♦ Scientific name: Sebastes schlegeli
♦ Common name: Black rockfish
Black rockfish inhabits shallow rocky shores from 10 to
100 meters in depth along the all coasts of the Korean peninsula, Japan, and
some parts of China. In Korea, the mature species produce directly their larvae
in May to June. The species grow to 25cm in body length by year 2 and 30cm by
year 3, feeding small fish, crustaceans, etc.
The black rockfish, Sebastes schlegeli has been studied
since 1986 by NFRDI for aquaculture purpose. Currently, its artificial seedling
cultivation method has been established. In reality, the rockfish together with
olive flounder, occupies leading species farmed in marine environment.
S. schlegeli seed production is done on hatchery in
cylindrical or rectangular indoor tank. By using mature fish about 3 years
old, birthing is done during April and May. Adult fish about 30¡40cm
big gives birth to about 100 to 200 thousands larvae. Early larvae is relatively
big at the size of 5.7mm and starts feeding from the moment of birth. For
first 10 days they are fed rotifer at the density of 10/§¢ as introductory
prey organism. From 11th days to about 30th days, Artemia is fed
at the density of 1¡3/§¢ and mixed feed is supplied from the third day on
and the size and quantity of the feed should increase gradually. The introductory
feed organism, rotifer and Artemia should be enriched with Chlorella.
When supplying mixed feed, green water and floor cleaning should be done
every 3 to 5 days to stabilize the water quality. Population density of
5000 to 7,500 larvae per §© is appropriate. When fry reach the size of
2§¯, population density of 2,300 per §© is appropriate and when fry reach
3§¯ the density should be adjusted to 2000. If the individual size of
fry differ greatly, they might feed on each other, thus selection and separation
is necessary. To prevent disease, medicate after every 2 to 3 weeks period
and when they reach the size of about 5§¯ they can be sold.
♦ Early seed production
First distinctive feature of current farming technique
is early seed production. With increase of nurseries in Taean and Boryung
in west coast, lagging production, more efficient use of farming facility and
the change of production type in existing flatfish farmers made the competitive
power to be vital. Therefore, nurseries try to produce high quality seedlings
earlier than other companies. Securing adult fish is important for such
early seed production. There are two methods of securing mature fish. The
first is buying 3 to 4 years old S. schlegeli (25§¯ to 35§¯, weighs 1.5 to 3§¸)
from Geomundo or Koje of south coast right before the birthing at high price
and the second is to buying the breeding fish around January and raising them
in increased water temperature of 13 to 14¡É and reduce the time of birth to
20 to 30 days. However, increased water temperature farming requires about
60 days of gradually increasing water temperature and fully equipped water temperature
facility thus serious consideration of fuel cost and economical efficiency is
necessary. With such efforts, seed production, which would normally be
produced in mid April, has been reduced to early April by 1 to 2 weeks.
Cage culture of Sebastes schlegeli. Majority of black rockfish are cultured in the net cage system.
♦ Seed production in open field
Recently, artificial sea cultivation has been active and
large scale plantation are being built around Chungnam Taean; west coast. However,
natural cultivation from abandoned open field is becoming an alternative method.
Originally, open sea fond had been used as Penaeus orientalis farm
but due to viral infection it had been abandoned and used as settling tank to
stabilize water quality. However, due to high coast and lowering quality
of seed production in dry land plantation, open field cultivation became a solution
for lower production cost and higher quality. Seed production method in
open field; fertilize water and mass-breed the prey organism and then barricade
one side to contain large number of breeding fish. After the birthing
let the seedlings grow in natural environment. This method has lower production
cost than the dry land cultivation and the feeding is dependent on natural prey
which also controls the population density. However, cost of acquiring breeding
fish is relatively high and difficulty of managing such large field under human
control is the downside of such method. Therefore, under current circumstance
of not having proven cultivation method, although there are differences among
different farms, open field cultivated seedlings are preferred due to their
stronger immune system and faster growth rate. Competition in the seed
production market will depend more on the quality of the seedlings rather than
the quantity in the future and the open field cultivation will probably become
more and more popular.
A cultivation of S. schlegeli means to purchase
seedlings and to raise them to 500g¡1kg in weight to be sold in market which
takes about 18 to 24 months. The fish are cultured in embankment and cages.
The cage cultures are more common. Quality good seeds show a strong
schooling behavior. The cage cultured for the species are of 5m X 5m or 10m
X 10m with 22¡27 nodes net cage for seedling s the size of 4 cm to 5 cm. When
they reach weight of 30¡75g use 18 net, 10 nodes for 75g¡135g, 7 net when
they grow over 135g. For cage method, appropriate population density is
700¡1000 per §© at the size of 4cm to 5cm and 300 ¡ 500 at past 8 cm. However
S. schlegeli do not like high temperature. Therefore, water should
be exchanged often and population density should be reduced to half the normal
level during summer. They should be fed 2¡3 times per day when they
are young. It should be reduced depending on their growth and by the time they
can be sold, they should be fed about 1¡2 times a day. During the winter with
low water temperature, every 2 to 3 days feeding scheme is advised. For
S. schlegeli cultivation, recording of seedling provider and the condition
should be done. In case of sick fish or abnormality fast and appropriate
response is necessary.
There are many efforts to prevent and cure diseases during
cultivation and summer period needs more attention during high water temperature.
To prevent disease, it is recommended to temporarily stop feeding and
provide fresh feed. Preventing contagious viral infection by maintaining
appropriate population density and fast responses, such as providing oxygen
and medication during red tides or outbreaks, are necessary. Especially,
careless overdose of additives that are sold recently can have serious impact
on the immune system so they must be used with care. Such precaution and
diagnosis can prevent abnormality and mass suffocation which will result in
improved product quality and maximized profit.
♦ Prospect of cultivation technology and development for the future.
Difficulties of cultivating S. schlegeli comes from
problem of introductory prey organism, high cost of production, low profit from
over-production, and lowered quality of seedlings from mutation. The development
of introductory mixed feed is necessary to solve these kinds of problems and
research to find introductory micro feed to substitute natural prey is in progress.
Lower profit from over-production should be handled by the companies themselves
by changing production output and type. Lowering quality of seedlings
should be handled by researching into immunology and engineering with having
competitive edge by securing high quality seedlings in mind.
70% of current sea barricades are breeding S. schlegeli which might cause lowered
price from over-production. Therefore shipment timing agreements among companies
and advertising to buyers seem necessary for the future.
for the development goal for the future of S. schlegeli environment friendly
cultivation method is necessary. For this, development of high efficiency
- low pollution feed is necessary. For production of high quality seedlings,
change in open field cultivation method is needed. Also, with mutual trusts
of each farmers, adjustment of market sold timing, output, and production type
Culture tank for seed production of Sebastes schlegeli
Juveniles of Sebastes schlegeli
Selecting of juveniles