1. Scientific name: Crassostrea gigas
2. Common name: Pacific oyster
The Pacific oyster is characterized by high
tolerance against water temperature variation, high
reproduction, and high colonization in the wide range of
Pacific coastal waters, including Korea, Japan, and China.
4. Ecology & habitat
The oysters are characterized by an
extraordinary ability to adapt to variations in the water
temperature. The species has a high performance of
reproduction in a wide range of habitats. The most
influencing factors affecting the distribution of the
oyster species are temperature and salinity. In natural
waters, the species spawns several times over May to
September according to its ambient water conditions. The
oysters feed on planktonic algae and organic matters.
Oyster aquaculture has been a traditional
practice and has taken considerable parts of total
shellfish production in Korea. For the culture of Pacific
oysters, seeds are obtained both from wild collection and
hatchery. Hatchery based seeds are commercially available
from 1990s and now increasing numbers of oyster
aquaculturists are using the hatchery-based seeds. For the
wild seed collection, the seed spats are collected on
collectors which are suspended from lines within 1 meter
in depth. The seed collectors, a string of scallop shells
threaded on a wire of about 1 meter length, are put on
time when ready-to attach oyster larvae predominate in the
Seedlings are practiced twice a year. This is
location-specific, the first seedling for oyster farms in
Namhae, Yosu, and Koheung, the second seedling for the
farms in Tongyound, Kosung, and Koje. The first seedling
is practiced in June to July, while the second seedling is
in August to September. Simplified farming strategy from
the two seedlings are follows;
The first seedling: Seed production
(Jun∼Jul) → Growing (Jul∼Mar) → Harvesting (Nov∼Mar)
The second seedling: Seed production
(Aug∼Sep) → Hardening (Oct∼April) → Growing (Apr∼Nov)
→ Harvesting (Nov∼Apr).
6. Seed production
The hatchery-based seed production eliminates the laborious
works on the sea. In the hatchery-based seed production, selection of healthy
broodstocks and algal cultures are necessary. Temperature manipulation is
one of the routine factors to control the reproduction of the oyster. In the
hatchery, algal foods such as Isochrysis galbana, Pavlova luteheri, Chaetoceros
calcitrans, Tetraselmis suecica, Phaeodactylum tricornutum, etc. are served
singly or in a combination manner for the nutritional balance. Supply of algal
foods are functional to filtering rate of the oyster broodstocks. However,
filtering rate of the oysters are functional to food concentrations. For example,
the filtering rates of the oysters fed 50, 75, 100x106 I. galbana cells/L
were 3 : 2 : 1. Therefore, it is important to provide favorable amount of
food concentration to keep the broodstocks healthy.
The production of healthy oyster seeds is a recent issue
in Korean oyster farming industry. To meet this, mass seed
production in the hatchery is primarily required. The
total seeds the industry needs annually are amounting to
1,800 ten thousands cultch (a cultch is of 30 oyster
shells). However, in reality, most of the seeds still come
Marine hatchery of NFRDI has played a crucial role in the
hatchery-based seed production. The oyster seeds produced
in the hatchery have been preferred to wild ones. The
merits of the hatchery-based seed are in their faster
growth and better fatness over wild seeds. This merits
have prompted the oyster farmers to produced hatchery
seed, resulting in recent production of 60 ten thousand
cultch (a cultch is of 60 oyster shell) in the hatcheries
located in Tongyoung and Koje areas, the southern parts of
Selection of quality good brood stocks is the first step in the hatchery-based
seed production. Once selected, the brood stocks are cleaned and conditioned
for maturation in the maturation tanks. Good supply of algal food and temperature
manipulation are key conditioning factors for the brood stocks. Temperature
manipulation should be achieved on the basis of 1℃ increase per day
up to 25℃ from the ambient water temperature 7℃ at the time of the conditioning.
Flow-threw system is used for the conditioning and holding of adult oysters
in hatchery. The algal foods, Isochrysis galbana, Tetraselmis teterathele,
and Phaeodactylum tricornutum are fed on the basis of 20∼30x104
cells/mL. The fully matured oysters are very sensitive to environmental changes.
Therefore, a careful handling of the spawners is particularly required.
Gametes are usually obtained by means of stripping and/or stimulating ripe
oysters. Spawners stripped usually produce fewer eggs available than those
stimulated because the stripping method might include immature eggs. There
are some stimulators for inducing the spawning behavior of the oyster. Of
them, temperature manipulation provides most useful method. For the temperature
stimulation, the ripe oysters are suddenly exposed to elevated water temperature.
A sudden increase of water temperature from 20℃ to 29℃ in several hours
strongly stimulate the ripe oysters to spawn in a few hours. It is important
to clean the spawners before they are stimulated. As soon as all the spawning
activities of the oysters are finished, the adults are to be removed from
the spawning chamber. The size of the spawned eggs is 52㎛ on average in
diameter. After fertilization, the eggs should be washed with filtered and
sterilized seawater, using a washing net (with mesh size, 20㎛).
Eggs develop to embryos in 6 hours at 26℃, to trochophore within 12 hours,
and to straight hinge larvae within 18 hours The D-shaped larvae metamorphose
to pediveliger larvae in 12 days at 27℃under normal culture conditions.
Shell length of straight hinge larvae are 75㎛. At the stage, they become
much hardier over embryonic and trochophore larvae.
The culture densities are to be adjusted from 5 to 1 ml-1 depending
on larval sizes. The culture water should be changed on every day basis. The
larvae are to be fed continuously with mixed phytoplankton of Isochrysis
galbana, pavlova lutheri, Chaetoceros gracilis, and Tetraselmis teterathele.
For feeding, the algae should be maintained at density of 0.5∼3x104
cells ml-1 Feedings are based on three times a day.
Cultch is a set of substrates assembled together by 60
oyster shells to which pediveligers attach as soon as when
they metamorphose. Therefore, it is utmost important to
put the clutches in the larval culture tanks just before
the pediveligers metamorphose. Before setting the cultch
in the tanks, preparing the cultch assembled 60 oyster
shells on string and immersing the cultch strings in the
filtered seawater are necessary at least for 2 days.