I. Introduction Lates calcarifer (Bloch), commonly called the giant sea perch or seabass, is an economically important food fish in the tropical and subtropical regions of Asia and the Pacific. It is commercially cultivated in Thailand, Malaysia, Singapore, Indonesia, Hong Kong and Taiwan, in both brackishwater and freshwater ponds, as well as in cages in coastal waters. Because of its relatively high market value, it has become an attractive commodity of both large to small-scale aquaculture enterprises. However, the major constraint to rapid expansion of seabass culture has been the inconsistent supply of fry collected from the wild. It fluctuates widely from year to year, making forward planning for production difficult. In the early 1970's, Thai scientists have achieved success in the breeding of seabass under captive conditions. Completion of its life cycle has also been accomplished. Growth performance of the hatchery-bred fry has been shown to be comparable with that of fry collected from the wild. Thailand is presently producing more than 100 million fry annually (Anon. 1985), with the Satul Fisheries Station producing more than 30 million (Kungvankij 1984). Thus, the seabass culture industry in Thailand is now assured of sufficient and consistent supply of fry. In order to extend the technology of seabass culture, this manual is prepared to serve as a practical guide for extension workers and farmers. Its contents are based on research findings in addition to many years of accumulated practical experience and field observations.
Transcript
I. Introduction
Lates calcarifer (Bloch), commonly called the giant sea perch or seabass, is an economically important
food fish in the tropical and subtropical regions of Asia and the Pacific. It is commercially cultivated in
Thailand, Malaysia, Singapore, Indonesia, Hong Kong and Taiwan, in both brackishwater and freshwater
ponds, as well as in cages in coastal waters. Because of its relatively high market value, it has become an
attractive commodity of both large to small-scale aquaculture enterprises. However, the major
constraint to rapid expansion of seabass culture has been the inconsistent supply of fry collected from
the wild. It fluctuates widely from year to year, making forward planning for production difficult.
In the early 1970's, Thai scientists have achieved success in the breeding of seabass under captive
conditions. Completion of its life cycle has also been accomplished. Growth performance of the
hatchery-bred fry has been shown to be comparable with that of fry collected from the wild. Thailand is
presently producing more than 100 million fry annually (Anon. 1985), with the Satul Fisheries Station
producing more than 30 million (Kungvankij 1984). Thus, the seabass culture industry in Thailand is now
assured of sufficient and consistent supply of fry.
In order to extend the technology of seabass culture, this manual is prepared to serve as a practical
guide for extension workers and farmers. Its contents are based on research findings in addition to many
years of accumulated practical experience and field observations.
II. Biology
1. Taxonomy
Phylum Chordata
Sub-phylum Vertebrata
Class Pisces
Sub-class Teleostomi
Order Percomorphi
Family Centropomidae
Genus
Lates
Species
Lates calcarifer (Bloch)
The above is an accepted taxonomic classification of seabass or giant perch. Seabass has been placed
under several families by various authors in the past (e.g. the grouper family, Serranidae and family
Latidae, etc.) However, Centropomidae is the commonly accepted familya name of this species, and the
recognized generic name is Lates. Other names such as Perca, Pseudolates, Holocantrus, Coins,
Plectropoma, Latris, and Pleotopomus were also given by various authors who collected the fish
specimens from different areas. Bloch (Schneider 1801) stated that Lates calcarifer occured in Japan Sea
but named it as Holocentrus calcarifer.
The common local names of this species are listed below:
Common English name : Giant perch, white seabass, silver seaperch, giant perch, palmer, cock-
up seabass
India : Begti, bekti, dangara, voliji, fitadar, todah
2. Morphology and distinctive characters (after FAO 1974)
Body elongated, compressed, with deep caudal peduncle. Head pointed, with concave dorsal profile
becoming convex in front of dorsal fin. Mouth large, slightly oblique, upper jaw reaching to behind eye;
teeth villiform, no canine teeth present. Lower edge of preoperculum with strong spine; operculum with
a small spine and with a serrated flap above original of lateral line. Dorsal fin with 7 to 9 spines and 10 to
11 soft rays; a very deep notch almost dividing spiny from soft part of fin; pectoral fin short and
rounded; several short, strong serrations above its base; dorsal and anal fins both have scaly sheath.
Anal fin round, with three spines and 7–8 soft rays; caudal fin rounded. Scale large ctenoid (rough to
touch).
Colour: two phases, either olive brown above with silver sides and belly in marine environment and
golden brown in freshwater environment (usually juveniles). Blue-green or greyish above and silver
below (adult).
3. Distribution
3.1 Geographic distribution
Seabass is widely distributed in tropical and sub-tropical areas of the Western Pacific and Indian Ocean,
between longitude 50°E - 160°W latitude 24°N – 25°S (Fig. 1). It occurs throughout the northern part of
Asia, southward to Queensland (Australia), westward to East Africa (FAO 1974).
3.2 Ecological distribution
Seabass is a euryhaline and catadromous species. Sexually mature fish are found in the river mouths,
lakes (e.g. Songkhla lake) or lagoons where the salinity and depth range between 30–32 ppt and 10–
15m, respectively. The newly-hatched larvae (15–20 days old or 0.4–0.7cm) are distributed along the
coastline of brackishwater estuaries while the 1-cm size larvae can be found in freshwater bodies e.g.
rice fields, lakes, etc. (Bhatia and Kungvankij 1971). Under natural condition, seabass grows in freshwate
and migrates to more saline water for spawning.
4. Life history
Seabass spends most of its growing period (2–3 years) in freshwater bodies such as rivers and lakes
which are connected to the sea. It has a rapid growth rate, often attaining a size of 3–5 kg within 2–3
years. Adult fish (3–4 years) migrate towards the mouth of the river from inland waters into the sea
where the salinity ranges between 30–32 ppt for gonadal maturation and subsequent spawning. The fish
spawns according to the lunar cycle (usually at the onset of the new moon or the full moon) during late
evening (1800–2000 hours) usually in synchrony with the incoming tide. This allows the eggs and the
hatchlings to drift into estuaries. Here, larval development takes place after which they migrate further
upstream to grow. At present, it is not known whether the spent fish migrates upstream or spends the
rest of its life in the marine environment (Fig. 2).
Smith (1965) noted that some fish spend their whole life in freshwater environment where they grow to
a length of 65 cm and 19.8 kg body weight. The gonads of such fish are usually undeveloped. In the
marine environment, seabass attaining a length of 1.7 m have been recorded in the Indo-Australian
region (Weber and Beaufort 1936).
Fig. 1. Geographic distribution of Lates calcarifer. (After FAO 1974)
Fig. 2 Migration pattern of Lates calcarifer Bloch
5. Feeding habits
Although the adult seabass is regarded as a voracious carnivore, juveniles are omnivores. Analysis of
stomach content of wild specimens (1–10 cm) show that about 20% consists plankton, primarily diatom
and algae and the rest are made up to small shrimp, fish, etc. (Kungvankij 1971). Fish of more than 20
cm, the stomach content consists of 100% animal prey: 70% crustaceans (such as shrimp and small crab)
and 30% small fishes. The fish species found in the guts at this stage are mainly slipmouths or or pony
fish (Leiognatus sp.) and mullets (Mugil sp).
6. Sex determination
Identification of the sexes is difficult except during the spawning season. There are some dimorphic
characters that are indicative of sex (Fig. 3).
Snout of the make fish can be slightly curved while that of the female is atraight.
The male has a more slender body than the female.
Weight of the female is heavier than males of the same size.
The scales near the cloaca of the males are thickers than the female during the spawning season.
During the spawning season, abdomen of the female is relatively more bulging than the males.
7. Sexual maturity
In the early life stages (1.5–2.5 kg body weight) majority of the seabass appear to be male but when
they attain a body weight of 4–6 kg majority become female. After culture period of 3–4 years, however,
in the same age group of seabass both sexes can be found and identified as mentioned above. In a fully
mature female, the diameter of the oocysts usually range from 0.4 ro 0.5 mm.
8. Fecundity and spawning
The fecundity of seabass is related to the size and weight of the fish. Gonad samples obtained from 18
females of body weight ranging from 5.5 to 11 kg gave a range of 2.1 to 7.1 million eggs (Wongsomnuk
and Maneewongsa 1976) as illustrated in Table 1. Observations by the Australian Department of
Agriculture (Anon. 1975) showed that a 12 kg fish had 7.5 million eggs; a 19 kg fish 8.5 million and a 22
kg. fish, 17 million.
Figure 3 Photograph of adult male and female seabass
Table 1. Relationship between size of fish and number of eggs from the gonads of seabass (Lates
calcarifer Bloch). (After Wongsomnuk and Maneewongsa 1976)
Total length
(cm) Weight No. of fish Fecundity (million eggs)
Range Average
70 – 75 5.5 3 2.7 – 3.3 3.1
76 – 80 8.1 5 2.1 – 3.8 3.2
81 – 85 9.1 4 5.8 – 8.1 7.2
86 – 90 10.5 3 7.9 – 8.3 8.1
91 – 95 11.0 3 4.8 – 7.1 5.9
Seabass spawn all year round (Kungvankij 1984) with the peak season occuring during April-August and
large number of fry (1 cm in size) can be collected during May to August (Bhatia and Kungvankij 1971).
Based on studies of spawning activity under tank conditions, mature male and female fish separate from
the school and cease feeding about a week prior to spawning. As the female attains full maturity, there
ia an increase in play activity with the male. The ripe male and female then swim together more
frequently near the water surface as spawning time approaches. The fish spawns repeatedly in batches
for 7 days. Spawning occurs during late evening (1800–2200 hours).
9. Embryonic development
First cleavage occurs 35 minutes after fertilization. Cell division continues every 15 to 25 minutes and
the egg develop to the multi-celled stage within 3 hours. Its development passes through the usual
stages: blastula, gastrula, neurola and embryonic stages. Embryonic hear starts to function in about 15
hours and hatching takes place about 18 hours after fertilization at temperatures of 28–30°C and
salinities of 30–32 ppt (Table 2, Fig 4a & b).
10. Larvae
Newly-hatched larvae have total length ranging from 1.21 to 1.65 mm averaging 1.49 mm. The average
yolk sac length is 0.86 mm. One oil globule is located at the anterior part of the yolk sac which causes
the hatchling to float almost vertically or about 45° from its usual horizontal position. Initial
pigmentation is not uniform; the eyes, digestive tract, cloaca and caudal fin are transparent. Three days
after hatching, most of the yolk sac is absorbed and the oil globule diminishes to a negligible size. At this
stage, the mouth opens and the jaw begins to move as the larva starts to feed.
Table 2. Embryonic development of seabass eggs (Kungvankij 1981).
Embryonic stage Hours & minutes after spawning
Hours Minutes
Fertilization - 5
2-cell - 35
4-cell - 55
8-cell 1 10
16-cell 1 30
32-cell 1 50
64-cell 2 20
122-cell 3 -
Blastula stage 5 3-
Gastrula stage 7 00
Neurola stage 9 10
Embryonic stage 11 50
Heart functioning 15 30
Hatch out 18 -
Fig 4a. Development of egg
Fig 4b. Development of egg
There are at least two pigmentation stages in seabass larvae. At 10–12 days after hatching, the
pigmentation of larvae appears dark gray or black. The second stage occurs between 25–30 days old
where the larvae develop into fry. In this stage, the pigmentation changes to a silvery-coloration.
It has been observed that only healthy fry of this stage (20–30 days) swim actively. They are always
lighter in color. Unhealthy post larvae have dark or black body coloration.
11. Growth
The growth rate of seabass follows the normal sigmoid curve. It is slow during the initial stages but
becomes more rapid when the fish attains 20–30 gm (Table 3). It slows down again when the fish is
about 4 kg in weight.
Table 3. Age, average body length and weight of seabass under tank conditions.
Age
(days) Average length
(mm) Average body weight
Fertilized eggs 0.91
0 1.49*
1 2.20
7 3.61
14 4.35
20 9.45
30 13.12 0.1
40 17.36 0.5
50 28.92 1.2
60 32.85 3.5
90 93 9
120 145 50
150 210 120
180 245 280
210 310 330
* newly-hatched larvae
III. Hatchery Design
The basic considerations in establishing a fish hatchery are: (i) which site is suitable, (ii) what is the area of the site and the facilities required in relation to the goals or objectives of the hatchery, and (iii) how will the hatchery be managed.
It is of primary importance to conduct a feasibility study to determine the suitability of the site. This should be done prior to the establishment of the hatchery.
There are three factors which must be considered in designing a fish hatchery: (i) species, (ii) production target, and (iii) level of financial input. In addition, the facility requirements will depend on the nature of organization to run the hatchery. For government pilot projects, some laboratory support facilities are required. Otherwise, it may not be necessary as in commercial projects.
The design of the hatchery will also depend on its objectives. Experimental facilities or production-oriented system for commercial purposes or the combination of both may be incorporated in the design.
The hatchery can be an independent enterprise which is entirely self-sufficient in terms of facilities and manpower or as part of a bigger organization which utilizes its facilities and technical know-how. The hatchery can be an independent enterprise by itself or vertically integrated with other aquaculture enterprises in an organization.
1. Criteria in the selection of sites for seabass hatchery
1.1 Seawater supply
The seawater used in a hatchery should be clean, clear and relatively free from silt. The water quality should be good with minimal fluctuation in salinity all year round. Suitable sites are usually found near sandy or rocky shore. Sites which are not suitable for hatchery include areas which are heavily influenced by rain or turbulence. River mouths should be avoided as abrupt salinity change occurs after a heavy rainfall. An added advantage of having a site on rocky shores is that good quality seawater is relatively near the shoreline. This reduces the cost of piping installation and pumping. The hatchery site should also be free from any inland water discharges containing agricultural or industrial wastes.
1.2 Accessibility
Ideally, a hatchery site should be selected in areas where there are active fish farming operations so that the fish larvae produced can be easily transported and distributed to the grow-out ponds and cages. The site chosen for a hatchery must have easy access to communication and transportation channels.
1.3 Availability of power source
A fish hatchery cannot be operated without electricity. Electricity is essential to provide the necessary power to run the equipment and other life support systems of the hatchery. Hence, the site must have a reliable source of power. Installation of a standby generator is absolutely necessary especially in areas where there are frequent and/or lengthy power failures and fluctuations.
1.4 Topography
The ideal site should be spacious, situated on flat to gently sloping grounds, well drained and not susceptible to floods, strong wave and tidal actions. It should also be on compact soil and accessible by paved road.
1.5 Acquisition
It is advisable to pay attention to land values early in the site selection phase to ensure that the site is available for purchase or lease and at a price consistent with the project budget. Since land with the above characteristics is generally also desirable for other activities, it may be competitive for alternate land usage.
2. Hatchery size
Hatchery design is aimed at achieving certain production targets which in turn determine the size of the hatchery. The capacity is based on an approximate ratio between tank for production of natural food (algae and rotifer) and larval rearing tank. The spawning tank depends on the larval requirement which is based on the number of spawners.
Based on hatchery techniques and practices in Thailand for seabass fry production, the following assumptions are made and used for estimating tank capacities:
a. By means of environmental manipulation or induced spawning, the species spawns monthly for 6 months (whole spawning season).
b. Survival rate of larvae from Day 1 to 50 is 15%. 50-day old larvae have an average length of 3 cm.
c. Production rate of 50-day old larvae in larval rearing tank is 5 per liter. d. Larval rearing period is 50 days. One larval rearing tank can be utilized only for 3
runs in a single spawning season. Since fish spawn monthly, there should be two sets of larval rearing tanks to accomodate monthly production of larvae.
e. All the 50 day old larvae are stocked in earthern ponds and nursery cages. f. The tank capacity for natural food production is the same as that for larval rearing
tank. The proportion of algal culture tank and rotifer culture tank is 2:1. g. The tanks for conditioning of selected spawners, for spawning, natural food
production and larval rearing are often located outdoor.
The following is an example of estimating roughly the tank capacity required for spawning, larval rearing and natural food production:
Production target : 2 million (50-days old larvae)
Spawning season period : 6 months
Spawning frequency : monthly
Production target/month : 2,000,000 larvae/month = 340,000 larvae
Production of 50-day old larvae in the larval rearing tank
: 5,000 larvae/ton
Total capacity of larval rearing tank needed
: 340,000 larvae/5000 larvae/ton = 68 tons
Larval rearing period : 50 days
Total capacity of larval rearing tank for monthly production
: 68 tons × 2 sets = 136 tons
Use of natural food/crop : 15–20 days
Natural food culture tank : 68 tons
Ratio of algal culture tank and brachionus culture tank
Total eggs requirement : 14,000,000 larvae/70% hatching= 20,000,000 eggs
Average number of eggs/spawner
: 1,000,000 eggs/spawner
Number of females needed : 20,000,000 eggs/1,000,000 eggs/spawner=20 female spawners
Stocking rate in spawning tank (1 male:1 female)
: 20 males + 20 females at 1 fish/5 tons=200 tons
3. Holding tanks
The holding tanks in the seabass hatchery are used for various purposes such as for broodstock conditioning and subsequent spawning, incubation, larval rearing and production of natural food. The design of various types of holding tanks are shown in Table 4 and Fig. 5, 6 and 7.
4. Floor space requirement
The figure given in Table 5 describes the space requirements of production hatchery (Fig. 8) with capacity of 2 million 50-days old larvae for a six-month operating season.
5. Seawater system
Seawater can be drawn directl from the sea or from the sump pit. If the source of water is relatively clear, the water can be pumped directly into the overhead filter tank and stored in the reservoir or storage tank. Water is then gravity-fed to various culture tanks through delivery pipes. However, if the water is turbid and contains a high concentration of suspended solids, it must be pumped first into a sedimentation tank where the suspended
solids are allowed to settle down. Only the clear upper portion water is pumped into the filter tank. In some areas where water source is far from the shoreline and during low tide where large quantity of water is needed continuously, the sump pit or tube well can be constructed inshore near the hatchery. The sumps pit is connected to an underground pipe which is situated towards the water source. The water continuously enter the sump pit through the underground pipe even during low tide. Water is then pumped directly from the sump pit or tube well (Fig. 9). Water from the sump pit or tube well is usually clear because the water is filtered naturally through a layer of sand before entering the pipe so that it can be used directly. However, if vary clear and clean water is required, it should be pumped through the filter tank before use.
Table 4. Tank facilities and capacity used in seabass hatcheries.
Stage Facility Stocking density
Volume needed
(ton)
Unit vol. (ton)
No.unit Size, shape, construction material
Adult spawning 1 fish/5 200 50
4 square concrete tank 6m × 6m × 1.5m capacity of 50 tons with water & aerasystem (Fig.5)
Eggs incubation 100 eggs/liter
14 1 14 circular w/flat or conical shape bottom; 1000 1 capacity fiberglass tank (Fig. 6)
Larvae larval rearing tank
20–50 larvae per liter
150 15 10
rectangular concrete tank (1m × 1.5m × 10m) of hollow block cement w/ mild aeration (Fig.7).
Natural food starter tank 6 1 6 circular tank flat bottom 1000 liters fiberglass tank
Phytoplankton algal culture tank
40 10 4 square concrete tank (3 × 3 × 1.2m) with aeration
Zooplankton Brachionus culture tank
20 10 4 rectangular concrete tank (1.5 × 3 × 1.2)
Fig. 5. A broodstock development/spawning tanks.
Fig. 6. An incubation tank.
Fig. 7. A larval rearing tank.
Table 5. Space requirement of finfish hatchery with a production capacity of 2 million 50-days old larvae.
Facility Dimension (m) Area (m2)
Staff office 5 × 4 20
Algal culture room 5 × 4 20
Wet laboratory 8 × 10 80
Spawning tank 25 × 6 150
Larval rearing tank 17 × 10 170
Phytoplankton culture 12 × 3 36
Zooplankton culture 6 × 3 18
Dry laboratory 5 × 4 20
Fig. 8 Lay-out of seabass hatchery
Fig. 9 Seawater intake thru sumpit.
Pump specification must be chosen properly since the size of the pump depends on the total water requirement per day and maximum pumping time. Figure 10 indicates the total head suction pipe, discharge value and pump horsepower. With these data, pump specification required for such hatchery facilities can be derived.
IV. Seed Production and Hatchery Techniques
Through many years of experience in seabass fishing, fishermen observed that the spawning season of seabass occurs during the southwest monsoon wind (April-August) when rainfall is slight. This is confirmed by the availability of wild fry (1 cm in size) in natural collecting grounds during May to August in Southern Thailand (Bhatia and Kungvankij 1971) and in the Philippines (Cuachon, V. Personal communication, 1984).
Artificial propagation of seabass was first achieved in Thailand in 1971 (Wongsomnuk and Maneewongsa 1972) by stripping the ripe and running spawners which were collected from natural spawning ground. In 1973, Wongsomnuk and Maneewongsa successfully induced the cultured broodstock to spawn in captivity by hormonal injection. Kungvankij (1981) successfully induced seabass to spawn by environmental manipulation and has identified the pattern of environmental conditions necessary for natural spawning.
The breakthrough in completion of the life cycle of seabass in captivity has greatly enhanced mass production of seabass. Figure 11 illustrates production procedures from the collection of wild spawners, broodstock development, hatchery operations including spawning, incubation, larval rearing and finally, grow-out.
1. Broodstock development
There are two sources of seabass broodstock: wild-caught adults and from ponds/cages (2–6 years old fishes averaging in weight from 3 to 5 kg). It is advantageous to use pond or cage-reared broodstock as they are already used to culture conditions being easier to condition and develop them into broodfish. However, when 2–3 year old cultured stocks are not available, wild-caught adults can be used, but they must be first acclimatized under cage or pond condition for at least 6 months before being used as spawners.
2. Wild broodstock collection
The fishery worker must constantly strive to minimize stress in handling captive broodstock. Efforts to capture seabass should be confined to areas where they are known to occur. The selection of a suitable gear or method of capture must also be considered.
Unless the fish are abundant in an area, the effort and cost required for capturing fish will become astronomical.
Fig. 10 Waterflow (Q) and pump h.p. vs. size and length of intake line (After Nash et al 1980)
Fig. 11 Flow Chart of Seabass Culture
Fishing gears found to be effective in collecting broodstock are as follows:
(a) gill net
Gill net can be utilized in both stream and lake. This is set perpendicular to the current. One end of the net is supported and marked by a float while the opposite end is controlled from the boat. Gill net can be used in any depth and is, therefore, very effective in capturing seabass. Since seabass swim in mid-water, the suitable mesh size of seabass gill net is 8–12 cm. However, gill nets often cause damage to the eyes and gills of the fish.
(b) seine net
This gear can be operated when a school of fish is observed. The net is set by surrounding the fish school. Seine nets are more preferable than gill nets because it is less injuring to the fish.
(c) hook and line
Fish collected by this method usually have a high mortality rate due to extreme stress caused during capture.
3. Conditioning of wild broodstock
Captured fish are placed immediately in transport tanks and taken directly to the hatchery or holding cages. Anaesthetic is not necessary if the fish are shipped in live tanks or in aerated transport containers. Upon arrival at the hatchery, the fish are treated with antibiotic such as oxytetracycline. If antibiotic is applied directly into the water, absorption is effected across the gills and the skin of the fish. The recommended concentrations of antibiotics are: 2 ppm for the dripping method for 24 hours and 20 mg per 1 kilogram of fish for the injection method.
In nature, seabass is a carnivorous and feeds voraciously on live fish. However, in captivity, they can be conditioned to feed on dead fish. After recovery from initial injuries resulting from capture, seabass can be trained to feed on fresh marine fish. It often takes a few days before the fish gets used to the new diet. It is important to throw the feed piece by piece as the seabass never eat the food when it settles to the bottom of the tank. The uneaten feed should be removed to prevent water pollution.
4. Broodstock maintenance
The fish, whether cultivated or wild-caught, can be maintained as broodstock in cages and concrete tanks.
(a) cages
Floating cages are usually used for broodstock development. Cages made of polyethelene netting materials are attached to GI pipe or wooden frames kept afloat by styrofoam drum and anchored within a calm bay or sheltered marine environment (Fig. 12). The size of the cages varies from 10 to 100 sq.m. in surface area with a depth of 2 meters (dimensionn: 5 × 5 × 2m or 10 × 10 × 2m). Smaller cages are more suitable because they are easier to
maintain and manage (such as in changing of net and harvesting). The mesh size of a broodstock cage varies from 4–8 cm. Stocking density of fish is 1 per cubic meter of water.
(b) concrete tanks
The size of concrete tanks used for holding broodstock depends on the size of the hatchery. It is advisable to use a bigger tank to allow the fish ample space for swimming. Generally, tank volume ranges from 100–200 tons (5 × 10 × 2m and 10 × 10 × 2m). Stocking rate in broodstock tank is 1 fish for every 2 cubic meters of water. Good water quality in broodstock tanks should be maintained. A water change of about 30–50% daily is recommended.
5. Feeds and feeding
Broodfish are fed once daily with fresh fish given at the rate of 5% of total biomass. Trash fish should be clean and fresh. As a normal practice, feeding is done at about 1600 hours.
6. Spawning and fertilization
6.1 Selection of spawners
The selection of spawners from the broodstock should be done months before the beginning of natural spawning to allow ample time for the fish to be conditioned to environmental and diet controls. Spawners are normally selected based on the following criteria:
fish should be active fins and scales should be complete free from disease and parasite free from injury or wounds males and females of similar size groups are preferred spawner should be at least 4–5 kg in body weight and should not be less than 3
years old
Selected spawners are then transferred to the pre-spawning tank. The ratio of male and female stocked in the pre-spawning tank is 1:1.
6.2 Care of spawners in pre-spawning tank
Immediately after stocking in the pre-spawning tank, the feeding is reduced from 5% to 1% of the total body weight and fed once a day.
Fig. 12 Floating cage for broodstock development
This is to prevent the fish from getting fat which can result in poor gonadal development. The feed given should be fresh marine fishes such as sardine, yellow stripe thread fin, etc.
Water in the spawning tank should be maintained in good condition. This can be achieved by changing the water about 50–60% daily.
7. Spawning of seabass
Presently, there are two major techniques employed in mass production of seabass fry in Southeast Asian countries: artificial fertilization and induced spawning.
7.1 Artificial fertilization
Spawners are caught in natural spawning grounds near the mouth of the river or in salt water lakes like Songkhla where the water depth is about 10–20m. Gill net and seine net are commonly used. Normally, the fishermen will net the fish during spring tide 2–3 days before the new moon or full moon until 5–6 days after the new moon or full moon at about 1800–2200 hours, at the time of the rising tide.
The degree of maturity of the collected spawners should be immediately checked. If the female has ripe eggs and the male is in the running stage, stripping is done in the boat. The fertilized eggs can then be transported to the hatchery for subsequent hatching. In cases where only the male is caught, the milt is collected by stripping into a dry glass container. Milt is then stored in an ice box or refrigerator. The milt can maintain its viability after a week in cold storage (5–15°C). The preserved milt should be made available for immediate use when a ripe female is caught.
The dry method of fertilization is normally used in this case. The eggs are stipped directly from the female to a dry and clean container where the milt is added. A feather is used in mixing the milt and eggs for about 5 minutes. Filtered seawater is then added into the mixture while stirring it and then allowed to stand undisturbed for 5 minutes.
7.2 Induced spawning
Two methods are normally used for inducing seabass to spawn in captivity, e.g. hormonal injection and environmental manipulation. Both methods would induce the fish to spawn naturally in the tank. This results in a monthly spawning until the gonads are spent.
7.2.1 Induced spawning by hormone injection
After stocking seabass broodstock in the pre-spawning tank for two months, the fish are inspected twice a month during spring tide, Ovarian maturity of the female is measured as follows: the eggs are sampled from the female through the use of a polyethelene cannula of 1.2 mm in diameter. The fish is either anaesthetized or inverted gently with a black hood over the head. The cannula is inserted into the oviduct for a distance of 6–7- cm from the cloaca. Eggs are sucked orally into the tube by the operator as the cannul is withdrawn. The eggs are then removed from the cannula and egg diameter measurement is made. When the seabass eggs reach the tertiary yolk globule stage or have a diameter of 0.4–0.5 mm, the female is ready for hormone injection. In males, only those with running milt are chosen.
The hormones usually used to induce spawning in seabass that produce reliable results are:
Puberogen HCG + pituitary gland of Chinese carp
Puberogen consists of 63% follicle stimulating hormone (FSH) and 34% Leutinizing hormone (LH). The dosage usually applied is 50–200 IU/kg of fish. The fish will spawn at about 36 hours after injection. If no spawning occurs, the second injection is applied 48 hours after the first injection (Fig. 13). The dosages of second injection should be double from that of the first injection and can also be given 24 hours after the initial injection. The
male is usually injected at the sasme time as the female with a dosage of 20–50 IU/kg of fish. The fish will normally spawn within 12–15 hours after the second injection.
Homogenized pituitary glands of Chinese carp are used at 2–3 mg/kg of fish mixed with Human Chorionic Gonadotropin (HCG) at 250–1,000 IU/kg of fish. The time interval of application and spwning are the same when using puberogen (Fig. 14).
Before injection, the spawner should be weighed and the hormone requirement computed. Spawners should be injected intramuscularly below the dorsal fin. After injection, they should be transferred from pre-spawning tank to the spawning tank. Twenty four hours after first injection, response of the fish to the hormone treatment is often manifested by the swelling of the belly. If the fish is expected to spawn within the nenxt 12–15 hours, a milky white scum (fatty in texture) will appear on the water surface of the spawning tank. If not, a second injection should be given.
Seabass that are induced to spawn by hormone treatment will always spawn within 12 hours after the second injection. The schedule of injections for subsequent spawning must be synchronized with the natural spawning time of the fish which occurs in late evening between 1800 to 2000 hours.
Time/strategy
First day Second day Third Day
0800am 0800 pm
0800am 0800pm 0800am 0800pm 0800am
1 1st injection
spawned eggs collection
2 1st injection
2nd injection
spawned eggs collection
3 1st injection
2nd injection spawned Eggs collection
Figure 13. Possibility of hormone injection interval of seabass.
Fish Sex Weight Hormone used
Dossages Time interval
Remarks
1. Female 5.2 Puberogen 100 IU/Kg. of fish
only one injection
Ovulation 36 hours after injection Fertilization rate is 70% hatching rate 80%, larvae were healthy.
2. Female 4.4kg. Puberogen 100 IU/Kg only one injection
Ovulation 36 hours after injecting Fertilization rate is 50% hatching rate is 30% larvae weak.
3. Female 4.8 kg. Puberogen 1st 50 IU/kg. 24 hours Ovulation 12 hours after final injection, fertilization rate is 80% hatching rate 70%, larvae healthy.
4. Female 5.5 kg. Puberogen 1st 50 IU/Kg 24 hours Ovulation 15 hours after final injection Fertilization rate is 70% hatching rate 80% larvae healthy.
5. Female 5.8 Puberogen 1st 50 48 hours Ovulation 12 hours after final
24 hours Ovulation 12 hours after final injection fertilization rate 80% hatching rate 70% larvae healthy.
10. Female 4.8 Kg HCG+GTH
1st 500 IU + 3mg/Kg 2nd 500 IU + 3mg/Kg
24 hours Ovulation 12 hours after final injection fertilization rate 80% hatching rate 80% larvae healthy.
Figure 14. The response of seabass to various type of hormone, dossages and time interval.
7.2.2 Induced spawning by environmental manipulation
Based on field observations and analysis of natural phenomena that occur during spawning period of seabass, techniques were developed to stimulate the fish to spawn in captivity. The following steps are necessary:
changing the water salinity to simulate fish migration decreasing the water temperature to simulate the decreased water temperature after
rain lowering and subsequent addition of fresh seawater to the tank in order to simulate
the rising tide, and follow the moon phase.
Initially, the salianity of water in pre-spawning tank is prepared at 20–25 ppt before stocking the selected spawners. After stocking, 50–60% of water is changed daialy until 30–32 ppt is reached. This will take about 2 weeks. This will simulate the migratin of fish from its growing grounds to the spawning grounds.
Constant monitoring of fish is required to detect pre-spawning behaviour. When the fish is observed to display its silver belly, this is an indication that it is ready to spawn.
The female fish separate from the school and cease to feed one week prior to spawning. Two or three days before the new moon or full moon, as the female approaches full maturity, there is an increase in play activity. The ripe male and female swim together more frequently near the surface as spawning time approaches.
At the beginning of the new moon or full moon, the water temperature in the spawning tank is manipulated by reducing the water level in the tank to 30 cm deep at noon time and exposing to the sun for 2–3 hours. This procedure increases water temperature in the spawning tank to 31–32°C. Filtered seawater is then rapidly added to the tank to simulate the rising tide. In effect, the water temperature is drastically decreased to 27–28°C.
The fish spawn immediately the night after manipulation (1800–2000 hours) or if no spawning occurs, manipulation is repeated for 2–3 more days, until spawning is achieved.
Whether the fish is induced by hormone treatment or environmentally manipulated to spawn, they would continue to spawn for 3–5 days after the first spawning provided the environmental factors that stimulate spawning are present, e.g. new or full moon, changes in salinity and temperature, etc. Since seabass spawn intermittently (by batch), the same spawner will continue to spawn during full moon or new moon for the next 5 to 6 months (Table 6).
Table 6. Monthly fish eff production and hatching rate of seabass by environmental manipulation at Satul Fisheries Station, Thailand
Month Tank No. No. of Eggs No. of Yolk Fish Hatching Rate
April 25–28 5 5,200,000 4,200,000 80.76%
May 23–27 5 6,120,000 4,710,000 76.9%
June 22–25 5 7,860,000 6,150,000 78.2%
July 20–24 5 11,240,000 9,450,000 84.1%
July 23–25 4 1,350,000 550,000 40.7%
August 22–26 5 13,510,000 10,900,000 80.1%
August 24–27 4 2,540,000 1,750,000 68.8%
September 22–23 4 1,730,000 1,000,000 57.8%
October 20–22 4 2,520,000 1,917,000 76.7%
November 19–21 4 390,000 272,000 69.7%
December 18–21 4 1,700,000 1,215,000 71.5%
January 16–17 4 200,000 86,000 43%
February 12–20 4 1,438,000 1,140,000 79.3%
March 15–18 4 3,770,000 2,960,000 78.5%
April 14–17 5 6,640,000 4,890,000 73.6%
May 13–15 5 14,000,000 11,950,000 85.4%
T O T A L 80,400,00 63,140,000 78.4%
7.3 Egg collection and incubation
Fertilized eggs of seabass range in size from 0.8–1 mm. They float in the water column (pelagic) and are very transparent.
Eggs in spawning tank can be collected and transferred to incubation tanks by either of the following procedures:
a. The spawning tanks are supplied with continuous flow of seawater. The overflowing water carry the eggs into a small tank (2 × 0.4 × 0.3 m) containing a plankton net (200μ mesh). This is usually set in the afternoon. Seawater should start to flow after the fish have spawned. Eggs are collected and transferred to larval rearing tanks the following morning (Fig. 15).
b. The eggs are collected from the spawning tanks using a fine mesh (200μ) seine net the morning after spawning (Fig. 16).
The collected eggs should be washed repeatedly through a series of filter screens to remove debris (organic detritus, plankton, etc.) that have adhered to the eggs. The eggs are then placed in graduated cylinders for density estimation. Normally, fertilized eggs float while the unfertilized eggs settle to the bottom of the container. Unfertilized eggs are later removed by siphoning.
Fertilized eggs are then transferred to incubation tank at the density of 100 eggs/liter. The eggs will hatch at about 17–18 hours at 26–28°C after spawning. Dead eggs which settled at the bottom are removed by siphoning. The newly-hatched larvae are carefully collected the following morning by scooping them with a beaker and immediately transferred to larval rearing tanks.
Hatching rate of seabass eggs by environmental and hormonal manipulation ranges between 40–85% and 0.1–85%, respectively
7.4 Larval rearing
The rearing tanks are commonly fabricated from plastic, fiberglass, wood or concrete. A typical larval rearing tank is rectangular in shape and located outdoor. Its volume ranges from 8–10 tons (7 × 1.2 × 1m or 10 × 1.5 × 1m). The tanks are usually protected from strong sunshine and heavy rains by a roof tile cover. The usual stocking density for newly-hatched larvae in rearing tank is between 50–100 larvae/liter.
Fig. 15 Egg collection
Fig. 16 Egg collecting by seine net.
7.5 Rearing environment
Good quality seawater at 30–31 ppt is required for larval rearing. Water temperature is also important and should range from 26–28°C to promote fast growth of larvae.
Larval tanks are prepared one to two days prior to the transfer of newly-hatched larvae. Filtered seawater are added to the tanks and very mild aeration is provided. After stocking, unicellular algae (Tetraselmis sp. or Chlorella spp.) are added to the tank and maintained at a density of 8–10 × 103 or 3–4 × 104 per ml for Tetraselmis sp. and Chlorella spp., respectively. These algae serve a dual purpose: as a direct food to the larvae and rotifer and a water conditioner in the rearing tank.
The following day after stocking, the bottom of the larval rearing tank should be cleaned and everyday thereafter. This is done by siphoning unfertilized eggs, faeces, dead larvae and uneaten food accumulating on the bottom of the tank. About 20% of tank water is changed daily for the first 25 days of the rearing period, then increased to 40–60% per day for the remaining culture period. Since seabass can also be cultured in freshwater, it is recommended to reduce the salinity of rearing water when the larvae is still in the hatchery, before it is transferred to the freshwater environment. Beginning from the 20th day, salinity can be lowered gradually until freshwater condition is reached on the 50th day.
7.6 Feeds and feeding
Larval feeds and the feeding scheme used most successfully are indicated and illustrated in Table 7 and Figure 17, respectively.
For the first three days after hatching, the larvae are not given any feed as they still obtain nutrients from the yolk sac. However, unicellular algaeChlorella sp. or Tetraselmis spp.) are added at the first day of rearing to maintain good water quality as well as feeds for Brachionus.
Beyond three days, when the yolk material has been fully absorbed and the mouth of the larvae are fully developed, rotifers (Brachionus plicatilis) are introduced as feed. A density of 3–5 rotifers per ml is maintained. Rotifers are given three times a day for about a week.
After the addition of Brachionus to the larval rearing tanks, pure culture of Tetraselmis sp. and Chlorella sp. should be added daily to maintain the required density of 8–10 × 103 or 3–4 × 104 cell/ml, respectively. A week after the larvae begin to feed, the larval density is then reduced to about 20–40 larvae per liter. The diet is switched to brine shrimp (Artemia sp.) nauplii for about 10 days. Thereafter, sub-adult and adult Artemiaare fed to the fish fry for 20–30 days or until the fish reach 50 days in age. After the fry attain body length of 12–15 mm (about 30 days old) ground fish meat can then be used as larval feed.
Organism Density/amount size of particle
Time of feeding fedding frequency
Phytoplankton (chlorella sp/Terraselnia sp)
4–5 × 103/ml 1–2 × 103/ml
5–20μ Day 1–5 1 time/day
Rotifer (Brachionus plicatilis)
3–5/ml 50–175 μ Day 3–12 4 times/day
Brine shrimp Nauplii (Artemia sp)
2–3/ml 250μ Day 10–23 2–3 times/day
Sub-adult and adult Brine (on demand) 1 mm-1 an Day 20–60 2–3 times/day
shrimp (Artemia sp)
Mince fresh marine fish (Sardine sp)
On demand 2 mm-5 mm Day 30/until harvest
2–3 times/day
Table 7. Type and size of larval feed
Fig. 17 Feed and Feeding Scheme Followed in the Rearing of the Seabass Larvae (L. calcarifer)
7.7 Larval feeds
One of the key factors that ensures success in seabass hatchery operation is the timely production of the needed food organisms in sufficient quantity.
7.7.1 Phytoplankton culture
Algal species used in seabass hatchery are Chlorella sp., Tetraselmis sp. and Isochrysis sp. All stages of the phytoplankton culture procedure are conducted in the phycology lab or algal room, except for large scale culture (more than 1 ton per tank) which are done outdoor.
The culture flow chart is shown in Figure 18. The hatchery should maintain pure stock (inoculum) of algae throughout the year. Sub-culture should be started each month so that larger cultures can be started as required.
To develop a culture, the required ration of starter (inoculum) to flask culture (1 liter) is 40–50 ml of each algal species per 1 liter of flask culture. From flask culture the algae are then used as inoculum for the larger culture in carboys (20 liters). The volume of ratio of the flask to carboy culture system is 1:10. Furthermore, they are inoculated with culture from flask to an initial density of 104 cell/ml. The culture cycle in the carboy is for 2–3 days and yields a final density of 1 million cell/ml.
The algae from 20 liters carboy are then inoculated to 200-liter aquarium tanks or transparent fiberglass tank. The volume ratio of carboy to aquarium tank is 1:10. From the aquarium tanks, the algae can now be directly used as starter for outdoor mass culture in 1 ton or 10 tons tank. The volume ratio will be 1:15. The average culture cycle in unicellular
algal mass prpoduction is between 3–5 days. Often, the average final cell density attained is 106 cell/ml. As a normal practice, the use of aquarium tank and mass culture tank are limited during the larval rearing season only.
The basic equipment needed in algal production are flask, carboy, test tubes, etc. Prior to any use, they are cleaned by rinsing with freshwater and sterilized in an autoclave. Vigorous aeration in flask, carboys, aquarium and mass culture tanks are required throughout the culture period.
Fig. 18. Feed Production Flow Chart
The culture media for these algae are as follows:
Conway medium (Walne 1974)
g/liter
Sodium nitrite 100
EDTA disodium salt 45
Boric acid 33.6
Sodium phosphate, monobasic
20
Ferric chloroide, 6 hydrate 1.3
Manganous chloride, 4 hydrate
0.36
Trace metal solution * 1 ml
Vitamin mix * 100 ml
Distilled water (to make) 1000 ml
NB: Use 1 ml conway medium and 1 liter of seawater
* Trace metal
Zinc chloride 2.1 g
Cobalt chloride, 6 hydrate 2.1 g
Ammonium molydate, 4 hydrate 2.1 g
Copper sulfate, 5 hydrate 2.0 g
Distilled water 100 liter
NB: Acidify with 1 N HCL until solution is clear
* Vitamin mix
Vitamin B12 10
Vitamin B1 10
Distilled water 200 ml
For aquarium tank, TMRL enrichment can be used.
TMRL g/liter distilled water
KNO3 100 g
NaHPO4 12H2O 10 g
FeCl36H2O 3 g
Na2SiO3.9H2O 1 g
NB: Use 1 ml of this solution in 1 liter of seawater
Rotifer is one of the most important feeds required for early stages of seabass larvae. It is rich in nutrient and small in size for the larvae to consume. In the larval rearing of seabass, rotifers in rearing water should be maintained at a density of 3–5 ml for at least 10 days.
Rotifer are usually reared either in concrete of fiberglass tanks. The size of the culture tank ranges from 1 ton to 50 ton tanks. The tanks are initially filled with Chlorella culture at a density of > 100,000 cell/ml. It is then inoculated with rotifer from another culture tank to a density of 10 rotifer/ml. Since the rotifer feeds on the algae, daily addition of the algae is neessary. Normally, rotifer production reach a peak density of between 100–200 ml in about 7–8 days. After 2 days of feeding with algae, rotifer can also be fed with marine yeast as a substitute diet
Harvest of rotifer from mass culture tanks are done after a period of 7–8 days. While some are used directly as larval feed, a portion must be saved for future inoculation of other tanks.
A production system partly using an incubator for the nauplii of Artemia will give reliable daily yield. The time for incubation ranges from 24–48 hours depending on the strain of Artemia. The cysts are stocked in the incubator unit at a density not to exceed 0.75 gm/liter. Strong aeration should be provided throughout the incubation period.
In an attempt to lower production cost of seabass fry, an intensive culture of Artemia should be adopted. The culture system normally practiced in the production of adult Artemia is raceway system. Artemia are stocked into rearing tanks at a density of 5–10/ml. The nauplii are fed daily with algae or commercial rice bran powder. After a growing period 7–10 days, they become sub-adult. Some may now be used as feed of seabass fry, while the rest are cultured to adult size.
7.8 Grading
In the rearing of seabass fry under confined conditions, the competition among the individuals for feed and space results in uneven growth of the fish especially if the stock is poorly managed. Heaavy mortalities will also occur due to cannibalism and stress on small or weaker fry. Since seabass is a voracious carnivore, proper grading should be done to avoid cannibalism. The graded fish must be reared separately.
Cannibalism in seabass larvae is distinctly rampant from the time the larvae starts to feed on Artemia (day 10 larvae). Grading is usually done a week after the fish started to feed on Artemia and every week thereafter. Grading trays used are normally made of plastic basin with many holes bored through the bottom, or made of netting with a wooden frame (Fig. 19). Each tray has a specific hole or mesh size to allow specific size of fish to pass through. The size of the hole/mesh of netting in each vessel varies from 0.3–10 mm.
Fish are placed in the trays which are floated in the newly prepared larval rearing tank. The smaller size fish can pass through the hole to the new tank. The remaining fish in the vessel
are transferred into another tank and likewise graded with the use of a bigger hole or mesh tray. This procedure sorts out fish to several sizes and simplifies management.
7.9 Diseases
The other important factor which causes high mortality in culture systems is disease. The most common symptoms of disease of seabass fry are:
loss of appetite loss of scales change of body colour from gray to black occurrence of white spot on the body
Treatment should be done immediately if any of these symptoms occur. Suitable treatments include:
a. For white spot:
Immersion of the fry in water at reduced salinity of 15–20 ppt with the addition of 20 ppm formalin for 1–2 hours.
b. For bacterial infection
Immersion in 3 ppm oxytetracycline for 10 hours.
Both treatments are followed by a flow-through of fresh seawater (100–150% of total water). The treatment is done once a day for 3–5 days until the larvae have regained normal colour and appetite.
Fig. 19 Grading vessel for grading of seabass fry.
V. Culture of Seabass
Seabass has been commercially cultivated in brackishwater and freshwater ponds and marine cages in many Southeast Asian countries. While the cagae culture technology is now established, grow-out techniques in pond are still are still in the developmental stages. Although considerable progress has been made over the past ten years, many problems remained unsolved.
The major problems that are always encountered during culture period are: (a) cannibalism during young stage (1–20 g), (b) dependence on trash fish as a main diet which has a very limited supply in many countries.
Despite some imperfections, the basic techniques of seabass culture are now developed and have been considered economically viable.
1. Culture techniques
As mentioned above, cannibalism is one of the most serious problems in seabass culture. High mortality is often encountered when uneven sizes of the fish are stocked. This has been noted to occur mostly where the fish are very young (1–20 cm in length, the first two months of culture). To minimize this problem, culture of seabass should be approached in two phases i.e. the nursery phase and the grow-out phase.
1.1 Nursery
The main purpose of the nursery is to culture the fry from hatchery (1–2.5 cm in size) to juvenile size (8–10 cm). This can solve the problem of space competition in the nursery tanks. Beyond the nursing period, the juveniles can be graded into different size groups and stocked in separate grow-out ponds. It has been observed that the juveniles from the nurseries perform better in terms of growth and survival than those stocked directly into the grow-out ponds.
Nursing the fry in concrete tanks is not recommended as accumulation of excess feed on the bottom of the tank cannot be avoided. Such accumulation can cause bacterial disease. In addition, constant contact with the tank wall results in wounded fish and subsequent bacterial infection
1.1.1 Nursery pond design
Nursery pond size ranges from 500 to 2000 m2 with water depth of 50–80 cm. The pond has separate inlet and outlet gates to facilitate water exchange. Pond bottom should be flat and sloping towards the harvesting or drainage gate. Inlet and outlet gates are provided with a fine screen (1 mm mesh size) to prevent predators and competitors from entering and fry from escaping the pond.
Fry ranging from 1–2.5 cm are suitable for stocking in the nursery ponds. Stocking density is between 20–50 individuals per square meter.
1.1.2 Pond preparation
A wellprepared pond is important as predators and competitors can endanger the stocked fry.
Some farmers still practice very crude farming techniques of drying the pond bottom and immediately filling with water and stocking fry directly for nursing. Feeding is entirely
dependent on supplementary feed such as chopped or grounded trash fish and is done twice daily in the morning (1800 hours) and afternoon (1700 hours). In this method, the survival rate and growth rate are low.
To enhance production, the following improved pond preparation techniques are done: The nursery pond must be drained and dried until the bottom soil cracks to release toxic gases, oxidize mineralized nutrients, eradicate some pests and predators. In cases where the pond cannot be completely drained, derris root (rotinone) may be applied at the rate of 20 kg/ha toeradicate unwanted species. Derris root is prepared by cutting them into small pieces, crushing and soaking in water overnight. Only the solution is applied to the pond. If derris root is not available, a mixture of 50 kg/ha of ammonium sulfate (21-0-0) with lime at a ratio of 1:50 will be sufficient to weed out unwanted species. The mixture is applied to the portions of pond with water. The use of any chemicals or inorganic pesticides is not recommended because the residual effect remains for many years and can reduce the pond production. If pond soil is acidic, the pond bottom should be neutralized with lime before letting the water in.
Production techniques of juvenile in nursery ponds have been improved recently at Satul Fishery Station, Thailand. The improved technique is based on the live food production in the pond supplemented with chopped or grounded trash fish. After neutralizing pond bottom by liming, organic fertilizer (chicken manure) is applied at the rate of 500 kg/ha. Then water depth is gradually increased for the propagation of natural food. Two to three weeks prior to stocking, newly-hatched Artemia nauplii are inoculated into the pond (1 kg of dry cyst/ha). Artemia will utilize the natural food as feed for growth and will reach adult stage within 10–14 days. The fry are immediately stocked at the rate of 20–50 individual per square meter.
Another approach to the improved technique is to stock Artemia nauplii in the separate pond and grow them into adult. Adults could be harvested daily to feed the fry.
1.1.3 Nursery pond management
Although seabass can be cultured in either freshwater or saltwater, fry must be acclimatized to the salinity and temperature prevailing in the pond on stocking to prevent loss.
Acclimatization is done in the following manner: transfer the fry to a tank, then gradually add nursery pond water. This can be completed within one day or more depending on the salinity difference. If the temperature and salinity in transport bag does not differ by more than 5°C and 5 ppt with the pond water, acclimation can be done by floating the bag in the pond for sometime to even out temperature difference. Pond water is then added gradually until both salinity become equal and the fry can be released.
Seabass fry are stocked in the nursery pond at a density of 20–50 fry/m2. Stocking is usually done in the erly morning (0600–0900 hours) or early evening (2000–2200 hours) when the temperature is cooler.
Water replenishment is needed to prevent deterioration of pond water quality due to the decomposition of uneaten feed or excess growth of natural food. Normally, 30% of pond water is changed daily.
Supplementary feed is given daily. The feed used for nursing seabass is chopped and grounded (4–6 mm3) trash fish, normally at the rate of 100% of biomass given twice daily in the first week (at 0900–1700 hours), gradually reduced to 60% for the second week and 40% in the third week. This has been found to be most effective feeding strategy for ponds without artemia inoculation.
The application of supplementary feed is a vital operational activity that should be done properly, if not, contamination of culture water and wastage of feeds result. Although the seabass in nature prefer live food, the fish can be trained to feed on dead animal. Prior to feeding, the fish should be attracted by sound (such as tapping a bamboo pole in the water) to induce them to form a school. Feeding time and place should be fixed. After the fish have formed a school, small amounts of feed are introduced by spreading into the water within the school of fish fry. It must be remembered that seabass never eat the feed when it sinks to the pond bottom. Therefore, feeding should be slow. When the fish are filled to satiation, they disappear thus feeding should be stopped. The same procedure should be followed at every feeding time. The first few days after stocking, feeding should be 5 to 6 times a day to teach them to accept dead feed. Once the fish is accustomed to it which takes about 5–7 days, feeding frequency is reduced to twice daily. In nurseries where Artemia is the main diet, once the Artemia population has thinned down, chopped or grounded trash fish can be supplemented using above described practice.
The nursing period lasts about 30–45 days until fingerling stage (size 5–10 cm). At this stage, they are ready for transfer to grow-out ponds.
1.1.4 Nursing in net cages
Nursing of seabass fry (1–2.5 cm to 8–10 cm) in cages is an approach to the nursery phase. The method has been successful since conducive environmental conditions such as flow through water, necessary for good health and growth of fish are used. It is likewise easy to maintain and require very little capital investment.
1.1.5 Nursing cage design and investment
The most convenient cage design is a rectangular cage made of synthetic netting attached to wooden frames. It is either (a) kept afloat by styrofoam, plastic or metal drum, or (b) stationary by fastening to a bamboo or wooden pole at each corner. The size of cages vary from 3 cubic meter (3 × 1 × 1m) to 10 cubic meters (5 × 2 × 1m). The mesh size of the net used for nursery cages is 1.0 mm. The cages may be installed in the river, coastal area or in a pond. Suitable sites for net cages should be free from biofoulers since the mesh size of a nursery cage is very small. Cages are easily damaged in strong currents and clogging by biofoulers. (Fig. 20)
1.1.6 Nursery cage management
Seabass fry (1–2.5 cm in size) are stocked in the nursery cage at the rate of 80–100 per square meter.
Stocking and feeding activity are the same as in nursery pond culture practice.
The net cages should be checked daily to ensure that the cages are not damaged by animals such as crabs or clogged with fouling organisms. Cleaning of the cages should be done every other day by brushing. This will allow water to pass through the cages naturally.
Fig. 20 Floating nursery cage for seabass.
After the nursing period of 30–45 days (in pond or cages) or when the fry have reached 5–10 gm, these are ready for transfer to grow-out ponds. Prior to stocking in grow-out ponds, grading procedure should be applied. Fish are graded into several sizes. It will give maximum advantage if the various sizes are stocked in separate ponds to prevent cannibalism.
1.2 Grow-out
The grow-out phase involves the rearing of the seabass from juvenile to marketable size. Marketable size requirement of seabass vary country to country e.g. Malaysia, Thailand,
Hong Kong and Singapore. The normally accepted marketable size of seabass among these countries and region is between 700–1200 g while in the Philippines, marketable size is between 300–400 g. The culture period in grow-out phase also vary from 3–4 months (to produce 300–400) to 8–12 months.
2. Cage culture
Cage culture of seabass is quite well developed in Thailand, Malaysia, Indonesia, Hong Kong and Singapore. The success of marine cage culture of seabass and its economical viability have contributed significantly to large scale development of this aquaculture system
2.1 Suitable site for cage culture
Criteria for selecting a suitable site for cage culture of seabass include:
a. Protection from strong wind and waves. The cage culture site should preferably be located in protected bays, lagoons, sheltered coves or inland sea.
b. Water circulation. The site should preferably be located in an area where influenc of tidal fluctuation is not pronounced. Avoid installing cages where the current velocity is strong.
c. Salinity. Suitable site for seabass culture should have a salinity ranging from 13–30 ppt.
d. Biofouling. The site should be far from the area where biofoulers abound. e. Water quality. The site should be far from the sources of domestic, industrial and
agricultural pollution and other environmental hazards.
2.2 Design and construction of net cages
In general, square and rectangular cages with size varying from 20 to 100 m3 are preferable because they are easy to construct, manage and maintain. Seabass cages usually are made of polyethelene netting with the mesh size ranging from 2 to 8 cm. The choice of mesh size depends on the size of the fish (Table 8).
There are two types of cages used in seabass culture:
(a) Floating cages
The net cages are attached to wooden, GI pipe or bamboo frames. The cage is kept afloat by floating material such as metal, plastic, styrofoam drum or bamboo. The shape of the cage is maintained with the use of concrete weights attached to the corners of the cage bottom (Fig. 22).The most manageable size for a floating cage is 50 m3 (5 × 5 × 2m). This cage dimension is easy to change when clogged with fouling organisms.
(b) Stationary cages
The cage is fastened to the bamboo or wooden poles installed at its four corners (Fig. 21). Stationary cages are popularly used in shallow bays since they are easy to install.
2.3 Cage culture management and techniques
Prior to stocking seabass juvenile in cages, fish should be acclimatized to the ambient temperature and salinity prevailing in the cages. The fish should be graded into several size groups and stocked in separate cages. The stocking time should be done in the early mornings (0600–0800 hours) or late in the evening (2000–2200 hours) when the temperature is cooler.
Stocking density in cages is usually between 40–50 fish per cubic meter. Two to three months thereafter, when the fish have attained a weight between 150–200 g, the stocking density should be reduced to 10–20 fish per cubic meter. Table 9 shows the growth of seabass under varying densities in cages. There should be spare cages as these are necessary for transfer of stock and to effect immediate change of net in the previously stocked cage once it has become clogged with fouling organisms. Changing cages allows for grading and controlling stock density.
2.4 Feeds and feeding
Feed is the major constraint confronting the seabass culture industry. At present, trash fish is the only known feed stuff used in seabass culture. Chopped trash fish are given twice daily in the morning at 0800 hours and afternoon at 1700 hours at the overall rate of 10% of total biomass in the first two months of culture. After two months, feeding is reduced to once daily and given in the afternoon at the rate of 5% of the total biomass. Food should be given only when the fish swim near the surface to eat.
mesh size size of fish
0.5 cm 1–2 cm
1 cm 5–10 cm
2 cm 20–30 cm
4 cm bigger than 25 cm
Table 8. The choice of netting mesh size of fish.
FIG. 21 STATIONARY CAGES
FIG. 22 FLOATING CAGES
Table 9. Monthly growth of seabass at different stocking densities in cages.
Culture period (month) Stocking density
16/m2 24/m2 32/m2
0 67.80 g 67.80 g 67.80 g
1 132.33 g 137.53 g 139.20 g
2 225.20 g 229.10 g 225.50 g
3 262.88 g 267.50 g 264.11 g
4 326.15 g 331.97 g 311.50 g
5 381.08 g 384.87 g 358.77 g
6 498.55 g 487.06 g 455.40 g
Since the supply of trash fish is insufficient and expensive in some countries, its use is minimized by mixing rice bran or broken rice to the trash fish (Table 10). However, even with these cost cutting measures, feed cost remain quite high.
A very recent development on improving the dietary intake of seabass is the introduction of moist feed. So far, the use is still on experimental stage. The feed composition recommended is presented at Table 11.
2.5 Fish cage management
Regular observation of cages is required. Since fish cages are immersed under water all the time, they are vulnerable to destruction by aquatic animals such as crabs, otter, etc. If damaged, they should be repaired immediately or replaced with a new one.
In addition to biofouling, the net walls of cages are subjected to siltation and clogging. Biofouling is unavoidable since the net walls usually represent a convenient surface for attachment by organisms such as amphipod, polycheate, barnacles, molluscan spats, etc. These could lead to clogging and reduce exchange of water and may result in unnecessary stress to the cultured fish due to low oxygen and accumulation of wastes. Feeding and growth would likewise be affected.
To date, mechanical cleaning of fouled nets is still the most efficient and cheap method. In areas where fouling organisms are abundant, rotational usage of net cage is highly recommended.
3. Pond culture
Although methods of pond culture of seabass have been practiced for over 20 years in Southeast Asia and Australia, not much has been done on the commercial scale. At present, culture of seabass in brackishwater pond has been identified in some countries as having tremendous market potential and high profitability. These, however, can be achieved if conditions are met such as adequate fry supply, availability of suitable site and properly designed fish farm. Supply of fry from the wild is very limited. As with cage culture, it is one of the constraints in the intensification of seabass culture in ponds. However, with the success in artificial propagation of seabass, fry supply may largely come from this source in
the future. A comparison of hatchery bred and wild fry cultured in ponds did not show very significant difference in growth rate(Table 12).
There are two culture systems employed in pond culture of seabass:
(a) Monoculture
Monoculture is that type of culture where a single species of animal is produced, e.g. seabass. This culture system has a disadvantage. It is entirely dependent on supplementary feeding. The use of supplementary feed reduces profit to the minimal, especially where the supply of fresh fish is limited and high priced.
Table 10. Combination of feed stuff.
Ingredient Percentage
Trash fish 70%
Rice bran or broken rice 30%
Table 11. Combination of moist diet
Ingredient Percentage
Fish meal 35%
Rice bran 20%
Soy bean meal 15%
Corn meal 10%
Leaf meal 3%
Squid Oil (or fish oil) 7%
Starch 8%
Vitamin mix 2%
Table 12 Comparison of growth rate of seabass (Lates calcarifer) culture in pond between wild fry and hatchery bred fry at stocking density of 3/m2.
Wild Hatchery bred
B.L. B.W. B.L. B.W.
Stocking 10.5 cm 40.44 g 5.2 cm 5 g
1st month 13.0 88.9 7.6 12.0
2nd month 16.4 204.2 10.6 26.02
3rd month 20.9 276.3 15.2 118.1
4th month 23.4 326.5 19.5 220.9
5th month 24.1 385.2 21.8 280.6
6th month 28.2 453.5 23.2 349.6
(b) Polyculture
This type of culture approach shows great promise in reducing if not totally eliminating the farmers' dependence on trash fish as food source. The method is achieved by simply incorporating a species of forage fish with the main species in the pond. The choince of forage fish will depend on its ability to reproduce continuously in quantity sufficient to sustain the growth of seabass throughout the culture period. The forage fish must be such a species that could make use of natural food produced in the pond and does not compete with the main species in terms of feeding habit such as Oreochromis mossambicus, Oreochromis niloticus, etc.
4. Criteria in the selection of site for seabass culture
4.1 Water supply
The site should have enough good water quality supply all year round. Water quality includes all physico-chemical and microbiological characteristics of water being used for culture of seabass. The following are the parameters normally considered as suitable water supply:
Parameter Range
pH 7.5–8.5
Dissolved oxygen 4–9 ppm
Salinity 10–30 ppt
Temperature 26–32°C
NH3 less than 1 ppm
H2S less than 0.3 ppm
Turbidity less than 10 ppm
4.2 Tidal fluctuation
Area best suited for seabass should have moderate tide fluctuation range between 2–3 meters. With this tidal characteristic even for ponds as deep as 1.5 meters, complete drainage during low tide can be done. In addition, the pond can readily admit water during spring tide.
4.3 Topography
It is advantageous if the selected site is mapped topographically. This would reduce development and operational costs such as for water pumping.
4.4 Soil
Ideally, the soil at the proposed site should have enough clay content to ensure that the pond can hold water. Area with acid sulphate soil should be avoided.
4.5 Accessibility
Accessibility is an important consideration in site selection for logical reasons. Overhead cost and delay in the transport of material and product may be minimized with good site accessibility.
Other factors in the selection of site that should be considered include availability of seed, labour, technical assistance, market demand and suitable social condition.
5. Pond design and construction
Seabass ponds are generally rectangular in shape with size ranging from 2000 m2 to 2 hectares and depth of 1.2 to 1.5 meters. Each pond has separate inlet and outlet gate to facilitate water exchange. The pond bottom is entirely flat levelling toward the drainage gate (Fig. 23).
6. Pond preparation
Preparation of grow-out ponds is similar to the procedure followed in pond system. In monoculture, the fish are stocked immediately after neutralizing the pond soil with lime. Ponds are filled immediately after pond preparation.
In polyculture, after the pond soil is neutralized, organic fertilizer (chicken manure) is applied at the rate of 1 ton per hectare. Then water depth is gradually increased for propagation of natural food. When abundance of natural food are observed, selected tilapia broodstocks are released to the pond at the rate of 5,000–10,000 per hectare. Sex ratio of male to female is 1:3. The tilapia are reared in pond for 1 to 2 months or until tilapia fry appear in sufficient number. Seabass juveniles are then stocked.
Seabass juveniles (8–10 cm in size) from nursery are stocked in the grow-out pond at the rate of 10,000–20,000 per hectare in monoculture and 3,000–5,000 per hectare in polyculture system. Prior to stocking, juveniles are acclimatized to pond culture and salinity conditions. Stocking the fish in uniform sizes will be most ideal and should be done at cooler times of the day.
Fig. 23 Pond lay-out for seabass culture.
7. Pond management
Due to the need of maintaining natural food in ponds, water replenishment in polyculture system should be minimized. Water change should be done once in three days for about 50% of capacity.
However, in monoculture where supplemental feed is given daily, there are chances that excess feed may pollute the water. Hence, daily water replenishment is necessary.
8. Feeds and feeding
Supplementary feed is not required in the polyculture system, but in monoculture, daily feeding is a normal practice. The method of supplying feed in ponds follows often the practice employed in cage culture.
VI. Financial Analyses of Sea Bass Culture Industry
Seabass farming generally requires a higher level of management than a conventional aquaculture enterprise. Moreover, it requires quite different types of management skills. However, at this early stage of development, seabass production, both fingerlings and marketable fish enterprises, show the promising future of the industry.
Fig. 24 shows the typical cost and return analysis for one private hatchery unit under experienced management in Thailand. The advantage of hatchery in Thailand is that, the hatchery can dispose excess newly hatched larvae to the farmers who are operating small nursery for their own use. They in turn, can sell their surplus to near-by farmers. The hatchery in this case, maintains a certain number of larvae that it requires. The financial analysis shows that net income of 0.5 USD per 1 USD investment can be obtained, thus, fully demonstrating its commercial viability. Operating cost of hatchery operation is fairly high, it contributed about 67% of the total cost. The cost of Artemia cyst (26.3%), salary skilled and experienced workers (15%), and electricity (9.5%) are major expenditures of the enterprise. Meantime, interest rate from a sizable expenditures for fixed cost (23.7%) because of high bank interest rate.
Despite the high operating cost, profitability of the enterprise can still be increased. By enhancing the survival of larvae, there will be corresponding increase in larval production. Hence, the same facilities can be used to produce many more fingerlings during hatchery season. With a production of 2 million 2.5 cm size larvae, the capital cost involved and the facilities used can be substantially reduced without deminishing the output. However, this will depend on the skill and experience of the hatchery technicians.
Grow-out of seabass has also proven to be an economically viable industry in the Southeast Asian Region. The cost benefit analysis in Fig. 25used the data obtained from private farm both pond and cage culture in Thailand. Pond culture is based on one hectare pond with stocking desnsity of 3/m2, while cage culture based on one farm which consisted of ten 5 × 5 × 2 m cages at Satul Province with stocking density ofm 40/m2 or 1000 fish per cage. The marketable size is 500–600 g, culture period is 6 months at 2 croppings per year. Feeding depends entirely on marine trash fish with food conversion rate ranging from 8 to 10:1.
Item Value
A. Income
Newly hatched larvae (1 day old) 10 M. (1,000/2 USD.) 20,000
0.5 cm. larvae (15 days old) 2 m. (1,000/6 USD.) 12,000
2.5 cm. larvae (40–50 days old) 2 M. (1,000/100 USD) 200,000
Sub-total A 232,000
B. Fixed Cost
Land Cost (10,000 × 18% interest) 1,800 (1.2%)
Hatchery construction (50,000 × 10% depreciation) 5,000 (3.3%)
Figure 24 The cost and return of seabass hatchery base on Thailand condition Fry are sold in several stages to prevent over stocking.
In Fig. 25, it shows that profitability of seabass culture is highly influenced by the cost of feed, 42% and 43%; seed, 27% and 16%, of the total expenditure of pond and cage culture respectively. Interest rate form a sizeable expenditure of 16% and 19%, respectively, for the two culture systems because of high bank interest rate. It is apparent that the annual net income from cage culture is almost the same as pond culture despite the total operating cost of the latter is about double of cage culture. The difference is attributed to the fact that
in cage culture, there is no land cost and stocking density is higher. However, the life span of cage is shorter and lasts about three years. With profit margin of 26.3% and 28.5%, for pond and cage culture systems, respectively, seabass culture has shown to be one of the lucrative aquaculture enterprises.
Conclussion
Seabass (Lates calcarifer) culture enterprise is one of the most dynamic and potentially profitable segments of the brackish and marine water fish farming industry in Southeast Asia. It is a desirable fish with good flesh texture and taste, high market value and market value and demand. It can be reared both in freshwater and seawater conditions. In the past 5 years, over 10,000 farmers engaged in cage culture of seabass and over 20,000 hectares of land have been established in the Region for intensive pond production of the species.
In as much as seabass can now be artificially propagated, the investment for hatchery production is justified which ensures sufficient seed supply to the farming industry. Although the seabass culture industry appears to have a bright prospect, it is still confronted with several constraints. Foremost of these constraints is the shortage of Artemia cyst and also has become very expensive. Artemia naupluii play a significant role in hatchery production for both finfish and crustacean as an important live food organism. It is, therefore, necessary that a cheap and suitable alternative live food organism with similar nutritional value as Artemia must be found and cultured. Otherwise, artificial feed must be used but so far no suitable formulation has been developed yet.
At the present time in some countries, all culture systems for seabass use trash fish as feeds. As the supply of trash fish is limited and expensive, these factors have hampered the growth of the industry. Thus, in addition to finding suitable alternative live food organisms, research work on artificial feeds must be conducted as subsitutes for trash fish.
Another factor that constraints the seabass industry is the lack of organization, communication and marketing channel to date. As a result, prices in the market of food fish as well as fry and fingerlings are not steady.
As long as the above factors remain unsolved, these will slow down the growth of the seabass industry as a whole.
Item Pond Cages
A. Income
Marketable fish 14,000 Kg × 3 USD 42,000 8,000 Kg × 3 USD 24,000
Sub-total A 42,000 24,000
B. Fixed Cost
Land cost (5,000 × 18% interest) 900 (2.7%) lease 10
Figure 25 Comparison of cost and return between pond and cages culture in Thailand.
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