by
Mr. Wichien Sakares
Rayong Brackishwater Fisheries Station
Brackishwater Fisheries Division
Department of Fisheries
Bangkok, Thailand
Sea Bass, Lates calcarifer (Bloch), is an important fish in Thailand. It is cultured in commercial scale in many parts of the coastal provinces. Due to its excellent taste, less bony character and high nutritional value there is a great demand for this species in the market. Cost of 1 kg. of sea bass ranges from 50 – 80 Baht.
In the past, the farmers had to depend mainly on the fry collected from natural grounds. In 1973 sea bass fry were produced in Government Fisheries Station by artificial breeding and from 1975 seed production of sea bass was successful in some Brackish Water Fisheries Station and private hatcheries by natural breeding in captivity.
1. Seed Production of Sea Bass
There are three methods for breeding of sea bass in Thailand.
1.1 Artificial breeding
Artificial breeding of sea bass was done by Songkhla Fisheries Station in 1973. It was the first time of sea bass breeding in Thailand. At present this method is done only in southern part of Thailand, especially in Songkhla Lake region. The farmers (private farm) will go to the mouth of Songkhla Lake and use gill net for catching sea bass. They will select spawners for stripping. Artificial breeding will be done on board the ship and fertilized eggs will be transported to the hatcheries. The season of sea bass breeding is range from April to October.
1.2 Natural spawning
Sea Bass can be spawned in captivity. Although in the southern part of the country this species could be spawned naturally in suitably constructed ponds. The age of broodstock are over three years and body weights are more than three kg.
1.3 Natural spawning by using hormone injection
This method was favourite in eastern part of the country. Especially at Rayong Brackish Water Fisheries Station and most of private farms.
a. Rearing of brood stock
The brood fish of Lates are reared in earthen pond at the stocking density of 100 fish per rai. (1 ha - 6.25 rai). The age of brood stock are ranges from 3 – 8 years and body weights are ranges from 3 – 10 kg. Carangid fish meat combined with Vitamin E. 1 capsule per fish is provided as feed. Ideal conditions for brood stock rearing are temperature 28 – 30°C, salinity 30 – 32 ppt and pH 8.3.
b. Hormone injection
Puberogen is used at the rate of 50 units/kg in females and 25 – 30 units/kg in males for induced spawning of sea bass. The fish are injected in the morning at about 9.00 am. Injected fish will be moved to the spawning pond and the ratio of male and female is 1:1. Generally 24 – 30 fish are released into 150 – 200 tons of cement spawning pond. At Rayong Brackish Water Fisheries Station the spawning pond is rectangular in shape with an average depth of 1.3 1.5 m. With aeration by continuous sprinkling of sea water through pipes. Spawning takes place in 36 hrs. Eggs are collected with scoop nets at night or early morning. Good eggs are 0.8 mm. in size and they possess and oil globule. Eggs float in sea water (30 – 32 ppt.). Collected eggs are transferred to circular hatching tanks at the rate of 50.000 -80,000 eggs/ton of water. Slow aeration is used. They will hatch in 15 – 17 hrs at the temperature of 28° – 30°C. Shells and unfertilized eggs are siphoned out.
c. Larval rearing
Newly hatched larvae depend on the yolk from the yolk sac attached to their bellies. On the 2nd day evening or 3rd day morning they are fed with excess rotifer until the 15th day. Chlorella is also added to the tank to serve as feed for rotifer. From the 12th day of hatching quantity of rotifers is decreased to 25% and Brine shrimp (Artemia salina) is fed until the 25th day. Stocking density after 15th day is 20,000 – 25,000 fry/ton of water. After 20 – 22 days of hatching the fry have to be graded into different sizes for further rearing. Smaller fry are fed with Artemia and the others are fed with fish meat and Artemia until 45th day. The stocking of bigger fry is 4,000 – 5,000 fry/ton of water and reared in bigger cement tanks of circular or rectangular type.
Table 1. Feeding diagrame of sea bass fry.
| Age (day) | Finds of feed in % | ||||
| Chlorella | Rofifer | Artemia | Mysid | Fish Meal | |
| 1 | - | - | - | - | |
| 2–4 | 75 | 25 | - | - | - |
| 5–9 | - | 100 | - | - | - |
| 10–14 | - | 75 | 25 | - | - |
| 15–21 | - | 25 | 75 | - | - |
| 22–25 | - | - | 50 | 25 | 25 |
| 26–30 | - | - | 25 | 25 | 50 |
| 30–45 | - | - | - | 25 | 75 |
The size of fry on the 30th – 45th day is about 1.5 – 2.5 cm. Most of the hatcheries distribute sea bass fry to the farmers at the size of 1.5 – 2.5 cm. in length or at the age of 30 – 45 days. Therefore the farmers have to carry out nursery rearing of the fry until they reach 7.5 – 15 cm. in length or about 3 – 4 months in age.
II. Nursing of sea bass fry. (From 1.5 – 2.5 cm. to 7.5 – 15 cm.)
2.1 Nursing in tanks
The best nursing of sea bass fry is nursing in tanks. Cement tanks supplied with exygen and drain pipes are used for nursing of the fry. Finely chopped fish is provided as food for feeding 2 – 3 time/day. After feeding in the afternoon, organic wastes of dead organisms and left-over food should be siphoned out. Water should be changed every day after feeding. Owing to the cannibalistic nature of the fish, size selection or grading is necessary during the whole nursery period. Especially during 1st month of nursing the fry are graded at least once a week. The stocking density of 1.5 – 2.5 cm. in length of the fry can be 4,000 – 5,000 fry/ton of water. The stocking density will be decreased during the nursery period. The stocking density of 10 cm. in length of the fingerlings are 200 fish/ton of water. Salinity of the water is range from 0 – 32 ppt. The period of nursing is about 1 – 3 months.
2.2 Nursing in earthen pond
Earthen ponds are constructed in coastal area. The size of the ponds are vary from 50 – 400 m2 and 0.8 – 1.0 m. deep with one or two sluice gate. The stocking density can be 10 – 20 fry/m2. The fry are fed with minced trash fish. The feeding frequency is 3 to 4 times a day. The fish are fed to satiation in each feeding.
2.3 Nursing in nylon net cage
1.5 – 2.5 cm. fry are stocked in nylon net cages each having an area 1 – 2 m2 and depth of 1m. The suitable mesh size is 3 – 5 mm. 300 – 500 fry can be stocked in 1 m2. Fry are fed with finely chopped trash fish. Feeding and management are similar to nursing fry in the tank. The net cages should be protected by another net cage of bigger mesh size around them.
III. Culture of fingerlings up to maketable size
The maketable size of sea bass in Thailand is 500 – 800 gm in weight. This size is more expensive than the smaller and bigger size. Two types of culture are favourite.
3.1 Culture in earthen pond
Generally ponds are located in the coastal area and within the tidal influences but some ponds are located in freshwater area. The size of the fish ponds are vary from 1,600 – 30,000 m2 in area and 1 1.5 m. depth. The ponds are provided with an inlet and an outlet. Pond should be dried at least 1 week prior to stocking with fingerlings. Water is pumped in during the high tide through a filter net. Fingerlings are stocked at the rate of 1 – 2 fingerling/m2 at the size of 7.5 – 10 cm. The fish are fed with minced fish in the first months. After 1 month, fresh trash fish are chopped into small pieces and fed to the fish twice a day the fish are fed to satiation in each feeding. In the past time, the fish of 50 – 100 gm in weight were cultured with low stocking. The yield was low about 400 kg/rai/ 6 months. The survival rate was about 60%. In 1983, the experiment of the Rayong Brackish Water Fisheries Station showed that sea bass 6.0 – 10 cm. in length can be cultured in the earthen pond with the stocking density of 2 fish/m2. The period of culture was 6 months. The yield was 1467.24 kg/rai and survival rate was 92.86%. The average body weight of the final fish were about 500 gm and food conversion ratio was about 3.5:1.
Table 2 The results of the experiment on Sea Bass cultured in earthen pond by Rayong Brackish Water Fisheries Station in 1983. (period of culture was 6 months).
| Pond No. 1 | Pond No. 2 | Total | Average | |
| Area (m2) | 1,700 | 2,040 | 3,740 | 1,870 |
| No. of fish were stocked | 3,400 | 4,080 | 7,480 | 3,740 |
| Average size of initial fish in length (cm.) | 6.21 | 6.21 | 12.42 | 6.21 |
| Average size of initial fish in weight (gm.) | 4.25 | 4.25 | 8.50 | 4.25 |
| Final mean individual weight of fish (gm.) | 506.10 | 521.72 | 1027.82 | 513.91 |
| Growth rate (gm/day) | 2.77 | 2.86 | 5.63 | 2.82 |
| Survival rate (%) | 94.32 | 91.40 | 185.72 | 92.86 |
| Yield (kg.) | 1,641.53 | 1,771.66 | 3,413.19 | 1,706.60 |
| Yield/rai* (kg) | 1,544.95 | 1,389.53 | 2,934.48 | 1,467.24 |
| Food conversion ratio | 3:16:1 | 3.24:1 | 6.40:2 | 3.20:1 |
3.2 Culture in net cage
Floating net cages can be used for rearing sea bass to marketable size. Generally this method is used in lake or in estaury and open sea where the wave action is minimal. Generally the size of the cages are 5 × 5 × 3 m. 10 × 10 × 3 m. Suitable mesh size is 2.5 cm. for fingerlings of size 10 – 15 cm. Stocking rate varies from 10 – 15 fish/m2. A yeild of 8 – 15 kg/m2/annum at 200 – 375 kg. from 5 × 5 × 3 and 800 – 1,500 kg. from 10 × 10 × 3 m. cage has been obtained.
In central part of Thailand the maketable size of sea bass are range from 500 – 800 gm. So sea bass cage culture ought to be more stocking. Rayong Brackish Water Fisheries Station have been researched on sea bass cage culture with high stocking density from 1980 – 1983. The results showed that sea bass 10 – 15 cm. in length could be cultured in floating net cage with the stocking density of 100 fish/m2. The fish were fed with chopped trash fish twice a day.
The fish were fed to satiation in each feeding. The cages were made of polyethylene netting of 2.5 cm. mesh size. The frame of the cages were 0.75 inch steel pipe and were rectangular in shape. The yield was 59.31 kg/m2/6 months and suvival rate was 94.50%. The average body weight of the final fish were about 625 gm. and food conversion ratio was about 4.6:1.
