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The Natural Pearl Shell Populations in French Polynesia

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The Natural Pearl Shell Populations in French Polynesia , --..-1. i . ..... .. .- . , . ! k ?. '4 Excerptsfiom an article by Don Kirkwood, published in Business Queensland, 23/8/93 Australia's A$200-300 m pearl industry could be He claims that, using techniques developed in threatened by competition from Asia if it does not Australia, the Indonesian industry is establishing upgrade its techniques, according to Bruce Stevens, hatcheries, and may well be able to produce higher- managing director of Reefarm Hatcheries. 'We've grade pearls in future. got to improve our genetic stock', he says. 'We've just collected our stock from the wild. There's been He says 'Australian pearl farmers have high labour no selective breeding'. costs, but hatcheries can produce spat for A$3 each, compared with A$20 each if they're gathered Reefarm hatcheries produce the high-quality bou- from the wild.' tique pearl, the highest-priced pearl in the world, a fetching from A$400 to A$ 80,000 each. Excerptsfiom Atlas de la Polynésie française (chapter written by André Intes) Past over-harvesting of natural pearl-shell stocks developed both by the Fisheries Service and by the has caused widespread depletion of oyster-pro- Institute for the Promotion of Aquacultural and ducing lagoons at the same time as the develop- Maritime Activities (EVAAM), will become the ment of pearl culture requires increasing quantities only supply source for professionals, as in Japanese of oysters. Studies carried out before 1960 showed pearl farming. a progressive decline in stocks and proposed mea- sures to conserve or rehabilitate this resource. None Having stated these development prospects, the of these recommendations have been implemented, resource's biology as well as its numerical strength probably because almost all were based on tech- remainincompletely understood andmustbemore niques similar to those used in oyster farming (spat closely studied in order to promote pearl culture. collection, oyster beds, husbandry), in which local This is a very difficult problem, however, for there communities had no faith. are as many stocks as there are lagoons and each one has it own characteristics. Only when there was no choice but to consider the resource completely exhausted, and when the ini- For the moment, the scattered geographic locations tial grafting tests demonstrated the product's high of the lagoons makes a Territory-wide assessment commercial value, the techniques proposed, by the impossible, but relatively complete information is Fisheries Service in particular, were quickly available on some stocks, such as the one inTakapot0 adop ted. atoll in the Tuamotu Archipelago. This atoll has always been among the principal pearl-shell pro- Since then, exploitation has undergone a complete duction centres. Its maximum annual production transformation, with objectives and techniques has been estimated at about 400 mt. In the 1950s, being radically changed. Only the targeted resource harvests remained large and exceeded 100 mt dur- 'i.e. natural stocks' has remained the same. These ing some seasons (1955,1957). During 1982-1983, are still exploited because controlled breeding tests ORSTOM and EVAAM carried out a study and carried out from 1976 to 1979, and those currently assessment of this stock. The findings are described under way, have not yet allowed large-scale pro- below. duction of spat. Stock distribution In 1983, fishing by skindivers still supplied nearly 80 per cent of oysters for grafting, but this harvest- Pearl oysters live in shallow depths of up to 60 m, ' ing or' adult animals w'll be eliminated soon. Spat clinging by their byssus to the coral substrates collection, for which effective\. techniques have been which form their'biotope in the atoll lagoon. In this SPC #8 Pearl Oyster Information Bulletin April 1995 I biotope, abundance varies according to ecological the 0-20 m layer is considered as a single unit. It is parameters and fishing pressure. These factors de- probable that this gradient inversion between the termine the distribution of the pearl-shell stocks. reserve area and the fishing areas is linked to both legal and illegal harvesting. Mean densities, even if they do not allow abun- dance in the lagoon to be determined absolutely, The deepest area of the lagoon, with a depth of are ..a good way of describing stock distribution.. - more than 40 m, is limited to Area 2 and showed only low densities. These trends by area were also Density measurements found in the other strata, with the highest densities observed between 20 and 40 m, that is to say, for all Densities are determined by divers who count oys- practical purposes, beyond the reach of most skin ters along transects. Each strip surveyed by a diver divers. measures 2.5 m in width, i.e. 5 m for a team of two divers. The number of pearl oysters found every These observations tend to show 'that the still rela- 10 m is noted. The typical 50 m long line is consid- tively productive lagoons, either overall or in spe- ered to be the basic density survey unit and corre- cific locations, have densities of around one oyster sponds to a sampling area of 250 m2. per 10 m2. Surveys carried out in 1984 over the entire area of These average densities allowed Takapoto to be the lagoon have allowed density distribution in compared with other lagoons where similar stud- Takapoto Lagoon to be described in terms of depth ies were carried out between 1982 and 1984, as and location. Vertically, depth layers or strata 10 m shown in Table 2. thick were used. Horizontally, there are three fish- ing zones, with a reserve area making up a fourth, Table 2: Mean densities of pearl oysters perm2in at the southern end of the lagoon. some French Polynesian atolls This two-dimensional division is of practical inter- est for stock exploitation; legal fishing is carried out Atoll Mean Standard by free divers, leaving the more shallow strata density error more exposed to harvesting. Fishing seasons are 0.09 opened on a rotational basis in the various areas. Scilly (Society Islands) 0.10 The overall results obtained have been expressed in Takapoto (Tuamotu Group) 0.09 0.07 term of mean densities in Table 1. Gambier Islands 0.02 0.05 The reserve area retums the highest densities, espe- Hikueru (Tuamotu Group) 0.01 0.0 1 cially in the surface layer. In Area 1, the average Manihi (Tuamotu Group) 0.01 0.0 density gradient increases with depth. The same 1 trend was observed in Areas 2 and 3, especially if Depth Mean number of oysters per square metre #2 I Reserve Area #1 Area Area #3 Mean 0.24 0.04 0.03 0.09 0-10 m 0.07 0.02 0.02 0.06 10-20 m 0.12 0.08 0.17 0.07 0.07 20-30 m 0.11 0.10 30-40m - 0.13 0.14 0.08 0.12 > 40 m - - 0.02 - 0.02 I 0.09 0.06 0.05 0.09 J April 1995 SPC Pearl Oyster Information Bulletin #8 sa These lagoons are divided into two groups, whose sampling effort proportionate to the size of the densities vary considerably. The first group (Scilly lagoons surveyed, which can vary from 80 to 300. - Takapoto) recorded one pearl oyster per 10 m2, km2.The presence of coral structures shows up the while in the second group (Gambier - Hikueru - recording as irregularities in the relief and a thicker Manihi), nomore than one or two pearl oyster were trace. observed for every loom2. In Takapoto, 12 transverse radials were used to In Manihi and Hikueru, the margin of error is very estimate the area of the lagoon floor. The area of the small and corresponds to a relatively uniform stock biotope favourable to oyster growth represented where all the densities observed were low. about 65 per cent of the developed area of the lagoon floor, of 83 km?. The situation is different in the Gambier Islands, however, because high densities do occur, particu- This relatively large area should not be surprising, larly in the area of Aukena (one pearl oyster per 2 as it is known that more than 400 coral patches and m2), at depths of less than 10 m. pinnacles rise above the sediment to emerge at high tide and that the number of lesser-sized structures, These observations tend to show that lagoons which especially those invisible from the surface, is even are still relatively productive sustain densities of greater. The surface area of coral slopes and walls is around one pearl oyster per 10 m2,either overall or thus considerable. in specific locations. Using information on the extent of the biotope and Density is very important for reproduction, be- average densities by strata, the stratified sampling cause successful spawning depends heavily on the technique allows the size of the lagoon population number of gametes released and on simultaneous to be calculated (figures are given in Table;3). spawning by the parental stock. These two aspects, called mass effect (the quantity of sexual cells) and Scilly Lagoon in the Society Islands is twice the size group effect (spawning by one animal induces that of the Takapoto lagoon, and has higher mew den- of its nearest neighbours) are fundamental ele- sities, but as thebiotopeis proportionally muchless ments of stock management. developed, its total population is only 5.5 million specimens compared with 7.5 million in Takapoto. Stock assessment In addition to the density measurements described above, it is important to know the extent of the pearl oyster biotope in order to estimate the stock's size. Estent of the biotope The limited penetration depth of satellite telemetry techniques does not allow all hard lagoon sub- strates to be surveyed.
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