NETCAGE CULTURE OF SEABASS (Lates calcarifer) IN THAILAND

Wichian Yongprapat ¹

I. Introduction

Lates calcarifer (Bloch), commonly called seabass or giant sea perch, has been cultured in Southeast Asia for more than ten years, in marine, brackish, and fresh waters. Cage culture of seabass in coastal waters is popular in Thailand, Hong Kong, Taiwan, Malaysia, Singapore and Indonesia. The method is simple and highly profitable when compared to pond culture. Thus, cage culture of seabass has expanded rapidly in the last five years particularly in Thailand. However, the major constraints to rapid expansion, are disease outbreaks during the culture period and insufficient supply of trash fish.
Despite some imperfections, the basic technique of seabass cage culture has been developed and now considered economically viable.

II. Site selection for fish culture

The criteria for selecting a suitable site for seabass cage culture are the following:

1. Water salinity. This 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 standard size of the cages which are 5 m × 5 m and 2 m deep. The tidal range should allow the water depth to be at least 2 meters at the low water of spring tide.

3. Current and waves. The 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 waste and other environmental hazards.

5. Water circulation. The site should have an adequate water circulation to improve on poor water quality that could occur at certain times due to the decomposition of waste material, which often accumulate under the netcages.

¹ Satul Brackishwater Fisheries Station, Brackishwater Division, Department of Fisheries, Bangkok, Thailand

The minimum ranges permitted for some water quality parameters for cage culture of seabass, are shown in Table 1.

III. Nursery

Seabass fry and fingerlings are reared in concrete tanks up to the size of 2.5 cm. After that, they are transferred to nylon net cages until they attain 25 cm. This size is reached in 2 to 3 months.
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 carboy 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 (Fig. 1). The mesh size of the nylon net is 1.0 mm. However, after a month of nursing, they can be transferred to netcages with a mesh size of 0.5 cm. This allows water to pass through the cages more freely.
The stocking density is approximately 1,000 fingerlings per cage. Grading of fingerlings is done at least once a week during the nursery period. Stocking is done separately for each size group. This tends to minimize losses from cannibalism. Fingerlings of 2.5–5 cm are fed daily with ground trash fish at 8–10 % of body weight, 4 to 5 times a day.
The netcages should be checked daily to ensure that they are not damaged by crabs or clogged with fouling organisms. The cages are cleaned every other day by soft brushing in order to allow water circulation in the cage. The survival rate for the nursery period is usually 50–80 % . This depends on feeding, aquatic environmental conditions, and on the experience of the fish farmer.

IV. Rearing market-size 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 highly demanded by the local market and consumers in neighboring countries.

Figure 1. Nylon netcage for nursing seabass fingerlings.

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

a. Floating Cages

The netcages are hung from GI pipe, wooden or bamboo frames. The most convenient dimension for a cage is that of a rectangle and a volume of 50 m³ (5.5m × 6m × 2m). The cage unit is stabilized with concrete weights at each bottom corner (Fig. 2). The cage unit is anchored to the bottom. The cages may rock a little in strong wind and current. Floating cages can be set on coastal waters where tidal fluctuation is ample.

b. Stationary Cages

This type of cage is fastened to wooden poles at its four corners (Fig. 3). Stationary cages are usually set in shallow bays where the tidal fluctuation is narrow.
The mesh size of the nylon nets depends on the size of fish. Fingerlings are transferred to a nylon net (mesh size of 2.0 cm) for about 2 months, then moved to a cage net of 4.0 cm mesh size until harvest.
Stocking density for marketable fish culture varies from 100–300 m³ (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 water circulation and distance from sources of pollution.

Table 2. Growth of seabass at different stocking densities in cages (after Sakares, 1986).

Figure 2. Floating netcage.

Figure 3. Stationary netcage.

Trash fish is the main feed for cultured seabass. Trash fish should be fresh and clean. Trash fish used in Thailand are sardines and other small marine fish. The trash fish is 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 give the feed slowly and watch the fish. Feeding is stopped when the fish no longer come up to the surface as it indicates that the amount of feed given is enough.
Food conversion rates of seabass culture in Thailand decreases with stocking density. It ranges from 3–10. It also depends on the quality and quantity of trash fish. Normally, seabass can grow at an average weight increase of 1 kg/yr.
Survival rates to marketable fish are around 80–95 % in normal culture conditions.
The cages are checked every month or once in two months to ensure that they are not damaged by fouling organisms, crabs or flotsam. The cages are cleaned or changed every month. Therefore, fish farmers usually have spare nylon netcages. Changing cages also allows the farmer to check on the number and health of fish.

V. Financial Analysis for Small-Scale Fish Culture

Types of cages can be divided into two:

1. standard, which is made of GI pipe frames, and

2. 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 3 and 4).
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 realise a benefit of about 3,849 to 6,325 baht per cage per year (Table 5).

Table 3. Cost of investment for standard floating cages (5 × 5 × 2m).

Table 4. Cost of investment for ordinary floating cages (5 × 5 × 2m).

APPENDIX 1.

Coastal aquaculture in Thailand, 1985 (Thai Fishery census of Thailand).

¹ 1 Rai = 1,600 m^2
2 280 Rai = 17,920 caged (5 × 5 × 2m)

APPENDIX 2.

APPENDIX 3.

Production of fish culture, 1981–1985 in MT (Thai Fisheries Statistics).

APPENDIX 4.

³ = Sale to provinces other than Bangkok

APPENDIX 5.

APPENDIX 6.

REFERENCES

Department of Fisheries. 1987. Fisheries Record of Thailand 1985. No 4/1987. Fish. Stat. Subdivision, Dept of Fish. 94 p.

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

National Statistical Office and Department of Fisheries. 1987 and 1985 Marine fishery census of Thailand. 329p.

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

Sirikul, B. 1982. Aquaculture for seabass in Thailand, SCS/GEN/82/39, UNDP, 9–10p.

Sungkasem, P. 1982. The economics of seabass production, SCS/GEN/82/39, UNDP, 53–57p.

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

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