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culture 5 is done by the hanging oyster’s fixed on ropes or in baskets from special frames in the Mediterranean lagoons or on lines in the open sea. The
Brittany coastline is highly varied with numerous favorable bays and estuaries, and it has a longstanding tradition of flat oyster culture. Oysters are cultivated in
mesh bags 70 or in deep waters Buestel, et al. 2009. In the Bay of Quiberon, deep water culture has increased, with 100 farms producing about 10,000 t per
year from more than 2600 ha of concessions Buestel, 2007. In The United States, oysters can be grown either on or off the bottom of
the ground. Additionally, off-bottom techniques are being developed in Maryland on the Chesapeake Bay involving floatation devices made from PVC frames on
which these racks or bags can be placed Webster, 2007.
2.2 Factors affecting the oyster mortality
Oyster culture is affected by factors like temperature and salinity, water circulation, the presence and condition of substrate, productivity of appropriate
algal food, presence of predators and disease and protection from ice or storm that might damage culture facilities. The oyster farmers control many of these factors,
thus enhancing larval and spat production in the hatcheries. Whether oyster cultivation begins in a hatchery or in nature, eventually small oysters are used in
either on-bottom or off-bottom suspended culture FAO, 2004.
Figure 2. Factors affecting oyster mortality
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1. Temperature Pacific oysters live and grow in water with temperatures of 4 to 24
O
C, and are able to survive air temperatures as low as -4
O
C when exposed by the tide. Optimum temperature for water transport through adult oysters appears to be
about 20
O
C Pauley et al., 1988. The growth usually begins in April, reaching a peak in July and August, declines to allow level by November and December
Spencer, 1990. 2. Salinity
Although oyster grow well at wide range of salinity, flat oyster prefer salinity in the higher levels near to 30 psu and pacific oyster prefer low levels near
to 25 psu Spencer, 1990. Pauley et al. 1988 observed that optimum salinity for maximum water transport through the body of C. gigas is 25-30 psu, and pacific
oyster become increasingly sensitive to salinities below 20 psu. 3. Water circulation
Water circulation plays a paramount role in providing condition for feeding and cleansing, as well as for successful reproduction and for dispersal of
oyster larvae. In the area less than15 m deep with unstratified waters and having moderately rapid water exchange, that areas provide good condition oyster
Pauley et al., 1988. 4. Substrate
Oysters are unable move when faced with detrimental environmental conditions. Suspended bottom sediments can cause oysters to stop feeding or
expend considerable energy in separating mud and sand from edible particles. Siltation causes mortality as high as 22 in tray-reared oyster Pauley et al,
1988. Trays, particularly those with a small mesh, may quickly clog with silt. This can smother the oyster or cause them to grow more slowly because of the
poor exchange of water Spencer, 1990. 5. Algal food
Oysters feed by filtering microscopic algae phytoplankton and organic detritus from sea water. An adult oyster may pump up to ten liters of sea water per
8
hour through its body cavity, depending on its size, sea temperature and other environmental and biology factors. Some microscopic algae produce toxins which
accumulate in the flesh of mussel, oysters and clams. Shellfish or oysters containing the toxin can induce paralytic shellfish poisoning PSP in human who
eat them Spencer, 1990. 6. Predator and disease
Unprotected oysters and other small bivalves are eaten by various predators, especially drills, starfishes, crabs, fishes and to a lesser extent, by birds
Spencer, 1990. Introduction of Pacific oysters is not thought to have brought pathogens with them that have resulted to post-spawning physiological stress in
warm water when oyster are densely crowded. However, their transport to some countries for direct relaying in the sea has been inadvertently accompanied by a
number of pests and parasites including the Japanese oyster drill, Ocenebrillus inornatus, the oyster flatworm, Pseudostylochus ostreophagus, and copepod
parasite, Mytilicola orientalis. Bacterially related diseases of larvae and early juveniles are not uncommon in hatcheries and are most frequently attributed to
Vibrio spp FAO, 2009.
2.3 Oyster Culture in the Quiberon Bay
