yielded a bountiful harvest, and in financial circles the London Syndicate, who have obtained a ‘concession’ of the oyster-beds for twenty years from the Ceylon Government, are understood to be ‘doing very well.’