♦ Scientific name: Paralichythys olivaceus ♦ Common name: Olive flounder ♦ Ecology

The olive flounder accounts for more than 98 percent of the flatfish cultured in Korea. The flounder lives from eastward to westward in Korea coastal waters. The water temperature range for the optimal growth of the flounder is 15 to 25°C.

♦ Aquaculture

Olive flounder, Paralichythys olivaceus, is a temperate marine species cultured in coastal areas of Korea, Japan, and China with a recent annual production roughly amounting to 28,000 MT in these areas.
Flounder culture is totally based on the hatchery seeds, and is mostly practiced in the flow-through system of land-based facilities. Conditioning strain-good broodstocks for seed production is one of the key issues in the flounder aquaculture.
The flounder, together with black rockfish, has been a key marine finfish species cultured in this country since late 1980s. With an aid of the advanced aquaculture technology on this species, particularly on the conditioning technology of the broodstocks in captivity, the production of the species is totally under control. However, some items, such as how to effectively control diseases and how to get better broodstocks are on-going subjects which need continuous research in this country.
 Although the aquaculture for the olive flounder started from late 1980s, its commercial production was from the beginning of the 1990s in Korea. Soon after the industrialized production, the Korean production exceeded Japanese and maximized by the year 1997, thereafter showing a decreasing trend. In 2001, total production of cultured marine finfish was 29,00 MT, valued at 2,935 billion Korean Won, corresponding to 0.6% of total fisheries production in value and to 1.1% of total finfish production (Korea National Statistical Office 2001). The highest production of cultured fish was from olive flounder (56.1%), followed by black rockfish (31.6%), mullet (4.8%), sea bream (3.4%), and sea bass (3.0%).

Fries		Fertilized Egg
Larva (25th day) started metamorphosis		Embryogenesis
	Larva at 15 days after hatching	Hatching Larva

                                  Life cycle of olive flounder.

 
2001 production of cultured marine finfish in Korea

 
2001 regional proportion of the cultured olive flounder in Korea. Total production = 29,297 MT,
valued at 2,935 billion Korean Won. Data from Korea National Statistical Office (2001).

♦ Seedling production of Olive flounder in South Korea

Flounder does not display any external secondary sexual characteristics. It has a cyclical pattern of reproduction characterized by massive gonad development. Water temperature is an important factor for the maturation of gonads. Under natural conditions, spawning takes place between April and June, May being the peak spawning season in Korean coastal waters. In captivity conditions, vitellogenesis occurs at temperatures ranging from 10 to 15°C. Optimum temperature for spawning is 14 to 16 °C. As water temperature increases above 18°C, gonads start to degrade. Light intensity triggers gonad development and, under artificial conditions, light intensity and photoperiod should be controlled. Results from experimental studies showed that 400 to 600 lux and 14 to 16 hours of light a day are indispensable for maturation and development of gonads. Spawning occurs at night often, from about midnight to early morning.
 

♦ Breeding techniques

Flounder eggs are 0.83 to 1.1 mm in diameter. Both eggs and yolk sac larvae are particularly difficult to rear, compared with other marine fish. At the end of the yolk sac stage, larvae start feeding and then they metamorphose, the symmetrical larvae (shaped like round fish) becoming flatfish.
Development of flounder from fertilized egg to end of metamorphosis takes 60 to 70 days to produce a juvenile about 50 mm long. Survival varies from 30 to 40 percent, being best in batches fed rotifers, brine shrimps and micropellets.

 - Broodstock and spawning

    Up to now, farmers and researchers obtained flounder eggs from broodstock, which was either caught in the wild or selected from farmed fish at least three years old. Natural spawning season lasts from April to June, but under farming conditions, it occurs from April to May. For commercial purposes, control of rearing environmental factors makes it possible to have batches of fish ready to spawn at all months of the year, especially in autumn. Flounder broodstock is held in land based tanks rather than cages, so that they can be handled more easily for spawning. Water temperature is usually kept below 19°C. Before complete feeds were developed, broodstock was fed raw fish, which did not always provide the required nutrients and carried with it the risk of disease transfer. But now, fish are mostly fed a formulated feed, which is moistened just before feeding. Flounder do not release all their eggs at once. Eggs may be obtained many times from the same fish during its spawning cycle, at intervals of two to eight days. In natural populations, minimum spawning size is about 30 cm in two-year old fish. One spawner weighing 2.5 to 4 kg can lay a total of about 3 million eggs, through successive batches of 40 000 to 450 000 eggs each. The use of a hormonal treatment and of stripping is not required because ripen males and females can spawn naturally in captivity. Normally, it is expected that at least 90 percent of the eggs are fertilized and that over 80 percent of these fertilized eggs produce larvae. More and more broodstock being hatchery-raised fish, this gives the opportunity for improving flounder broodstock.

 - Egg incubation

 Ripe eggs can easily be collected from spawning tanks. Once fertilized, eggs are collected into screened containers (egg collectors) and transferred to incubation tanks. Hatching rate is higher in a darkened room. Water temperature is also important: hatching rate reaches 90 percent at 14 to 16°C but drops to 60 percent when water temperature rises to 22°C. The incubation system varies. Volume ranges from 1 m3 to 200 m3, depending on the number of eggs to be incubated. In general, small incubators are used for intensive systems, juveniles being later moved to larger tanks for weaning and nursing. Larger incubators are used not only for rearing fertilized eggs but also for the production of juveniles 20 to 30 mm long. A continuous water flow is generally maintained through 2 to 20 m3 incubators. Dead eggs and debris are removed daily to prevent bacterial and fungal contamination. Live eggs are disinfected immediately before hatching. At 14 to 16°C, hatching occurs after about 63 hours producing fragile yolk sac larvae from 2.13 to 2.35 mm long. The yolk sac is relatively large but there is no functional eye or mouth yet.

 - Yolk sac larvae development

 The yolk sac stage lasts for four to five days at 18 ¡É water temperature. During this period, larvae develop from their yolk reserves. They are sensitive to light and temperature. Larval development needs a light intensity of 400 to 600 lux, survival and growth being affected if light intensity is lower than 40 lux or higher than 1000 lux. At the water temperature of 20°C, larvae become females and to increase the proportion of males, water temperature should be maintained between 15 and 19°C. Survival during generally ranges from 50 to 70 percent. By the end of this period, larvae are about 3.8 mm long.

 - First feeding

About four to five days after hatching and just before the mouth of the larvae opens, rotifers should be distributed. In Korea, rotifer, Brachionus niloticus is widely used for the first four to 15 days. Then, brine shrimp nauplii are preferred.

 - Larval rearing

Larvae are fed brine shrimp (Artemia) nauplii hatched from dried eggs and/or copepods produced by commercial company. The quality of these living feeds was the subject of much research. Brine shrimp nauplii are an incomplete source of nutrients, responsible for low survival, incomplete metamorphosis and/or abnormal pigmentation. They have to be supplemented either by copepods or by enriched brine shrimp nauplii.

 - Metamorphosis

Flatfish start their life upright, like a round fish. They turn on to one side, which then becomes the belly, during metamorphosis. The eye and nostril on that side move up and over the head, joining the other eye and nostril on what now becomes the back. This extraordinary biological change usually occurs 35 to 40 days after hatching, depending on larval growth rate and water temperature. Not all larvae metamorphose at the same time. As bigger larvae start eating smaller ones, size grading becomes necessary. Therefore, synchronization of metamorphosis should be considered as a priority.

 - Weaning period

Weaning occurs when the diet of newly metamorphosed juveniles is changed from live food to artificial feeds. Fry fish are then 10 to 20 days old and they weigh between 20 and 100 mg. Both types of food are offered together, the supply of live food being gradually reduced. Feeding rate depends on average size of juvenile fish. Feeding frequency is five to six times a day. This process usually takes 30 days to be completed, by which time juvenile fish weigh about one gram. Expected survival is about 70 percent. As presently practiced, weaning is a somewhat cumbersome and expensive process because juveniles require large amounts of live food until they are weaned. As far as possible, live food requirements should be reduced by helping young fish to learn to accept inert food early. This also provides a means to offer additional nutrients which might be lacking in live food. With this technique, it is claimed that fish can be weaned at a weight of 150 to 200 mg with 90 percent survival. This is a good example of how technology for rearing marine fish larvae can be improved and made less costly in the future.

 - Nursing period

The purpose of nursing is to rear young flounders until they can be moved or sold to an on-growing system, but the size at which such transfer occurs can vary substantially. During this period, moistened pellets are fed at the rate of about 5 percent of body weight. To optimize growth until fish average 15 cm, water temperature should range from 18 to 23°C and light intensity from 500 to 1000 lux. Depending on location, different strategies are used in flounder hatcheries, either individually or in combination, to maintain optimum water temperatures during the nursing period. In early spring or late autumn, warmer water from a power station or a deep well is commonly used for this purpose. Recently, recirculation systems have become the method of choice.

 - On-growing period

Flounder are naturally docile and not easily agitated. As a result, they subject themselves to little stress under farming conditions and, therefore, do better than more excitable species. They also like crowding together, though, as flatfish, they do not fully use the water column as do round fish such as large yellow croakers. In fact, stocking densities for flounder are usually expressed in terms of kilograms per square metre, rather than kilograms per cubic metre as they are for round fish. Optimum stocking density for flounder varies from 20 to 30 kg per square meter. This does not appear to stress them and, in this respect, they are similar to other farmed flatfish such as the European turbot and Atlantic halibut.

Flounders accept dry formulated feeds well and convert them efficiently, the feed conversion ratio (the weight of distributed feed per unit weight gain) being equal to 1:1 or a little more. This might be due to an intrinsic virtue of flounder metabolism and/or to a sedentary life style. If such excellent feeding efficiency could be achieved in large scale commercial systems, it would provide flounder farmers with a significant advantage from the economic point of view.