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.