Summer mortality of Pacific
oyster in Dalian, China
Guofan Zhang
Institute of Oceanology
Chinese Academy of Sciences
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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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