The latest information on pearl farming in Tahiti

Introduction

Pearl farming is a form of aquaculture: through the cooperation between humans and bivalve mollusks, pearls are born. However, there are times when pearl oysters die en masse, a scourge that can push the industry to the brink of collapse.

In Myanmar, abnormally high mortality rates have been observed since 1983. A study identified the bacterium Vibrio as the cause.

The aim of this paper is to present information on these mass mortalities, to list and describe their causes and symptoms, and to propose suggestions based on literature and field experience.

The Phenomenon of Mass Mortality

In 1969–1970, mass mortalities affected the farms of Port Moresby (Papua New Guinea) and Kuri Bay and Smith’s Harbour (Australia). In many cases, almost all individuals died; in cages of ten oysters, only one survived (George, 1992).

At the time, the average mortality rate of Pinctada maxima was around 80%, whereas in Australia, since 1974, it had typically fluctuated between 30 and 60%. A three-year study (1980–1983) concluded that transport conditions (37 hours by boat, or 4–5 days onboard during the harvest season) and the high density in containers, which reduced water circulation and favored bacterial proliferation, were to blame. Vibrio harveyi was identified as the main agent (Dybdahl & Pass, 1985).

High mortalities (30 to 85%) were also reported in most regions of Indonesia in 1992–1994, likely related to climatic anomalies altering currents, temperatures, and plankton (Anonymous I, 1994).

In 1985–1986, at Takapoto (French Polynesia), both spat farms and grafted oyster farms suffered losses of 50 to 80% (Intes, 1995b).

The Akoya sector in China also experienced increased mortality: after 4–5 months, no nacre layer was covering the nucleus; bleached nuclei (China/Vietnam) were rejected or failed to induce secretion, and most oysters perished (Anonymous II, 1994).

In Japan, mortality — already high for a decade — peaked in 1996–1997: 150 million Akoya oysters died, with average rates ranging from 25 to 60% depending on location (Canedy, 1998; Anonymous, 1998).

Causes of the Phenomenon

Table 1 summarizes twelve main causes (alphabetical order) of increased mortality in pearl oysters reported in the literature.

Symptoms

Metabolic weakening of infected or moribund oysters manifests through numerous signs. The presence of one or more of the 16 symptoms in the table below indicates poor health.

Sometimes an oyster recovers: a clear demarcation line on the valves indicates a past infection that has been overcome.

Discussion

Infectious diseases are a limiting factor for marine invertebrate aquaculture. Under normal conditions, oysters can tolerate moderate stress but remain vulnerable to pathogens. The etiology remains poorly understood, but gross and histopathological examinations now provide useful references for diagnosing P. maxima diseases (Humphrey et al., 1999).

Beyond biological factors, physico-chemical parameters (salinity drops, temperature increases, cold/red tides, H₂S, domestic/industrial pollution) can trigger severe problems (Mizumoto, 1979; Anonymous I, 1994).

Natural disasters (hurricanes, earthquakes, tsunamis) strongly impact stocks: six hurricanes in the Tuamotu (1992–1993) devastated shallows and farms (Intes, 1995a); in Indonesia (1992), earthquakes and tsunamis weakened oysters (Anonymous I, 1994). To improve post-nucleation survival, nuclei (Japan/USA) are coated with antibiotics, with good results (Akiyama et al., 1998; Anonymous, 1999).

George (1992) suggests that mass mortality has been recurrent in Japan since 1960 and in South Seas farms working with Japanese specialists; the movement of technicians and instruments may spread pathogens (see also Aquilina, 1999). Hence the importance of systematic sterilization before and after each trip.

Transport-related mortalities can be reduced by improving water circulation, lowering container density, maintaining strict hygiene, and avoiding transport during the coldest months (Pass et al., 1987).

Transporting oysters to areas without natural colonies may introduce diseases, parasites, and predators present on the shells. Avoid transfers from infected or cyclone-affected areas where animals are weakened.

Braley et al. (1993) note that an oyster with an “unknown” disease may appear healthy, then in 2–3 days turn into an open shell with necrotic tissue; it is therefore difficult to certify that a stock is healthy.

Except for obvious cases (tsunami), causal agents often remain unidentified. As Mr. Koichi Takahashi (Mikimo America) said about the 1996–1997 episode in Japan: “all hypotheses are being considered; it is extremely difficult to determine the main cause” (Canedy, 1998).

It is essential to better understand the pearl oyster ecosystem: management of oyster numbers, spacing, maintenance, transport limitations, and monitoring of hydrological conditions. Significant water exchanges (open lagoons, bays/estuaries, exposed coasts) reduce the risk of water quality degradation (Anderson, 1998).

Suggestions

Based on field experience and literature, recommendations are proposed (Table 3).

Acknowledgments

Thanks to U Mange Toe (administrator) and U Khin Nyunt (Director General, Myanmar Pearl Enterprise) for their encouragement, and to Mr. Neil A. Sims, Mr. Martin Coereli, and Mr. Rand Dybdahl for the references provided.

Table 1: Causes of Increased Mortality in Pearl Oysters

Bacteria
Climate change
Poor farm management
Biofouling
Natural disasters (tsunami, earthquake, etc.)
Nucleus issues
Parasites
Pollution
Predators
Red tide
Rough handling
Viruses

Table 2: Symptoms of Physiological Weakening

Red/brown adductor muscle
Slowed adductor muscle reaction time (stimulation of mantle edge)
Soft, glassy, watery visceral mass
Increased mucous secretions
Malformed mantle lobes
Necrosis of the outer mantle
Heavy secretion of amorphous organic matter on nacreous valve edges
Brownish deposits inside the valves
Twisted/irregular shell growth
Temporary/permanent growth interruption
Swollen, blood-engorged ventricle
Swollen rectum
Growth stoppage → frequent death
Reproductive capacity lost/altered
Less use of crystalline style; decreased feces production
Altered pearl production: secretion of calcite instead of aragonite

Table 3: Recommendations for Pearl Farm Management

Pearl Oyster
Monitor any suspicious death: detect the first case early in a series.
Detect any shell/visceral mass anomalies: identify early warning signs of high mortality.
Do not transport oysters from one farm to another: prevent disease spread.

Farming Area
Align oyster rows with the current: improve water flow between rows and valves.
Ensure sufficient spacing between rows: maintain hygiene and proper food supply.
Store biofouling material away from culture areas: avoid accumulation and dead matter.
Monitor any abnormal abundance of predators: assess likely predation rates.

Grafting
Regularly sterilize all instruments (including gloves): prevent iatrogenic infections.
Sterilize traveling technicians’ equipment before/after each trip: avoid spreading pathogens.
Do not discard infected oyster meat into the sea; bury it: prevent new infections.

Other
Limit to 5 separations per cage (10 oysters/cage): reduce biofouling surfaces and competition.
Avoid any rough handling: minimize stress, especially in infected oysters.
Regularly monitor hydrological conditions: quickly detect any environmental change.
Study/analyze past cases: identify early warning signs of future problems.

According to staff from the Southern Marine Corporation (SOMMACO) and the Hikari South Sea Pearl Corporation in Busuanga, in the Calamian Islands of Palawan, hatchery techniques for pearl oyster farming remain experimental in the Philippines. Several obstacles are hindering large-scale production of pearl oyster larvae.

Augustin Badon, Deputy Director of Hikari, points out that only 70% of the rearing processes are currently mastered, while the remaining 30% still rely more on “art.”

“The situation differs from that of tiger prawn farming, where protocols are well established and solutions are known. For pearl oysters, we must improvise when problems arise. If we could apply proven hatchery techniques, we could produce five to six times a year,” he explains. Currently, production only takes place twice a year.

Progress at SOMMACO

Malou Sanchez, consultant for SOMMACO, reports that survival rates have climbed to 90% since the start of pearl oyster culture. “There are only seven pearl farming companies here, and we are ready to pool our resources. However, the complexity and cost of the techniques mean that the larvae produced are mainly used internally,” she explains. SOMMACO is now aiming for genetically improved broodstock and lower production costs.

The facilities were specially designed to optimize operations: pumps, filtration systems, algae culture tanks, and nursery areas are centralized, reducing movement and exposure to the elements. “Thanks to this setup, we never need to leave the building, even during a typhoon,” notes consultant John Hamiter.

Challenges of Rearing

Redentor Diaz, a technician at Hikari’s hatchery, notes that vibrios — pathogenic bacteria — are a major threat to the spat, highlighting the importance of water filtration and disinfection. Sudden changes in salinity or temperature can also affect plankton distribution, the larvae’s source of nutrients. He also stresses the need to maintain healthy broodstock and monitor feed quality to prevent contamination.

Despite these efforts, the survival rate of the 500,000 oysters set on longlines does not exceed 20%, according to Augustin Badon. To reduce pressure on natural stocks, only 20% of broodstock are taken from the wild, with the rest coming from hatcheries.

A Sector That Divides Opinion

Pearl farming is also controversial in Busuanga. Local fishermen accuse pearl farms of depriving them of traditional fishing grounds and obstructing navigation, especially at night. Farms occupy between 10% and 15% of the marine area, according to Badon.

But the economic benefits are significant: “Pearl farms provide regular income to locals while preserving the ecosystem,” says Ms. Sanchez. The longlines act as fish aggregating devices and help limit the use of destructive fishing techniques such as blast fishing or cyanide fishing, which benefits biodiversity. Operators even hope that these illegal practices will soon be banned with the support of local authorities, who collect fees on marine concessions.

Currently, foreign grafters are said to demand up to 50% of all grafted oysters, making it difficult for local operators to expand their farms and repay their loans.

This training initiative was launched by the Development Investment Board (DIB). Its first objective is to identify experts — through the Embassy of China in Wellington (New Zealand) and the South Pacific Trade Commission in Auckland — who will be able to travel to the atolls of Manihiki and Penrhyn, the country’s main pearl oyster production sites.