1. Scientific name: Crassostrea gigas

2. Common name: Pacific oyster

3. Distribution

   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.

5. Aquaculture

   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 seedling grounds.

   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 from wild.

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 Korean peninsula.

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.