The below mentioned article provides an overview on pearl fishery in India:- 1. Introduction to Pearl Fishery 2. Artificial Pearl Culture in India 3. Prospect of Pearl Fishery 4. Enemies of Pearl Oysters.
Introduction to Pearl Fishery:
In India the harvesting season mainly depends on the condition of the sea from where the oysters are collected. From November to May, the Palk Bay becomes turbid but in the Gulf of Mannar the water remains calm and clear.
In South India the fishing of pearl oysters is done by expert divers and they mainly come from the districts of Kanyakumari, Tirunelveli and Ramanathpuram, but in the Gulf of Kachchh the pearl oyster beds are generally exposed at the low tide and are collected by hand. When the pearls grow 90-102 mm, in 4-5 years they are ready for harvesting.
Exploitation of pearl fisheries is carried out under the supervision of State Governments. One-third of the catch by a diver form his wages and further a part goes to the crew and to the owner of the boat. The rest oysters share of the Government is auctioned. After meeting the expenses, usually 3 to 4 lakh rupees are realized per year.
The highest revenue collected by the Government of Tamil Nadu, was reported to be Rs. 908953 during the period 1926-1928. The pearl oyster fisheries in the Gulf of Kachchh fetch a few thousand rupees as revenue annually for its smaller areas. The pearl fisheries of the Gulf of Mannar have been exploited for a long time.
Measures taken by the Government of Tamil Nadu to obtain sustained yields from pearl oyster beds are:
1. Restriction on the size of oysters for harvesting and observance of the closed seasons;
2. Occasional cleaning of areas for the spats to settle; and
3. Establishment of sanctuaries to enable oysters to breed and repopulate the denuded area.
Artificial Pearl Culture in India:
The method and technique of the pearl culture may be divided into:
1. Collection and acclimatization of pearl oyster
2. Nucleus insertion
2(a) Making oysters ready for operation
2(b) Preparation of graft tissue
2(c) Preparation of a suitable nucleus
2(d) Nucleus insertion operation
2(e) Post-operational rearing
2(f) Enemies of pearl oysters.
1. Collection and acclimatization of pearl oyster:
Oysters for pearl culture are obtained in various ways:
(i) Pearl oysters are collected from the floor of the sea by expert divers and are sorted out to their age and health. The suitable oysters are kept undisturbed in shallow water on moderately rough bottom for 6-7 months. This is done to acclimatize the oysters in changed high temperature and low pressure.
(ii) Raising of pearl-oysters from spats (eggs of oysters) has gained tremendous popularity by placing cages for catching the drifting oyster eggs. The cage of spat collection is prepared by a heavy wireframe that measures 84 cm x 54 cm x 20 cm and with a 2 cm wire-mesh. The cage is dipped into hot coal-tar as an act against corrosion and is then dipped into a thin sand-cement mixture.
The sand-cement coating over the cage renders an adequately rough surface to which free floating oyster spats adhere. The spat collectors are kept suspended at a depth of 6 metres from floating rafts, anchored in areas where spats are found abundantly.
2. Nucleus insertion:
2(a). Making oysters ready for operation:
Oysters selected for operation must be strong and able to withstand the operational shock. Selected oysters are then kept under suffocation for a short period, not more than two hours. As a result, the oysters open its valves and starts gaping. A small bamboo is then placed vertically in between the two valves so that the animals cannot shut down its wives.
2(b). Preparation of graft tissue:
It is done with extreme care and precision. The graft is prepared from the frilled mantle edge of a living healthy oyster that has a proved merit of highly nacre secreting capacity. A small piece of mantle tissue measuring 7 cm x 75 cm is taken and the mucus is cleaned with the help of a sponge. The thick outer edge of the mantle is cut off and the tissue is made smooth.
The remainder thin portion of the mantle tissue is trimmed to the size of 2 to 3 cm long narrow piece. Finally, the tissue is cut into smaller square pieces of definite sizes (2-7 mm in diametre with a thickness of 2-5 µ) depending upon the size of the nucleus to be used. The graft tissue is kept in clean sea water under moderate temperature (17 °C to 22 °C). The graft is placed inside the host pearl oyster within 20 minutes.
Remarks:
Since the nacre-glands are located only on the outer surface of the mantle, the respective sides of the graft- tissue (mantle) must be definitely known.
2(c). Preparation of a suitable nucleus:
Although the pearl is formed around any type of foreign irritant, yet a calcareous nucleus has been proved to be the best for the purpose. Nacre is deposited around a calcareous object in the most satisfactory way. Bead-like polished shell pieces of about 2-7 mm in diameter of each selected species of bivalves are used.
Today, most of the nuclei beads are made in Osaka in Japan and sold to the pearl cultivators. In India, such type of molluscs (e.g., Xancus pyrum) for the preparation of ‘nuclei’ (shell) are available easily. Preparation of suitable nuclei and graft tissues are done in large scale from sacred chank (Xancus pyrum) giving excellent results.
2(d). Nucleus insertion operation:
Acclimatized gaping oysters are collected from the shallow water and are brought to the laboratory along with menthol mixed sea water. The oyster is fixed with a desk-clamp in such a way that its right valve is on the upper side. The mantle folds are smoothed, and the foot and the body mass are exposed. An incision is made on the foot and a small channel is prepared in the body mass.
A piece of graft tissue is then pushed down the channel at a point selected as the bed for the nucleus. The nucleus is subsequently inserted that remains in a position just above the graft tissue (Fig. 16.79). The pedal mass is then gently smoothed, the mucus secreted in profuse quantity help in the healing process.
After nucleus insertion, the bamboo pegs are removed and the oysters are then transferred to the rearing cages. These cages are suspended from rafts at a depth of 2-3 metres for about a week to recover from the operation shock. For insertion of a second nucleus into the same oyster, operation is performed from the left side, and the nucleus is inserted into the gonadal tissue. Insertion of a third nucleus is rarely tried.
Remarks:
Oysters after nucleus insertion are periodically examined from the rearing cages and the dead ones are removed. The animals often cast off the nuclei as a response to the operational shock. Use of X-ray photography is helpful in order to identify the specimens which have done so.
2(e). Post operational rearing:
The operated oysters are kept in a box lined with finely meshed nylon net and kept in sea water for 7 days. Then the oysters are placed in rearing cages, made of bamboo baskets which are suspended from the permanent culture rafts (Fig. 16.80). Each raft accommodates 2000 pearl oysters and within 18-20 months pearl formation becomes complete.
Prospect of Pearl Fishery:
Pearl fishery in India has a great prospect as compared with other pearl producing countries that have occupied the world’s pearl market.
In India the artificial pearl production has started at the Tuticorin Research Centre of CMFRI (Central Marine Fisheries Research Institute) with the collaboration scheme of the Tamil Nadu State Fisheries Department. In 1975, K. Alagaswami has proved that the growth rates of pearls is much faster in India than that of Japanese Sea.
The comparative features are:
Thus it is apparent that India has a good pearl manufacturing potentiality under the proper encouragement by the Govt. of India.
Enemies of Pearl Oysters:
Natural enemies of pearl oysters are octopus (devil fish), barnacles, eel, etc. In addition low salinity of the water caused by the heavy rains and red tide are also dangerous to the pearl oysters. Sometimes growth of sea weeds and barnacles kill many oysters.
