CHAPTER VIII
CAGE CULTURE OF SEABASS (LATES CALCARIFER) IN THAILAND

Siri Tookwinas*
Boonchu Charearnrid**

Lates calcalifer (Bloch) is commonly called the seabass or giant sea perch. It has been cultured in Southeast Asia for more than ten years, in marine, brackish, and fresh waters. The cage culture of seabass in coastal waters is the most popular in Thailand, Hongkong, Taiwan, Malaysia, Singapore and Indonesia. The method is simple and highly profitable compared to pond culture. Thus, the cage culture of seabass has expanded very rapidly in the last five years in Thailand. However, the major constraints to rapid expansion, are the disease outbreaks during culture period and insufficuent supply of trash fish.

Despite some imperfections, the basic techniques of seabass cage culture have been developed and now considered economically viable.

1. Selecting a Suitable Site for Cage Culture

Criteria for selecting a suitable site for cage culture of seabass are the following:

1. Water salinity (which should range from 10–31 ppt).

2. Tide and water depth. Water depth should be more than 2–3 meters. This is due to the usual size of culture cage which is 5 m × 5 m and 2 m deep. The tidal fluctuuation should allow the water depth to be at least 2 meters at the low water of spring tide.

3. Current and waves. Area should be protected from strong winds, waves and current. An ideal area would be in protected bays, sheltered coves and inland sea.

4. Water quality. The site should be relatively free from domestic, industrial and agricultural wastes and other environmental hazards.

5. Water circulation. The site should have enough water circulation to improve on poor water quality that could occur at some period in the culture due to the decomposition of waste material which often accummulate at the bottom under the net cage.

* Head, Satul Brackishwater Fisheries Station.
** Senior Fishery Biologist, Satul Brackishwater Fisheries Station.

The water quality parameters which are considered of minimum range for cage culture of seabass are shown in Table 1.

2. Nursery

Seabass fry and fingerlings should be reared in concrete tanks up to the size 2.5 cms or 1 inch. After that, they can be transferred for rearing in nylon net cages until they attain 25 cm or 10 inches in about 2 to 3 months of culture period.

The most convenient cage design is a rectangular cage made of synthetic netting attached to wooden, GI pipe or bamboo frames. It is either a) kept afloat by styrofoam, plastic carbuoy or b) stationary by fastening to a wooden or bamboo pole at each corner. The size of cage varies from 0.9 × 2.0 m and a depth of 0.9 m to 1.0 × 2.0 meters and a depth of 1.0 meter (Figure 1). The mesh size of the nylon net is 1.0 mm. However, after a month of nursing, they can be transferred to cages with nylon net with mesh size of 0.5 cm. This would allow water to pass through the cages more freely.

The stocking density is approximately 1,000 fingerlings per cage. Grading of fingerlings has to be done at least once a week during the nursery period. Stocking is done separately for each size group. This would minimize the losses from cannibalism. Fingerlings of 2.5–5.0 cms should be fed with ground trash fish at 8–10 percent of body weight daily or about 4 to 5 times a day. After that, they can be fed with finely chopped trash fish.

Figure 1. Nylon net cage for nursing seabass fingerlings.

The net cage should be checked daily to ensure that it is not damaged by crabs or clogged with fouling organisms. The cage should be cleaned every other day by soft brushing in order to allow water circulation in the cage.

The survival rate for the nursery period would be 50 to 80 percent. This would depend on feeding, aquatic environmental conditions, and the expertise of the fish farmers.

3. Rearing Marketable Fish

Fish are reared from juvenile to marketable size for another 5 to 20 months. The marketable size requirements of the seabass are between 700–900 g and 2,000–3,000 g. However, the size between 700–900 g is demanded by the local market and consumers in neighboring countries.

There are two types of cages used in seabass culture in Thailand:

a. Floating Cages

The net cages are hung on GI pipe, wooden or bamboo frames. The cage is kept afloat by styrofoam drum, plastic carbuoy or bamboo. The most convenient dimension for a cage is that of a rectangle and a volume of 50 cubic meters (5.5 m × 6 m × 2 m). The cage unit is stabilized with concrete weights at each bottom corner (Figure 2). The cage unit has to be anchored to the bottom. The cages might be rocked a little by strong wind and current. Floating cages can be set on coastal waters where tidal fluctuation is wide.

Figure 2. Floating cage.

b. Stationary Cages

This type is fastened to wooden poles installed at its four corners (Figure 3). Stationary cages are usually set in shallow bays where the tidal fluctuation is narrow.

The mesh size of nylon net would depend on the size of fish as shown in Table 2. Firstly, finglerlings should be transferred to a nylon net (mesh size of 2.0 cm) for about 2 months of culture period. Them they are moved to a cage net of 4.0 cm mesh size until harvest.

Figure 3. Stationary cage.

Stocking density for marketable fish culture varies from 12 sq. meters to 300 sq. meters (Table 3), depending on water quality and the environmental conditions of the culture site. Floating cages can be stocked with more than stationary cages. This is because floating cages are usually set in sites with better aquatic environmental condition such as deeper water, narrower fluctuation of water salinity, more rapid circulation and further away from sources of pollution.

Trash fish is the main feed for seabass culture. Trash fish should be fresh and clean. Trash fish used in Thailand are sardines and other small marine fish. The trash fish should be chopped and fed twice a day, in the morning and afternoon. The size must be suitable for the size of the mouth of the fish. The farmers should give the feed slowly and watch the fish. Feeding should be stopped when the fish no longer come up to the surface; it shows that the amount of feed is enough for them.

Food conversion rate of seabass culture in Thailand decreases with stocking density. They range from 3.0 to 10., 0. It also depends on the quality and quantity of trash fish. Normally, seabass can grow at an average of 1 kg/yr.

Survival rates for marketable fish culture would be about 80–95 percent in normal culture conditions.

The cages should be checked bimonthly or monthly to ensure that they are not damaged by fouling organisms, crabs or flotsam. The cages should be cleaned or changed every month. Therefore, fish farmers should have spare nylon net cages. Changing cages also allows the farmer to check on the number and health of the fish.

4. Financial Analysis for Small-Scale Fish Culture

Types of cages can be divided into two: a) standard which is made of GI pipe frames and b) ordinary which makes use of wooden frames. The total cost is 12,700 baht per cage of the standard type, and 4,400 baht per cage of the ordinary type. The cost per year is 4,051 for the standard type and 1,575 baht for the ordinary type (Tables 4–5).

A floating cage can culture 1,000 fish. The cost would be about 14,675 to 17,151 baht. The production or yield is about 350 kg/yr

Thus, the fish farmer can realize a benefit of about 3,849 to 6,325 baht per cage per year (Table 6).

REFERENCES

Kungvankij, P. et. al. 1986. Biology and culture of seabass (Lates calcarifer), NACA Training Manual Series No. 3, NACA/RLCP. Bangkok, 70 p.

Sakaras, W. 1986. Optimum stocking density of seabass (Lates calcarifer) culture in cages. ACIAR Proceedings No. 20. 172–175pp. Canberra Printing Co. Melbourne.

Tookwinas, S. et. al. 1987. Cage culture of brackishwater fish in Satul Province. Technical Paper, Brackishwater Fisheries Division, Dept. of Fisheries. 30p.

Tookwinas, S. 1985. Consideration aspects for coastal aquaculture survey. Thai Fisheries Gazette. 38(4): 243–249.