Stimulating investment in pearl farming in ... - World Fish Center

Page 155. ENVIRONMENTAL MANAGEMENT OF PEARL FARMS5.1. Pearl oyster healthThe pearl oyster industry is based entirely on shellfish health. The pearl itself is a product ofthe immune defences of the pearl oyster, which work to fight off any invasion or irritation ofthe soft tissues by enclosing it in a pocket of calcium carbonate and protein. Since significantenergy is required to lay down this defensive pocket, the most productive pearl oysters arethose that are in good health and with access to the energy reserves required.Pearl farming means that cultured pearl oysters grow in crowded and unnatural conditions,are repeatedly handled, and undergo surgery, all of which present physiological stresses thatthe oyster would not face in the wild. The energy requirements for pearl production have tobe balanced against the need to handle the oysters and grow them in conditions they did notevolve to fit. This balance is easier to achieve if overcrowding and disease can be minimised.Most disease problems in pearl oysters centre on opportunistic pathogens taking advantage ofoysters weakened by the stress of handling, pearl surgery and sub-optimal growingconditions. Mass mortalities have been experienced in Japan, Australia, French Polynesia andCook Islands. In all cases these appear to have resulted from oysters being weakened bynatural events (e.g., high temperature or cyclone-induced changes), sometimes coupled withpearl oyster spawning seasons, or by coral spawning or red tides, both of which deplete thewater of oxygen; and then succumb to bacteria or parasites that are normally toleratedwithout harm. Poor farming and handling practices such as stocking shell too densely orfailure to clean the shell of fouling organisms, both of which weaken the oysters by deprivingthem of nutrients and oxygen, also seem to have contributed to mass mortalities.Pearl oysters have not yet faced the types of contagious disease agents that have plaguedother types of mollusc culture. Although a blessing for the industry so far, this means thatthere has not been extensive research on pearl oyster pathology, and there is a lack ofinformation documenting normal versus abnormal parasites, pests and diseases for thesespecies.Once an epizootic occurs in an aquatic habitat, the chances of eradication and control arelimited. In fact there are no examples to date of any molluscan disease agent being activelyeradicated from an open-water system. The main purpose of environmental and healthmanagement is to minimise the risk of disease outbreaks on pearl farms and, if an outbreakoccurs on one farm, to reduce the likelihood of it spreading to other farms. This is addressedmainly by regulating the total number of shell that can be held on a farm, their stockingdensity, the distance between farms, and the transfer of shell from one area to another.Environmental management also aims to ensure that pearl farming does not have negativeimpacts on adjacent marine ecosystems or on other economic activities, such as fishing ormarine tourism. Pearl farming involves the use of boats and fuel, construction materials forfarm lines and over-water structures, cleaning fluids, transfer of shells between locations, andthe disposal of pearl oyster fouling waste and diseased or dead shells. Pearl hatcheries mayuse antibiotics, disinfecting agents and other chemicals, and if an infection occurs can serveto spread disease to wild shell and the natural environment. If not carried out in a responsibleand sensitive manner, therefore, pearl farming can have potentially negative environmental,economic and social consequences which can degrade local marine ecosystems, affect otherresource users, and cause a nuisance to residents in the area.