Previous Page Table of Contents Next Page


12.1 Culture conditions

Freshly operated oysters should be reared undisturbed for a few days. If kept in the laboratory, they should be placed in plastic troughs or FRP tanks, where seawater is allowed to flow gently. If no flow-through system is available, the seawater has to be changed frequently to overcome the narcotizing effect of menthol. When normalcy is resumed, the oysters slowly re-open their valves and commence their pumping and filtering activity. If the sea is not calm, it is desirable to rear the operated oysters under laboratory conditions till the wound heals completely. The normal duration of wound healing is only a day or two. However, if the surgery is rough or the incision is large, the nucleus could slip out of the oyster, if cultured in rough waters.

In Japan, newly operated oysters are hung in deep and calm waters for a period of 2–3 weeks, by which time they recuperate fully. Afterwards they are hung as in normal culture practices. However, in areas where the inshore waters are not calm, as in India, it is always advisable to keep the operated oysters for 3–4 days in the laboratory under observation and then transfer to the farm. If the oysters are suspended in rough sea immediately after the operation, they will be subjected to undue stress, which may lead to the dislodging of the nuclei.

Some Japanese pearl culturists examine the pearl oysters individually after recuperation by fluoroscopy, in order to check the condition of the inserted nucleus. Only those with the nucleus in the proper position are further cultured for pearl production, while the rejected ones will be retained as mother oyster for future use.

The quality of the pearl to be formed is influenced by several hydro-biological factors, such as primary production, temperature, current, trace metals content, etc. Coastal waters deeper than 5 m are usually favourable for the formation of high quality pearls. However, the culture site and depth at which the hanging culture structures should be fixed must be carefully chosen to have the most favourable conditions for the culture of the pearl oysters.

During the post-operation rearing period, the oyster density in the culture cages and culture grounds should be kept at a minimum. Over-crowding may cause adverse effects such as production of low quality pearls, slow formation of the nacre layer, shell damage and physical stress, as well as oyster mortality from diseases and parasites.

The oysters must be suspended in areas of high phytoplankton production and also at greater depths than the mother oysters. Settlement of undesirable organisms and silt on the oysters is responsible for the formation of poor quality pearls, slow formation of the pearl layer and oyster mortality. Hence the oysters should be monitored periodically and fouling organisms thoroughly eliminated. In Japan, when the seawater temperature drops below 10 °C during winter, the rafts are towed to warmer water bodies to allow the pearl oysters to over-winter.

The length of the culture period following the operation phase depends on the size of the nuclei inserted and the desired size of the pearls to be obtained. The Culture period ranges between 3–24 months for 2–7 mm diameter nuclei under tropical conditions. Periodic sampling of each oyster batch will give the basis for deciding on when to harvest. Pearls with a thin nacreous layer usually will not have good lustre and iridescence, and therefore command a lower market price.

Previous Page Top of Page Next Page