Summer mortality of Pacific

oyster in Dalian, China

Guofan Zhang

Institute of Oceanology

Chinese Academy of Sciences

gfzhang@ms.qdio.ac.cnDo not

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• More than 20

regions of oyster

production in

China from

Bohai to

Southern China

Sea

China

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Oyster farming in China

• The oyster farming has more than 1000 years in southern China, such as Fujian and Guangdong provinces. The seed was collected from the natural sea.

• Crassostrea hongkongensis in Guangdong and Gunagxi and the oyster C. angulata in Fujian are the main farmed oysters.

• C. gigas and C. talienwhanensis are tested for the aquaculture in Huanghai and Bohai seas.

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Bamboo raft farming in

southern China

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Bamboo raft farming

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Stone raft farming in southern

China

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Suspended raft farming in

northern China

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• Total output: 3,826,213 tons in 2005 in shell and 35.18% in total output of mariculture mollusks.

–C. gigas and C. talienwhanensis 797,621 tons in Liaoning, Hebei, Shandong and Jiangsu.

–C. angulata and C. sikamea 1,824,765 tons in Fujian and Zhejiang.

–C. hongkongensis 1,203,827 tons in Guangdong, Guangxi and Hainan.

–C. ariakensis (not accounted)

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• In northern China seas, there is a native oyster, C.

talienwhanensis, the production depends on the

natural seed capture.

• Japanese C. gigas introduced to Zhejiang, China in

1982 and then moved to North China to become a

dominant bivalve with suspended raft farming in

the late of 80’.

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The mortality in Dayaowan Bay

• In 1993, the production of living oyster C. gigaswas 4000 kg/raft and percentage of the soft tissue

reached 13% (optimum 16-24%).

• But from 1994, the raft number increased sharply

and then the mass mortality happened in 1995 in the

Dayaowan Bay and the vicinity.

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• In 1995, the mortality of 2 year-old group oyster reached 40-50%, some site 70-80% and a few site almost 100%.

• Alive individuals showed slow growth and soft part wizened.

• The output of soft part tissue reduced to 6-9%, some become to “water-oyster”.

• So investigation of mass mortality was carried out since 1995(Sui Xilin). Do n

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Investigated site

Bohai Sea

Huanghai Sea

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A

A

B

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• Dayaowan Bay (N38059’30’- 3902’50’’, E1210

49’30’’- 121054’40’’) locates at north of Yellow

Sea and south of Liaodong Peninsula near

Dalianwan Bay which is a famous commercial

harbour. It’s a semi-closed and horse-heel shape

bay, with 7km length and 3.15km width.

• Silt is the dominant sediment of the bay.

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Objectives

• The exchange of sea water depends on the tidal

current.

• Investigation on mass mortality in Pacific oyster

focus on relationship with cultural density and

environment.

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• Impact of environmental factors on the oyster

production had been investigated during 1995-

1997. Sampling depth was at 0.5 and 3.0 m and 1

time/month. July and August were 2 times/month.

• Temperature, salinity, transparency, pH, DO,

COD, NH+4-N, NO-

3-N, NO-2-N, PO4-P,

chlorophyll, plankton, sulphide in sediment and

organic substance were measured.

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Aquaculture history

• 1966: The mariculture beginning with kelp, mussel, Chinese Pacific oyster and zhikong scallop in Dayaowan bay.

• 1988: Japanese Pacific oyster with in 55 rafts

• 1989: 1000 rafts with oyster

• 1993: 2000 rafts

• 1994: 2500 rafts

• 1995: 3500 rafts

• 1998: 5000 rafts Do not

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Raft number in different sites

0

1000

2000

3000

4000

5000

6000

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

系列1 系列2A

B

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Output (kg)/raft in different sites

2000

2500

3000

3500

4000

4500

1993 1994 1995 1996 1997 1998 1999 2000

inside inlet

A

B

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Percentage of meat output

0

2

4

6

8

10

12

14

16

1993 1994 1995 1996 1997 1998 1999 2000

inside inlet

AB

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The relationship between raft

number and mortality

0

500

1000

1500

2000

2500

3000

3500

4000

4500

1992 1993 1994 1995 1996 1997 1998 1999

raft number

0

10

20

30

40

50

60

70

mortality(%)

raft number

mortality (%)

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Environment of farming sea

Year Tem. Sal. pH COD N P chlorella

1995 10.8-25.1 26.5-31.6 7.95-8.38 0.79-1.76 115-275 3.3-14.0 2.80-14.40

1996 15.4-24.1 30.0-34.0 8.05-8.32 0.54-1.44 91-321 5.2-28.7 0.15-7.13

1997 18.1-28.1 26.0-33.1 7.77-8.30 0.87-2.16 43-200 2.0-18.7 0.92-1.15

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Plankton

Time Zoo / (103ind. m

-3) Phyto / (10

4 ind. m

-3)

07-11, 1995 712 – 46,610 150 - 8,980

07-11, 1996 315 – 2,410 25 - 780

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• 1995, Mortality found in July to Sept and peak at

mid Aug. Heavy mortality in 2 year-old group with

50-60%, which is spawning group and the 1year-old

group was 10-30%.

• 1996, the mortality occurred mid August to late

October and the peak was Sept. The mortality

increased to 60-70% in the 2 year-old group, but the

1 year group staid 10-30% in mortality.

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• The mortality of farmed oyster was different in

sites A and B.

• 1995: 50-60% mortality in site A and 10-15% in

the site B.

• 1996; similar to that of 1995.

• Mortality was heavier in the upper layer than in

lower layer of the sea and for the single culture of

oyster compared to poly-cultureDo not

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• Turbidity favors the mortality. 1999, the

Dayaowan built a new harbor and produced a

great deal of suspended substances in the bay.

• The mortality affected more young oysters than

old ones. The mortality was 60-75% for one-year

old group and 30-40% in 2-years old group.

• Parasite hydra Eudendrium ehrenberg occurred in

the gill.

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• Organic pollution and water quality (1996) : the

organic carbon was 4,172 mg/L and 4,192 mg/L in

the surface layer in site A and B, and 5,132 mg/L

and 4,196 mg/L in the bottom layer water.

• Sulfide was 1,113 mg/g and the organic substance

was 612 mg/L the site A.

• Red tidal occurred in the farming sites during Aug.

to Sept. 1995 and total inorganic nitrate (DIN)

exceeded 200µg/L.

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• In Aug.- Sept. 1995, pH and COD were highest,

Inorganic phosphate (DIP) was 0.012 mg/L, which

is lower than the lower demand of phytoplankton

(0.018 mg/L).

• This shows that oyster farming sea resulted in an

abnormal situation of poor phosphate (DIP) and

abundant nitrite (DIN).

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• The relative content of NH4+ - N was 63% -97%, far higher than the nature sea.

– This partly resulted from the metabolic process of mass farming oyster and the fouling invertebrates : sea squirt Styela certusa and Giona intestinalis living in the raft and on the shell of oyster, which are competitors for the diet and from the production of waste(feces) which reduced the exchange of the sea waterDo n

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The reasons of the mass mortality

in summer

• Over carrying capacity : biomass increased rapidly

• Single cultivated species with high density

• Poor water quality due to the high density farming

• Physiological depression of the oyster during

reproductive seasons

• Parasite hydra Eudendrium ehrenberg in the gill

• The imperfect cultural techniques

• Lack of improvement oyster for farming

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• In fact the population of 2 year-old group decreased

sharply due to the mass mortality in 1999 and

ceased next year.

• So the most important is the maintenance of

available biomass of oyster based on the

environmental capability for the oyster aquaculture

and how to combine with the algae.

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• Other oyster mortality was found in different seas

and at different seasons. Most mortality was

relative to high density of farming and the sudden

change of environmental factors with reproductive

behavior.

• C. hongkongensis in Guangdong province

appeared large-scale mortality in 80-90% in some

seas and the rickettsia was found (Wu).

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