Rehabilitation of Pearl Oyster Beds in French Oceania
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Rehabilitation Of Pearl Oyster Beds In French Oceania Once rich in pearl oyster beds, many lagoons in French Oceania have become exhausted due to over- exploitation by man, and to natural causes. Practical measures to restore their productivity are detailed in this article. By GILBERT RANSON* The time of the year when the eggs are produced has been the subject of numerous observations which, however, do not agree with one another. In fact, the reproductive glands are only func tional from October to February, that The author, with native is, during the Austral summer. From assistant, preparing to March to September, the winter months, shoot a plankton net in the glands contain only remnants of Hikueru Lagoon. more or less degenerated sexual products. When a favourable set of circum stances occurs, an astronomical number of eggs and spermatozoa are released by the female and male pearl oysters. They sink to the bottom or are carried away by the currents. The eggs are rapidly TOURING a mission to French Oceania main points of the biology of the pearl fertilized. Within 24 hours an egg of I studied the conditions under which oyster. good conformation (only one-half to pearl oysters were living in the Hikueru two-thirds are) is transformed into a and Takuma lagoons. Shorter visits were Biology Of The Oyster small ciliate larva, capable of minute made to other islands—Kaukura, Apa- In pearl oysters the sexes are dis vertical movements, which remains near taki, Takapoto, Takaraoa, Raroia, and the surface of the sea. This larva is, Fakarava. I went also to Niau and tinct. Male and female oysters can be differentiated. Japanese scientists have however, unable to swim against the Makatea, where, although no oysters are slightest current, which carries it away. to be found, I was able through obser made very detailed studies of the Japan ese and Celebes pearl oyster, and have It is part of the plankton. All marine vations to understand better the evolu animals from the fish down to the tion of the other islands. found that they change sex from one year to the other. It is therefore impos smallest crustaceans eat these larvae. I recorded water temperatures and sible, from the mere examination of the For 21 to 25 days, a plaything of the salinities, collected plankton, studied the shell, to say whether the specimen is currents, it grows and undergoes many growth of the pearl oyster at various male or female. To do that, it is neces transformations. On the second day it depths, the nature and condition of bot sary to examine the reproductive pro is equipped with the embryo of a shell. toms, the direction of bottom and sur ducts of each under a microscope. Gradually its internal organs become face currents, and the evolution of the complete and our larva, measuring oyster's reproductive glands. The ratio of male to female oysters is highly variable, according to localities. about one-fiftieth of an inch on the 21st At the outset I found that 35 lagoons, In the Hikueru lagoons I have observed day, is near the end of its planktonic previously prosperous, now produced a preponderance of males in some places, life. It is about to undergo an internal only negligible quantities of pearl shell, of females in others, and more or less metamorphosis. It drops to the bottom or had completely stooped producing; 8 equal numbers of each sex elsewhere. and, provided it finds a suitable support had still an appreciable yield, and only A pearl oyster five to six years old on which to attach itself, it goes on 6 were rich. can produce some tens of millions of living. Otherwise, it dies or becomes a Let us examine the reasons for such eggs, while the number of spermatozoa prey to the innumerable enemies lying an evolution, after briefly reviewing the emitted by a male is more than ten times in wait. Myriads die in this way. greater. The young survivors still have to face When one year old the oyster already many new dangers, including fish, crus * Professor Ranspn is Deputy Director of the Laboratory of Malacology, National Museum of gives forth sexual products, but it is not taceans, molluscs and echinoderms. Natural History, Paris. certain that they are viable. Again many will die. Very few will Page 22 SPC QUARTERLY BULLETIN, JULY, 1955 Selling out collectors of mikimiki wood in Hikueru Lagoon. Several months later it was observed that considerable quan tities of young oysters—more than had been hoped for— had attached themselves to the collectors. reach the adult stage. For each million eggs spawned, only from one to ten will go on to perpetuate the species. The number of parent oysters necessary to ensure the survival of the species in a given locality can now be gauged. When the breeding stock fall below a certain number the total extinction of the popu lation will sooner or later become a fact. The larvae, which have been dispersed by the currents, attach themselves any where, even in places where the adults will vegetate for the rest of their exist ence. The adult stages can withstand severe conditions. However, only those living under very favourable conditions will give birth to normal progeny; their eggs will be well constituted. The par ticular zone where these conditions ob tain may be considered as the "nest of the species." If the breeding stock is men have no control. The former long before 1900: in 1863, Bovis was taken away from it, the species ceases should be brought to a stop, and we already requesting regulation of the to be. In the case of the pearl oyster of should try and solve the difficulties fishing. French Oceania, these favourable con caused by the latter. All the steps taken since, such as ditions are found only in the deep parts Attempts have been made through dividing some lagoons into three areas of the lagoons. successive regulations covering fishing which are each in turn opened to diving, operations, and also by a division of and the opening of other lagoons in Reasons For Lagoon Exhaustion some lagoons into "sectors" (measures rotation, are excellent, and those at In the light of these facts we are now excellent in themselves), to minimize the present in force will be maintained. They in a position to understand what has effects of over-exploitation. But these are aimed towards conserving pearl shell happened in the course of time in the steps were not sufficient. Nothing was resources in the lagoons. pearl-shell producing lagoons. attempted to obviate the untoward effects The evolution of the lagoons, with of the natural phenomena which hamper Establishment Of Reserves Essential regard to the problem under review, re the reproduction of pearl oysters. The However, these steps have not been sults partly from the disorderly activi main points were overlooked. sufficient. The main point has been ties of the men who have exploited them, I have mentioned that the spawning overlooked; the establishment of "natu and partly from the combined action of of pearl oysters in the Gambier and ral reserves" to assure, on a permanent natural phenomena over which these Tuamotu Islands takes place from Octo basis, the necessary breeding stock. ber to February. The fishing season It might be assumed that the parts of should not coincide with the breeding the lagoons which are closed to fishing SKETCH OF A FLOATING COLLECTOR season. This problem is easy to solve for two or three years contain enough SCHEMA O'ON COLLECTEUR MOBILE in the Tuamotus, where the water is progenitors, or that the fact that a never very cold (79° to 86° F.). I have lagoon is not open to diving for three Surface of foe sea^ ^SurfAce de /kau therefore proposed that in future the or four years ensures its restocking. lagoons of this Group be opened for This is not so. After the fishing Float "Q—fh/Zeur- diving from April to July only. season, pearl oysters are too few and too In the Gambier Group the problem far apart in these areas. The few male Fa.gqot of __^ . f&gor de bois or female oysters which have escaped de mikimiki is more complicated, as the seasons are mtkimiKi wood more definitely marked. In winter the the divers are often too far from each water is colder and diving is not pos other. A large number of eggs is never sible. After studying the question closely fertilized. I have proposed that an open season be In addition, almost all the old oysters declared from January 1 to April 30 in are gathered, and there remain on the all islands south of 20° S. bottom only young oysters from one to Wire cable -*. • fi'lin d'&CJer We have seen how, for every million three years old, which are probably not eggs released, only one to ten will capable of releasing viable sexual pro eventually become adult oysters. This is ducts. In order to have progeny, one the most important factor affecting the needs oysters four years old or more, in prosperity of a lagoon. Tens of large numbers and concentrated on some thousands of progenitors, concentrated favourable bottom. over a comparatively small area, are The necessity for a reserve in each necessary to ensure the continuity of the lagoon is obvious. From 50,000 to species. Over-fishing is fatal in that it 100,000 mature pearl oysters, living decreases the stock in a dangerous under good conditions, would produce Rock-*; Pierre Bottom of lago'ctn^ Fond du /agon ^ manner.