Pearls farming : oyster disease and other causes of death
While very little is known about pearl oyster diseases or their treatment, it is known that stress can bring on disease. Disease organisms are always present in the water, but healthy pearl oysters seem to be capable of fighting off disease. When disease appears on a farm, it is almost always a sign that the pearl oysters have been subjected to some form of stress, which weakens the pearl oyster so it can no longer fight off disease organisms.
New diseases can also be introduced to which pearl oysters have no resistance. In most cases where disease has been epidemic in farming areas, it appears to be related to crowding, poor farming practices or problems in transport. The best way to prevent diseases is to keep pearl oysters in good condition so they can resist diseases. Prevention is also important because there are no known cures for pearl oyster diseases.
Causes of stress that may bring on disease include:
- Polluted water from boats, villages, factories or the farm itself; Prevent stress by locating farms in areas with clean water or upcurrent of pollution sources. Do everything possible to keep the farm area clean, including not discarding the waste material from cleaning pearl oysters in this area.
- Crowded conditions with too many pearl oysters in the same area; Prevent crowding by keeping only 10-15 pearl oysters per chaplet, not crowding pearl oysters in lantern baskets, keeping main lines at least 66 ft (20 m) apart and farms at least 1650 ft (500 m) apart.
- Rough handling; Handle pearl oysters gently and work quickly to minimize the time they are out of the water.
- Exposure to heat and cold; Keep temperatures constant during handling and transporting by shielding pearl oysters from heat and by using coolers for extended transport periods.
- Nucleus implantation “grafting or seeding;” Grafting is a very stressful process. Pearl oysters must be handled gently during and after grafting.
- Minimize the risks by hiring a good technician and following his or her instructions for post-grafting care of the oyster
- Transporting to other areas;Transporting pearl oysters over long distances is not a good idea for several reasons. The trip itself issufficiently stressful that pearl oysters may become sick and die afterwards. Transferring pearl oystersbetween areas may also introduce new diseases.
- Additionally, there is evidence that pearl oysters fromdifferent lagoons may be genetically different. This means that pearl oysters are best adapted to theirnative lagoon environment and may be more susceptible to diseases when transplanted to a new area.
- Too many fouling organisms on the shell;Fouling organisms can weaken pearl oysters by competing for food or by boring into the shell. Preventthis by cleaning the pearl oysters regularly.
- Lack of food due to crowding or being held at the wrong depth;Keep the pearl oysters at a depth of 19-23 ft (6-7 m) or at depths where you have seen the best growth.Follow the guidelines for proper spacing given above to ensure that pearl oysters receive enough foodand oxygen through good water exchange.
Introduction of new disease from other areas.
Bringing in pearl oysters from other areas, particularly those areas where farming is intensive, may
introduce new diseases and should be avoided. If you do bring pearl oysters in from areas outside the
lagoon, set up a quarantine area far away from the farming area and keep pearl oysters there for a month so you can observe them before transferring them to your farm.
What to do if disease strikes
- Disease can strike without warning and kill many pearl oysters in a short period of
- Disease can spread rapidly and kill quickly. You may not notice that the pearl oysters are beginning to die until it is too late unless you visit the farm often. If you notice that more than a few pearl oysters have died recently, try to determine if some factor is stressing the animals.
- There should be very little mortality on a farm except during the month following grafting. Even after grafting, mortalities should not exceed 5-10% of grafted pearl oysters. Correct this if possible by following the guidelines listed above. Usually crowding, excessive fouling organisms on the shell and/or rough handling during transport are to blame. Take action immediately, so the situation will not worsen and jeopardize more of your pearl oysters
- If pearl oysters begin to die, avoid moving the possibly afflicted ones to areas with healthy pearl oysters and risking spreading the disease.
Other causes of death or poor health
- Pearl oysters can also suffer from predation or fouling that may ruin their commercial value, health or even kill them.
- Fouling organisms such as sponges and burrowing worms (polychaetes) can damage and weaken the shell by boring into it. Other fouling organisms may not directly attack the pearl oyster, but as they accumulate and become heavy, they prevent the pearl oyster from feeding normally and thus weaken it.
- Prevent damage from fouling organisms by regularly cleaning the pearl oysters. Some lagoon areas may have higher fouling rates. If your farm area suffers from a heavy degree of fouling, consider moving it to another area.
- Although more difficult, some animals also prey upon pearl oysters. These predators include certain kinds of snails, crabs, fish and octopus. Snails and crabs are most often a problem in lantern baskets or other containers. They must be removed by hand and killed. Fish and octopus can also attack larger pearl oysters, although small pearl oysters are still most vulnerable. Keep the spat in lantern baskets or cages if you have problems with mortality caused by these animals. Snails and crabs are most likely to attack spat because their shells are weaker.
Article source: The Basic Methods of Pearl Farming, Author: A Layman’s ManualMaria Haws, Ph.D. (Director, Pearl Research and Training Program, Pacific Aquaculture and Coastal Resources Center, University of Hawaii at Hilo, Hilo, HI 96720 USA, Center for Tropical and Subtropical Aquaculture, Publication No. 127, March 2002)
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