Popular Science Monthly/Volume 63/July 1903/The Pearl Fisheries of Ceylon
|THE PEARL FISHERIES OF CEYLON.|
UNIVERSITY COLLEGE, LIVERPOOL.
THE celebrated pearl 'oysters' of Ceylon are found mainly in certain parts of the wide shallow plateau which occupies the upper end of the Gulf of Manaar, off the northwest coast of the island and south of Adam's Bridge.
The animal (Margaritifera vulgaris, Schum. Avicula fucata, Gould) is not a true oyster, but belongs to the family Aviculidæ, and is, therefore, more nearly related to the mussels (Mytilus) than to the oysters (Ostræa) of our seas.
The fisheries are of very great antiquity. They are referred to by various classical authors, and Pliny speaks of the pearls from Taprobane (Ceylon) as 'by far the best in the world.' Cleopatra is said to have obtained pearls from Aripu, a small village on the Gulf of Manaar, which is still the center of the pearl industry. Coming to more recent times, but still some centuries back, we have records of fisheries under the Singhalese kings of Kandy, and subsequently under the successive European rulers—the Portuguese being in possession from about 1505 to about 1655, the Dutch from that time to about 1795, and the English from the end of the eighteenth century onwards. A notable feature of these fisheries under all administrations has been their uncertainty.
The Dutch records show that there were no fisheries between 1732 and 1746, and again between 1768 and 1796. During our own time the supply failed in 1820 to 1828, in 1837 to 1854, in 1864 and several succeeding years, and finally after five successful fisheries in 1887, 1888, 1889, 1890 and 1891 there has been no return for the last decade. Many reasons, some fanciful, others with more or less basis of truth, have been given from time to time for these recurring failures of the fishery; and several investigations, such as that of Dr. Kelaart (who unfortunately died before his work was completed) in 1857 to 1859, and that of Mr. Holdsworth in 1865 to 1869, have been undertaken without much practical result so far.
In September, 1901, Mr. Chamberlain asked me to examine the records and report to him on the matter, and in the following spring I was invited by the government to go to Ceylon with a scientific assistant, and undertake any investigation into the condition of the banks that might be considered necessary. I arrived at Colombo in January, 1902, and as soon as a steamer could be obtained proceeded to the pearl banks. In April it was necessary to return to my university duties in Liverpool, but I was fortunate in having taken out with me as my assistant, Mr. James Hornell, who was to remain in Ceylon for at least a year longer, in order to carry out the observations and experiments we had arranged, and complete our work. This program has been carried out, and Mr. Hornell has kept me supplied with weekly reports and with specimens requiring detailed examination.
The steamship Lady Havelock was placed by the Ceylon government at my disposal for the work of examining into the biological conditions surrounding the pearl oyster banks; and this enabled me on two successive cruises of three or four weeks each to examine all the principal banks, and run lines of dredging and trawling and other observations across, around and between them, in order to ascertain the conditions that determine an oyster bed. Towards the end of my stay I took part in the annual inspection of the pearl banks, by means of divers, along with the retiring Inspector, Captain J. Donnan, C.M.G., and his successor. Captain Legge. During that period we lived and worked on the native barque Rangasameeporawee, and had daily opportunity of studying the methods of the native divers and the results they obtained.
It is evident that there are two distinct questions that may be raised—the first as to the abundance of the adult 'oysters,' and the second as to the number of pearls in the oysters, and it was the first of these rather than the frequency of the pearls that seemed to call for investigation, since the complaint has not been as to the number of pearls per adult oyster, but as to the complete disappearance of the shell-fish. I was indebted to Captain Donnan for much kind help during the inspection, when he took pains to let me see as thoroughly and satisfactorily as possible the various banks, the different kinds and ages of oysters, and the conditions under which these and their enemies exist. I wish also to record my entire satisfaction with the work done by Mr. Hornell, both while I was with him and also since. It would have been quite impossible for me to have got through the work I did in the very limited time had it not been for Mr. Hornell 's skilled assistance.
Most of the pearl oyster banks or 'paars' (meaning rock or any form of hard bottom, in distinction to 'Manul,' which indicates loose or soft sand) are in depths of from five to ten fathoms and occupy the wide shallow area of nearly fifty miles in length, and extending opposite Aripu to twenty miles in breadth, which lies to the south of Adam's Bridge. On the western edge of this area there is a steep declivity, the sea deepening within a few miles from under ten to over one hundred fathoms; while out in the center of the southern part of the Gulf of Manaar, to the west of the Chilaw Pearl Banks, depths of between one and two thousand fathoms are reached. On our two cruises in the Lady Havelock we made a careful examination of the ground in several places outside the banks to the westward, on the chance of finding beds of adult oysters from which possibly the spat deposited on the inshore banks might be derived. No such beds, outside the known 'paars,' were found; nor are they likely to exist. The bottom deposits in the ocean abysses to the west of Ceylon are 'globigerina ooze,' and 'green mud,' which are entirely different in nature and origin from the coarse terrigenous sand, often cemented into masses, and the various calcareous neritic deposits, such as corals and nullipores, found in the shallow water on the banks. The steepest part of the slope from ten to twenty fathoms down to about 100 fathoms or more, all along the western coast seems in most places to have a hard bottom covered with Alcyonaria, sponges, deep-sea corals and other large encrusting and dendritic organisms. Neither on this slope nor in the deep water beyond the cliff did we find any ground suitable for the pearl oyster to live upon.
Close to the top of the steep slope, about twenty miles from land, and in depths of from eight to ten fathoms, is situated the largest of the 'paars,' the celebrated Periya Paar, which has frequently figured in the inspectors' reports, has often given rise to hopes of great fisheries, and has as often caused deep disappointment to successive government officials. The Periya Paar runs for about eleven nautical miles north and south, and varies from one to two miles in breadth, and this—for a paar—large extent of ground becomes periodically covered with young oysters, which, however, almost invariably disappear before the next inspection. This paar has been called by the natives the 'mother-paar' under the impression that the young oysters that come and go in fabulous numbers migrate or are carried inwards and supply the inshore paars with their populations. During a careful investigation of the Periya Paar and its surroundings we satisfied ourselves that there is no basis of fact for this belief; and it became clear to us that the successive broods of young oysters on the Periya Paar, amounting probably within the last quarter century alone to many millions of millions of oysters, which if they had been saved would have constituted enormous fisheries, have all been overwhelmed by natural causes, due mainly to the configuration of the ground and its exposure to the southwest monsoon.
The following table shows, in brief, the history of the Periya Paar for the last twenty-four years:
|Feb. 1880.||Abundance of young oysters.|
|Mar. 1882.||No oysters on the bank.|
|Mar. 1883.||Abundance of young oysters, 6 to 9 months old.|
|Mar. 1884.||Oysters still on bank, mixed with others of 3 months old.|
|Mar. 1885.||Older oysters gone, and very few of the younger remaining.|
|Mar. 1886.||No oysters on bank.|
|Nov. 1887.||Abundance of young oysters, 2 to 3 months.|
|Nov. 1888.||Oysters of last year gone and new lot come, 3 to 6 months.|
|Nov. 1889.||Oysters of last year gone; a few patches 3 months old present.|
|Mar. 1892.||No oysters on the bank.|
|Mar. 1893.||Abundance of oysters of 6 months old.|
|Mar. 1894.||No oysters on bank.|
|Mar. 1896.||Abundance of young oysters, 3 to 6 months.|
|Mar. 1897.||No oysters present.|
|Mar. 1899.||Abundance of oysters, 3 to 6 months old.|
|Mar. 1900.||Abundance of oysters 3 to 6 months old; none of last year's remaining.|
|Mar. 1901.||Oysters present of 12 to 18 months of age, but not so numerous as in preceding year.|
|Mar. 1902.||Young oysters abundant, 2 to 3 months. Only a few small patches of older oysters (2 to 22 years) remaining.|
|Nov. 1902.||All the oysters gone.|
It is shown by the above that since 1880 the bank has been naturally restocked with young oysters at least eleven times without yielding a fishery.
The ten-fathom line skirts the western edge of the paar, and the one hundred-fathom line is not far outside it. An examination of the great slope outside is sufficient to show that the southwest monsoon running up towards the Bay of Bengal for six months in the year, must batter with full force on the exposed seaward edge of the bank and cause great disturbance of the bottom. We made a careful survey of the Periya Paar in March, 1902, and found it covered with young oysters a few months old. In my preliminary report to the government written in July, I estimated these young oysters at not less than a hundred thousand millions, and stated my belief that these were doomed to destruction, and ought to be removed at the earliest opportunity to a safer locality further inshore. Mr. Horn ell was authorized by the Governor of Ceylon to carry out this recommendation, and went to the Periya Paar early in November with boats and appliances suitable for the work, but found he had arrived too late. The southwest monsoon had intervened, the bed had apparently been swept clean, and the enormous population of young oysters, which we had seen in March, and which might have been used to stock many of the smaller inshore paars, was now in all probability either buried in sand or carried down the steep declivity into the deep water outside. This experience, taken along with what we know of the past history of the bank as revealed by the inspectors' reports, shows that whenever young oysters are found on the Periya Paar, they ought, without delay, to be dredged up in the bulk and transplanted to suitable ground in the Cheval district—the region where the most reliable paars are placed.
From this example of the Periya Paar it is clear that in considering the vicissitudes of the pearl oyster banks, we have to deal with great natural causes which can not be removed, but which may to some extent be avoided, and that consequently, it is necessary to introduce large measures of cultivation and regulation in order to increase the adult population on the grounds, give greater constancy to the supply, and remove the disappointing fluctuations in the fishery.
There are in addition, however, various minor causes of failure of the fisheries, some of which we were able to investigate. The pearl oyster has many enemies, such as star-fishes, boring sponges which destroy the shell, boring molluscs which suck out the animal, internal protozoan and vermean parasites and carnivorous fishes, all of which cause some destruction and which may conspire on occasions to ruin a bed and change the prospects of a fishery. But in connection with such zoological enemies, it is necessary to bear in mind that from the fisheries point of view their influence is not wholly evil, as some of them are closely associated with pearl production in the oyster. One enemy (a Plectognathid fish) which doubtless devours many of the oysters, at the same time receives and passes on the parasite which leads to the production of pearls in others. The loss of some individuals is in that case a toll that we very willingly pay, and no one would advocate the extermination of that particular enemy.
In fact the oyster can probably cope well enough with its animate environment if not too recklessly decimated at the fisheries, and if man will only compensate to some extent for the damage he does by giving some attention to the breeding stock and 'spat,' and by transplanting when required the growing young from unsuitable ground to known and reliable 'paars.'
Those were the main considerations that impressed me during our work on the banks, and, therefore, the leading points in the conclusions given in my preliminary report (July, 1902) to the governor of Ceylon ran as follows:
1. The oysters we met with seemed on the whole to be very healthy.
2. There is no evidence of any epidemic or of much disease of any kind.
3. 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.
4. Many of the larger oysters were reproducing actively.
5. We found large quantities of minute 'spat' in several places.
6. 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.
7. A very large number of these young oysters never arrive at maturity. There are several causes for this:
8. They have many natural enemies, some of which we have determined.
9. Some are smothered in sand.
10. Some grounds are much more suitable than others for feeding the young oysters, and so conducing to life and growth.
11. Probably the majority are killed by overcrowding.
12. They should therefore be thinned out and transplanted.
13. This can be easily and speedily done, on a large scale, by dredging from a steamer, at the proper time of year, when the young oysters are at the best age for transplanting.
14. 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.
To the biologist two dangers are however evident, and, paradoxical as it may seem, these are overcrowding and overfishing. But the superabundance, and the risk of depletion are at the opposite ends of the life cycle, and, therefore, both are possible at once on the same ground—and either is sufficient to cause locally and temporarily a failure of the pearl oyster fishery. What is required to obviate these two dangers ahead, and ensure more constancy in the fisheries, is careful supervision of the banks by some one who has had sufficient biological training to understand the life-problems of the animal, and who will therefore know when to carry out simple measures of farming, such as thinning and transplanting, and when to advise as to the regulation of the fisheries.
In connection with cultivation and transplantation, there are various points in structure, reproduction, life-history, growth and habits of the oyster which we had to deal with, and some of which we were able to determine on the banks, while others have been the subject of Mr. Hornell's work since, in the little marine laboratory we established at Galle.
Although Galle is at the opposite end of the island from the pearl banks of Manaar, it is clearly the best locality in Ceylon for a marine laboratory—both for general zoology and also for working at pearl oyster problems. Little can be done on the sandy exposed shores of Manaar island or the Bight of Condatchy—the coasts opposite the pearl banks. The fisheries take place far out at sea, from ten to twenty miles off shore; and it is clear that any natural history work on the pearl banks must be done not from the shore, but, as we did, at sea from a ship during the inspections, and can not be done at all during the monsoons because of the heavy sea and useless exposed shore. At such times the necessary laboratory work supplementing the previous observations at sea can be carried out much more satisfactorily at Galle than anywhere in the Gulf of Manaar.
Turning now from the health of the oyster population on the 'paars,' to the subject of pearl formation, which is evidently an unhealthy and abnormal process, we find that in the Ceylon oyster there are several distinct causes that lead to the production of pearls. Some pearls or pearly excrescences on the interior of the shell are due to the irritation caused by boring sponges and burrowing worms. Minute grains of sand and other foreign bodies gaining access to the body inside the shell, which are popularly supposed to form the nuclei of pearls, only do so, in our experience, under exceptional circumstances. Out of the many pearls I have decalcified, only one contained in its center what was undoubtedly a grain of sand; and from Mr. Hornell's notes taken since I left Ceylon, I quote the following passage, showing that he has had a similar experience:
"February 16, 1903—Ear-pearls. Of two decalcified, one from the anterior ear (No. 148), proved to have a minute quartz grain (micro, preparation 25) as nucleus."
It seems probable that it is only when the shell is injured, as, for example, by the breaking off or crushing of the projecting 'ears,' thereby enabling some fine sand to gain access to the interior, that such inorganic particles supply the irritation which gives rise to pearl formation.
The majority of the pearls found free in the tissues of the body of the Ceylon oyster contain, in our experience, the more or less easily recognizable remains of Platyelmian parasites; so that the stimulation which causes eventually the formation of an 'orient' pearl is, as has been suggested by various writers in the past, due to infection by a minute lowly worm, which becomes encased and dies, thus justifying. in a sense, Dubois' statement that—'La plus belle perle n'est done, en définitive, que le brillant sarcophage d'un ver.'
To Dr. Kelaart (1859) belongs the honor of having first connected the formation of pearls in the Ceylon oyster with the presence of vermean parasites. It is true that Filippi seven years before (in 1852), showed that the Trematode Distomum duplicatum was the cause of pearl formation in the fresh-water mussel Anodonta, and Küchenmeister (1856), Moebius (1857) and others extended the discovery to some of the larger pearl oysters, and to other parasites; but it is probable that Kelaart knew nothing of these papers and that he made his discovery in regard to the Ceylon oyster quite independently. He (and the Swiss zoologist, Humbert, who was with him at a pearl fishery) found "in addition to the filaria and cercaria, three other parasitical worms infesting the viscera and other parts of the pearl oyster. We both agree that these worms play an important part in the formation of pearls; and it may yet be found possible to infect oysters in other beds with these worms, and thus increase the quantity of these gems."
Thurston, in 1894, confirmed Kelaart 's observation, finding in the tissues, and also in the alimentary canal, of the Ceylon oyster, 'larvæ of some Platyhelminthian (flat-worm).'
Garner (1871) associated the production of pearls both in the pearl oysters and also in our common English mussel (Mytilus edulis) with the presence of Distomid parasites; Giard (1897) and other French writers have made similar observations in the case of Donax and other Lamellibranchs; and Dubois (1901) has more recently ascribed the production of pearls in mussels on the French coast, to the presence of the larva of Distomum margaritarum. Jameson (1902) then followed with a more detailed account of the relations between the pearls in Mytilus and the Distomid larvæ, which he identifies as Distomum (Brachycælium) somateriæ (Levinson). Jameson's observations were made on mussels obtained partly at Billiers (Morbihan ), a locality at which Dubois had also worked, and partly at the Lancashire Sea-Fisheries Marine Laboratory at Piel in the Barrow Channel. Finally, Dubois has just published a further note in which, referring to the causation of pearls in Mytilus, he says (p. 178): "En somme ce que ce dernier [Garner] avait vu en Angleterre en 1871, je l'ai retrouvé en Bretagne en 1901. Quelques jours après mon départ de Billiers, M. Lyster Jameson, de Londres, est venu dans la même localité et a confirmé le fait observé par Garner et par moi." But Jameson has done rather more than that. He has shown that it is probable (his own words are 'there is hardly any doubt') that the parasite causing the pearl-formation in our common mussel (not in the Ceylon 'pearl oyster') is the larva of Distomum somateriæ, from the eider duck and the scoter. He also believes that the larva inhabits Tapes or the cockle as a first host before getting into the mussel.
We have found, as Kelaart did, that in the Ceylon pearl oyster there are several different kinds of worms commonly occurring as parasites, and we shall I think be able to show in our final report that Cestodes, Trematodes and Nematodes are all concerned in pearl formation. Unlike the case of the European mussels, however, we find so far that in Ceylon the most important cause is a larval Cestode of the Tetrarhynchus form. Mr. Hornell has traced a considerable part of the life history of this parasite, from an early free-swimming stage to a late larval condition in the file fish (Batistes mitis) which frequents the pearl banks and preys upon the oysters. We have not yet succeeded in finding the adult, but it will probably prove to infest the sharks or other large Elasmobranchs which devour Balistes.
It is only due to my excellent assistant, Mr. James Hornell, to state that our observations on pearl formation are mainly due to him. During the comparatively limited time (under three months) that I had on the banks I was mainly occupied with what seemed the more important question of the life-conditions of the oyster, in view of the frequent depletion of particular grounds.
It is important to note that these interesting pearl-formation parasites are not only widely distributed over the Manaar banks, but also on other parts of the coast of Ceylon, Mr, Hornell has found Balistes with its Cestode parasite both at Trincomalie and at Galle, and the sharks also occur all round the island, so that there can be no question as to the probable infection of oysters grown at these or any other suitable localities.
There is still, however, much to find out in regard to all these points, and other details affecting the life of the oyster and the prosperity of the pearl fisheries, Mr, Hornell and I are still in the middle of our investigations, and this must be regarded as only a preliminary statement of results which may have to be corrected, and I hope will be considerably extended in our final report.
It is interesting to note that the Ceylon Government Gazette, of December 22 last, announced a pearl fishery, to commence on February 22, during which the following banks would be fished:
The southeast Cheval Paar, estimated to have 49 million oysters.
The East Cheval Paar, with 11 millions.
The Northeast Cheval Paar, with 13 millions.
The Periya Paar Kerrai, with 8 million—making in all over 80 million oysters.
That fishery is now in progress, Mr. Hornell is attending it, and we hope that it may result not merely in a large revenue from pearls but also in considerable additions to our scientific knowledge of the oysters.
As an incident of our work in Ceylon, it was found necessary to fit up the scientific man's workshop—a small laboratory on the edge of the sea, with experimental tanks, a circulation of sea-water and facilities for microscopic and other work. For several reasons, as was mentioned above, we chose Galle at the southern end of Ceylon, and we have every reason to be satisfied with the choice. With its large bay, its rich fauna and the sheltered collecting ground of the lagoon within the coral reef, it is probably one of the best possible spots for the naturalist's work in eastern tropical seas.
In the interests of science it is to be hoped, then, that the marine laboratory at Galle will soon be established on a permanent basis with a suitable equipment. It ought, moreover, to be of sufficient size to accommodate two or three additional zoologists, such as members of the staff of the museum and of the medical college at Colombo, or scientific visitors from Europe. The work of such men would help in the investigation of the marine fauna and in the elucidation of practical problems, and the laboratory would soon become a credit and an attraction to the colony. Such an institution at Galle would be known throughout the scientific world, and would be visited by many students of science, and it might reasonably be hoped that in time it would perform for the marine biology and the fishing industries of Ceylon very much the same important functions as those fulfilled by the celebrated gardens and laboratory at Peradeniya for the botany and associated economic problems of the land.
- Abstract of discourse before the Royal Institution of Great Britain.
- Comptes Rendus, October 14, 1901.
- Comptes Rendus Acad. d. Sci., January 19, 1903.