by
W.L. Chan, B. Tiensongrusmee, S. Pontjoprawiro and I. Soedjarwo1)
1. INTRODUCTION
The farming of sea fish is not a new concept in Indonesia, where for centuries the culture of the mikfish in brackishwater pond has been known. Also for centuries marine fishermen have been keeping their catches live in bamboo containers as a means of preservation before the fish could be sold. The culture of seabass and grouper in floating netcages in the sea can thus be said to have been derived from such traditional practice. This has however received world-wide recognition in recent years as a means to increase fish protein on the one hand and to create need job opportunities on the other.
Seabass and grouper are selected species that are suitable to culture in inpoundment. Seeds or juveniles are available either from the wild stock or hatcheries through induced spawning. The fish have a commercial value with market demand fetching consistently high market values throughout the year. They have a relatively high average daily weight increment, hardy, capable of withstanding handling and other stresses in impoundment.
2. CULTURE SYSTEM
The species can be reared in floating and bottom netcages and ponds.
3. FACILITIES FOR CULTURE
3.1 Floating culture system
a. Raft
The function of the raft is to provide a frame-work for the rigging of the netcage and catwalk; and it is floated with floats providing a working platform of sufficient stability in the water. Stability is important as it prevents escapement of the fish and the loss of the facility. It may also be an indirect factor preventing the caged stock from being frightened thereby injuring their bodies and subsequent contracting disease infection.
In marine and brackish water environment, materials suitable for raft construction are wood and bamboo. There are many kinds of wood having high resistance to seawater and marine borers. Changai Pasir, Hopea odorata, is found to be an ideal material for the construction of rafts. This wood could last more than four years if it is regularly maintained and coated at least once a year with antifouling paints.
Bamboo could also be used for the construction of raft for calm water area. The cost of bamboo is normally much less than that of wood, but the bamboo poles have to be replaced once a year to ensure the security of the fish stock.
1) FAO/UNDP Seafarming Development Project INS/81/008
b. Float
Floats provide bouyancy to the raft and therefore also the netcage. The types of floats vary from bamboo, metal drum, ruber tyre filled with styrofoam, ferrocement box, and used fibreglass drum. The size and number of floats required for sufficient floatation of the raft an cage depend on the weight of the raft, netcage and people working on the raft.
c. Netcage
The netcage can be made of polyethylene netting having a thickness of 18 to 24 ply. Thicker netting is strongly recommended for netcage so that it could be more resistant to the potential damage of crabs or pufferfishes.
Nylon netting for cages is not recommended for tropical conditions because the exposed portion of the netting becomes brittle from long exposure to the sun. Polyethylene netting may have a life span of more than three years if properly maintained.
Galvanized and lead-coated wire netting could also be used for cage construction. However, it may have to be replaced after 1 – 2 years; it is also rather clumpsy to handle as it is rigid.
Plastic webbing could also be used where it is economical as regards production cost and durability.
d. Design
The design of floating netcages is determined by a number of parameters such as the nature of its operation. The size of a unit netcage depends on the cost of materials, the size of available materials, the purpose of the netcage, and other management considerations. Nursery activities normally require smaller size of netcage; whereas grow-out activities normally prefer larger size units. Smaller netcage can be more easily managed when compared with the bigger one. It is not economical to design a cage physically not manageable by the farmer alone in small-scale operation. In Malaysia the size of netcage of 3 – 11 m2 surface area are recommended for a family unit; and standard size of netcage of 1.80 m × 2.10 m × 1.50 m is recommended for a pilot project.
In Indonesia, the netcage size of 3.00 × 3.00 × 3.00 m is on trials. Whether such a size is economically suitable for the local conditions should be determined.
The shape of cages can be circular, hexagonal, rectangular or square. The shape of cages is normally determined by the behaviour of species, ease of construction and cost effect. For free-swimming species like the jack, yellow-tail and milkfish bigger space is required. Such fishes tend to aggregate in shoal and swim in a circular pattern. Therefore, circular or hexagonal cages may be more suitable than rectangular or square cages for such fishes. For demersal species such as groupers, seabass, snappers and others, which are less active, the shape of cages does not normally affect the survival of the fish. In the circumstances, square or rectangular cages have an advantage over circular or hexagonal ones in enabling greater ease of construction and management; whereas circular or hexagonal cages need special skills in construction and also expensive to build.
e. Rigging a cage
A properly constructed netcage should have an opening whose size, without stretching, should fit into the compartment in the raft for the accommodation of a netcage. In attaching the headrope of the netcage should rest on the inner material of the compartment, to which it is attached by threading a nylon rope. This makes the netcage stand out of the water and up to level with the raft. There should be no slag of the headrope.
It is advisable to provide a cover net to the cage to avoid escapement, poaching and many other unnecessary happening. A netcage must then be distended to maintain a full cube shape or shape of other design. For shapes having right angle corners, a frame of galvanised pipe with or without welding should be attached to the outer part of cage bottom. At each angle a rope is tied to each bottom cage corner and attached the corner of the compartment of the cage.
Other methods of distention are used. For strong current areas, netcage can be built around galvanised pipe frame. This is the purpose also of using galvanised wire netting. In normal areas, a rock hang down separate from the raft and attached to the bottom angle of a cage is also commonly used.
There are numerous other alternatives, but the main criteria are to be practical and functional whatever the rigging may be.
f. Positioning and anchoring
Floating netcages can be positioned by wooden stake, pole, sand bag, iron anchor or concrete slab depending on the depth of the water, the nature of the sea floor, tides, currents, and size of the cage. Wooden stake normally works in muddy sea floor. Concrete slabs of different sizes and shapes, sand bags and iron anchors also work well both over sandy and muddy substrates and practiced in many countries in Southeast Asia. Concave concrete slabs are also practical on muddy bottom. The size and number of anchores required to position a cage depends on the size of the cage and also aforementioned factors.
The mooring system should be strong enough to hold the rafts and to maintain the floating facilities in position in all weather condition. It is recommended that a raft be allowed to drift to a certain limit, when it is exposed to strong wind and current. In this case the original site should be marked so that the raft can be drawn back to it as required.
Since the conventional metal anchors are too expensive, the use of wood stakes with a thick rope passing through a hole of 10 – 20 cm below the cut end of the stake are more economical and practical. With a guide pole and applying simple pulley system, the stake can be driven right into the bottom soil by utilizing the weight of the boat. The rope is then attached to a float as a marker. It must be warned that any fixedly positioned device unable to provide a dragging force and to allow dredging of the raft, is extremely dangerous for waters having wide tidal range and typhoon weather conditions.
3.1 Bottom culture system
This system of culture can be divided into two categories, set netcage and netpen culture.
a. Set netcage
This system is suitable for the rearing of seabass and grouper particularly in shallow coastal waters and mengrove areas. It is practiced in Thailand. Netcages are made of nylon net in square or rectangular shape. The dimension and mesh size of the netcage depend on the number and size of fish to be cultured. Netcages of 2.0 × 1.0 × 10. m with a 0.2 cm mesh are designed for the nursery of the seabass and grouper fry typecally 1.5 – 2.0 cm; whereas netcages of 2.0 × 4.0 × 2.0 m with 1.5 cm mesh size of nylon twine No. 6, are used for the rearing of juvenile. For growout netcages, the sizes varies from 5 – 10 m in length 5 – 10 m in width, and 3 m in depth. Mesh size used for growout netcages is 5.0 – 7.5 cm made of nylon twine No. 15 or No. 18. The netcages are tied to bamboo poles driven into the bottom at a depth of 1 – 2 m. In this system the net bottom tend to be near the ground or set on the seafloor. This culture system has disadvantages in that organic matters are trapped in the substrates, and when released could cause oxygen depletion and in serious cases eutrophication.
b. Netpen
This system refers to a type of culuture unit where one part of the natural water body is enclosed by netting resting on the bottom. The shape can be circular, rectangular or square. For ease of operation and management, rectangular shape is recommended. The netting materials can be kuralon or nylon. The net is tied inside of the enclosure. Areas of enclosure can range from 1 to 5 ha with a water depth or 2 – 3 m. The poles are staked in mud at depths of 0.5 – 1.0 m or more depending on the substratum. In the Philippines this system of culuture is widely used despite encounterance enormous damages to structures and stocks by typhoons.
c. Pond culture system
Like the milkfish, the seabass and grouper can also be reared in earth ponds. The pond requirements followed the same pattern as those coastal ponds. The size of rearing pond can range from 0.5 to 3.0 ha with a water depth of 1.20 – 1.50 m. The pond will have better exchange system if there are two sluice gates. One serves as an inlet with the other in an opposite direction as the outlet. The water control structures can be made of concrete or wood. It can be an ordinary open flume structure as those used in shrimp and milkfish pond with a netting screen or monk type system or as simple as concrete culverts with control valves for regulation of water flow.
4. CULTURE TECHNIQUES
Culture of the seabass and grouper can be divided into two phases, nursery and grow-out phase.
4.1 Nursery phase
There are two types of nursing phase. One is nursing from fingerling to juvenile, another one is nursing juvenile prior to releasing in the grow-out netcages or ponds.
a. Nursery from fry to fingerlings
Most of hatcheries distribute seabass and grouper fry to fish farmers at the size 1.5 – 2.0 cm in total length at the age of 35 – 45 days. This size of fry needs further nursery care in pond or nursery netcage until they reach a size of 7.5 – 10.0 cm in length in 3 – 4 month. However if the seeds are collected from the wild stock, their size tends to be bigger and more uneven. Normally the wild fry collected from natural grounds have a size range between 5.0 – 15.0 cm. In that case the fish can be transferred directly into the grow-out netcages after acclimatization to water conditions within impoundment environments.
Nursing of seabass and grouper fry can be done either in floating netcages, set netcages or in nursery ponds. The fry size between 1.5 – 2.0 cm can be stocked at 150 – 250 fry/m3. The fry after acquiring an average size of about 5 cm are stocked at densities of 30 – 50 pcs/m3 During the nursing phase the fish will accept a minced fish diet. If formulated feed is given, it needs time for the fish to accept the feed, especially fish fry obtained from hatcheries. It will then be necessary to wean the fry of their hatchery diet as soon as possible. For minced trash fish about 8 – 10 percent body weight is given per day. Grading need to be done twice a week to remove the fast growing from slow growing fish. Grading is to promote average growth of the fry stock and to reduce losses through cannibalism. The choice of grader is also important to avoid further losses (Chan, 1982).
b. Nursery of juvenile prior to releasing into grow-out netcage and ponds.
This is to condition wild and hatchery fingerlings for stocking. Involving fingerlings between 7.50 – 10.0 cm. This standard practice is to acclimatize the fingerlings to the new environments (such as salinity fluctuations, new type of feed) and to strengthen individuals through quarantine and the sorting or removal of the injured fish through handling. This period of nursing requires about 1 – 4 weeks depending on the sources and condition of fry. The stocking density practised is between 15 – 60 juveniles/m3. After the fish show normal behaviour in swimming, readily accepting new type of feed and free from deseases, the fish can then be transferred into the grow-out netcages and ponds for further growth.
4.2 Grow-out phase
This phase of rearing is to grow the fish from juvenile to marketable size. The period
of rearing ranges from 6 to 12 months depending on the optimal value specific
to the market concerned. In Malaysia and Singapore, the marketable size ranges
between 600 – 800 g, Thailand and Indonesia prefer the bigger fish, 800 – 1000
g. In the netcage the stocking density of grouper can range from 15 to 60 fish/m3
depending on tidal range, current, turbidity, water depth, water quality, feed,
management skill, and techniques of stock manipulation.
In earthpond stocking of grouper of 3 – 5 fish/m2 is accepted. The production of
grouper in netcages varies from 3 – 40 kg/m3/yr depending on quality of feed given,
water quality and culture skill.
Seabass can also be raised in netcages and earth pond at the density implied to the grouper. In earth pond, at stocking density of 3 fish/m2, yields ranging from 3,250 – 3.750 kg/ha/yr can be obtained (Sirikul, 1982).
From the field trials in Indonesia situation, it appears that both seabass and grouper can attain a growth rate of 3 – 5 g/day if good feed and proper management are provided. The main constraint in seabass and grouper culture in netcages and ponds in Indonesia is very difficult to obtain good trashfish at low price. Other alternative feeds such formulated feed both semi-moist and pelletize, and mussel are being tested. As far as the natural feed concerned, the green mussel seems to be a potential one. Besides it is good natural feed, fresh and containing high protein, the price is also relatively lower than trashfish. Grave et al. (1979) showed that trout fed with the blue mussel (Mytilus edulis) sustained a better growth rate than those provided with ordinary commercial trout pellet.
5. DISCUSSION
From the aforementioned discussion, it can be seen that the rearing of seabass and grouper is promising. However the level of perfomance depends on management skills and experience of the farm operator. The design of production programmes also requires an understanding of the characteristics of the market, since the production cost for differring annual gross yield planning differs significantly for different market. Summary of different production programmes of the grow-out of seabass and grouper can be found in WBL/85/WP-6.
LITERATURE CITED
Grave, H, A. Schultz, and R. Van Thielen. 1979 The influence of blue mussel, Mytilus edulis and krill Euphausia on growth and proximate composition of rain bow trout Salmo gairdneri. Pro. World Symp. on Finfish Nutrition and Fish Feed Technology. Hamburg 20 – 23 June 1978 Vol. 1 : 575–586.
Chan, W.L. 1982 Management of the nursery of seabass fry. In: Report of training course on seabass spawning and larval rearing Songkhla, Thailand, 1 – 20 June 1982 SCS/GEN/82/39:34 – 37.
Sirikul, 1982 B. Aquaculture for seabass in Thailand. In: Report of training course on seabass spawning and larval rearing Songkhla, Thailand, 1 – 20 June 1982. SCS/GEN/82/39:9 – 10.
