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
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%).
Larva (25th day) started metamorphosis
Larva at 15 days
Life cycle of olive flounder.
Production trend of farmed olive flounder in Korea . Data from Korea
National Statistical Office (2001).
production of cultured marine finfish in Korea
2001 production proportion of cultured marine finfish in Korea. Total
production = 29,297 MT, valued at 2,935 billion Korean Won. Data from Korea National Statistical
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
Production of olive flounder in Korea by facility (2001). Total
production = 29,297 MT, valued at 2,935 billion Won (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 กษ 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
- 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.
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.