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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 10
Fig 11

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:

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

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 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/strategyFirst daySecond dayThird Day
0800am0800 pm0800am0800pm0800am0800pm0800am
11st injection  spawnedeggs collection  
21st injection 2nd injectionspawnedeggs collection  
31st injection   2nd injectionspawnedEggs collection

Figure 13. Possibility of hormone injection interval of seabass.

FishSexWeightHormone usedDossagesTime intervalRemarks
1.Female5.2Puberogen100 IU/Kg. of fishonly one injectionOvulation 36 hours after injection Fertilization rate is 70% hatching rate 80%, larvae were healthy.
2.Female4.4kg.Puberogen100 IU/Kgonly one injectionOvulation 36 hours after injecting Fertilization rate is 50% hatching rate is 30% larvae weak.
3.Female4.8 kg.Puberogen1st 50 IU/kg.24 hoursOvulation 12 hours after final injection, fertilization rate is 80% hatching rate 70%, larvae healthy.
4.Female5.5 kg.Puberogen1st 50 IU/Kg24 hoursOvulation 15 hours after final injection Fertilization rate is 70% hatching rate 80% larvae healthy.
5.Female5.8 Kg.Puberogen1st 50 IU/Kg.
2nd 50 IU/Kg
48 hoursOvulation 12 hours after final injection fertilization rate 75% hatching rate 60% larvae healthy.
6.Female6.5 Kg.Puberogen1st 50 IU/Kg.
2nd 50 IU/Kg
48 hoursOvulation 12 hours after final injection Fertilization rate 60% hatching rate 85%, larvae healthy.
7.Female4.5 Kf.HCG1st 500 IU/Kg.
2nd 1,000 IU/Kg
24 hoursOvulation 12 hours after final injection fertilization rate 30% hatching rate 65% larvae weak.
8.Female6.2 Kg.HCG1st 500 IU/Kg.
2nd 1,000 IU/Kg
24 hoursOvulation 12 hours final injection fertilization rate 30% hatching rate 65% larvae weak.
9.Female5.5 Kg.HCG+GTH1st 500IU + 3mg/kg
2nd 500IU + 3mg/kg
24 hoursOvulation 12 hours after final injection fertilization rate 80% hatching rate 70% larvae healthy.
10.Female4.8 KgHCG+GTH1st 500 IU + 3mg/Kg
2nd 500 IU + 3mg/Kg
24 hoursOvulation 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:

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

MonthTank No.No. of EggsNo. of Yolk FishHatching Rate
April 25–2855,200,0004,200,00080.76%
May 23–2756,120,0004,710,00076.9%
June 22–2557,860,0006,150,00078.2%
July 20–24511,240,0009,450,00084.1%
July 23–2541,350,000550,00040.7%
August 22–26513,510,00010,900,00080.1%
August 24–2742,540,0001,750,00068.8%
September 22–2341,730,0001,000,00057.8%
October 20–2242,520,0001,917,00076.7%
November 19–214390,000272,00069.7%
December 18–2141,700,0001,215,00071.5%
January 16–174200,00086,00043%
February 12–2041,438,0001,140,00079.3%
March 15–1843,770,0002,960,00078.5%
April 14–1756,640,0004,890,00073.6%
May 13–15514,000,00011,950,00085.4%
T O T A L 80,400,0063,140,00078.4%

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:

  1. 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).

  2. 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

Fig. 15 Egg collection

Fig. 16

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 algae Chlorella 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 Artemia are 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.

OrganismDensity/amountsize of particleTime of feedingfedding frequency
(chlorella sp/Terraselnia sp)
4–5 × 103/ml
1–2 × 103/ml
   5–20μDay 1–51 time/day
(Brachionus plicatilis)
3–5/ml50–175 μDay 3–124 times/day
Brine shrimp Nauplii
(Artemia sp)
2–3/ml     250μDay 10–232–3 times/day
Sub-adult and adult Brine shrimp
(Artemia sp)
(on demand)1 mm-1 anDay 20–602–3 times/day
Mince fresh marine fish
(Sardine sp)
On demand2 mm-5 mmDay 30/until harvest2–3 times/day

Table 7. Type and size of larval feed

Fig. 17

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

Fig. 18. Feed Production Flow Chart

The culture media for these algae are as follows:

Conway medium (Walne 1974)g/liter
 Sodium nitrite100     
 EDTA disodium salt45    
 Boric acid33.6
 Sodium phosphate, monobasic20   
 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 chloride2.1 g
 Cobalt chloride, 6 hydrate2.1 g
 Ammonium molydate, 4 hydrate2.1 g
 Copper sulfate, 5 hydrate2.0 g
 Distilled water100 liter

NB: Acidify with 1 N HCL until solution is clear

* Vitamin mix

 Vitamin B1210
 Vitamin B110
 Distilled water   200 ml

For aquarium tank, TMRL enrichment can be used.

TMRLg/liter distilled water
KNO3100 g
NaHPO4 12H2O   10 g
FeCl36H2O     3 g
Na2SiO3.9H2O     1 g

NB: Use 1 ml of this solution in 1 liter of seawater

Enrichment for outdoor culture

46–0-0 (Urea)12
21-0-0100 g

7.7.2 Rotifer culture

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:

Treatment should be done immediately if any of these symptoms occur. Suitable treatments include:

  1. 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.

  2. 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

Fig. 19 Grading vessel for grading of seabass fry.

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