1. Scientific name: Anadara broughtnonii![Photo of ark shells](ark.files/uni2dae.gif)
2. Common name: Ark shell
3. Distribution
Anadara broughtnonii is one of the most economic species
belonging to Anadarinae which distribute in Pacific coasts including Korean
coastal waters. The species has common appearance characteristics with A. satowi,
but they differ in shell thickness and vertical distribution. A. broughtonii
distribution goes down to 50 meters in depth, while A. satowi is limited within
30 meters in depth.
4. Aquaculture
The spawning seasons of the arkshells are normally July to
October with major spawning season of July to August on the southern coasts
of Korea. For A. broughtonii, the spawning occurs mostly from August
to September. The size of the spawned eggs is about 55 ??in diameter. The
species grows fast; the eggs spawned in September reach 31.6??65.9 mm by year
1 and 48.7? ?8.1 mm in shell length.
![Arkshells and a harvesting vessel](ark.files/picd1a4.gif)
Arkshells and a harvesting vessel
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![Ark shell spawner](ark.files/picd1a5.gif)
Ark shell spawner
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For the seed production, the wild brood stocks weighing
100 to 200g (7??0cm in shell length) are purchased and kept in captivity with
supplement of algal foods for maturation. During the maturation conditioning,
a special care should be given not to have the brood stocks spawned in the conditioning
tanks. Water temperature over 23??can stimulate the spawners to spawn.
Besides water temperature, exposure to air, chemicals,
electric shock, and presence of sperm can be stimulators to spawning of the
shell. For water temperature stimulation, sudden increase of water temperature
to 5??has been routinely used in the ark shell hatchery. The fertilized eggs
are to be washed with the aid of egg washing net (mesh size, 20??. Larvae of
ark shell are growing under water temperature of 24 26??and illumination
of lower than 100Lux. For the best growth, the D-shaped larvae should be contained
in densities 5??0/mL. Water exchange is performed on every two-day basis.
The ark shell larvae metamorphose to attaching larvae when
they reach 260??in shell length. Normally, it takes about 20 days at 25??until
the metamorphosis. Just before the metamorphosis, substrates for larval attachment
are to be inserted in the larval culture tanks. The attached larvae move to
nursery ground before they go to the farming ground.
The ark shell larvae are to be fed continuously with a mixture
of Isochrysis galbana, Chaetoceros calcitrans, and Monochrysis lutheri.
The algal densities supplied depend on the sizes of the larvae, normally 0.5??x104cells/ml
The procedure for the seed production ark shell practiced in the Namhae Marine
Hatchery of NFRDI
5. Recent problem in the ark shell aquaculture.
Ark shells are one of the farmed species facing difficulties in the production.
The prolonged practice of the aquaculture brings local failures of wild seed
production. The production decrease is particularly evident since 2000. There
might be several reasons for the local failure of the seed production. One of
the most significant reasons is local appearance of unhealthy brood stocks in
the seeding grounds. Unit production is also shows a decreasing trend, worsening
farming ecology in locations. NFRDI shellfish research team are continuously
monitoring the health of the wild population. Fluctuation environmental factors
are also monitored, including water temperature, salinity, dissolved oxygen,
kinds of dissolved organic matters, food organisms, and chemical contaminants
both of organic and inorganic matters in the seed and farming grounds. Hatchery-based
seed production is also increasing. All these efforts have been practiced cooperatively
with local fisheries institutes.
Chart below shows
Farmed production of ark shell 1970 through 2001.
Year
|
1970
|
1980
|
1990
|
1995
|
1998
|
1999
|
2000
|
2001
|
Aqua-
culture
|
Q'ty
|
怨?
|
0
|
2,301
|
17,758
|
9,357
|
23,029
|
8,550
|
10,618
|
7,359
|
K'nam*
|
0
|
1,669
|
2,787
|
979
|
1,652
|
231
|
44
|
173
|
Busan
|
0
|
19
|
14,355
|
8,005
|
21,375
|
8,319
|
10,574
|
7,186
|
Others
|
0
|
583
|
616
|
373
|
2
|
0
|
0
|
0
|
Unit Prod.
(Ton/ha)
|
0
|
0.2
|
2.2
|
1.2
|
2.9
|
1.1
|
1.4
|
1.0
|
*K'nam: Kyungnam province (Data: MOMAF Fisheries Statistics))
|