Seabass has been commercially cultivated in brackishwater and freshwater ponds and marine cages in many Southeast Asian countries. While the cagae culture technology is now established, grow-out techniques in pond are still are still in the developmental stages. Although considerable progress has been made over the past ten years, many problems remained unsolved.
The major problems that are always encountered during culture period are: (a) cannibalism during young stage (1–20 g), (b) dependence on trash fish as a main diet which has a very limited supply in many countries.
Despite some imperfections, the basic techniques of seabass culture are now developed and have been considered economically viable.
As mentioned above, cannibalism is one of the most serious problems in seabass culture. High mortality is often encountered when uneven sizes of the fish are stocked. This has been noted to occur mostly where the fish are very young (1–20 cm in length, the first two months of culture). To minimize this problem, culture of seabass should be approached in two phases i.e. the nursery phase and the grow-out phase.
The main purpose of the nursery is to culture the fry from hatchery (1–2.5 cm in size) to juvenile size (8–10 cm). This can solve the problem of space competition in the nursery tanks. Beyond the nursing period, the juveniles can be graded into different size groups and stocked in separate grow-out ponds. It has been observed that the juveniles from the nurseries perform better in terms of growth and survival than those stocked directly into the grow-out ponds.
Nursing the fry in concrete tanks is not recommended as accumulation of excess feed on the bottom of the tank cannot be avoided. Such accumulation can cause bacterial disease. In addition, constant contact with the tank wall results in wounded fish and subsequent bacterial infection
Nursery pond size ranges from 500 to 2000 m2 with water depth of 50–80 cm. The pond has separate inlet and outlet gates to facilitate water exchange. Pond bottom should be flat and sloping towards the harvesting or drainage gate. Inlet and outlet gates are provided with a fine screen (1 mm mesh size) to prevent predators and competitors from entering and fry from escaping the pond.
Fry ranging from 1–2.5 cm are suitable for stocking in the nursery ponds. Stocking density is between 20–50 individuals per square meter.
A wellprepared pond is important as predators and competitors can endanger the stocked fry.
Some farmers still practice very crude farming techniques of drying the pond bottom and immediately filling with water and stocking fry directly for nursing. Feeding is entirely dependent on supplementary feed such as chopped or grounded trash fish and is done twice daily in the morning (1800 hours) and afternoon (1700 hours). In this method, the survival rate and growth rate are low.
To enhance production, the following improved pond preparation techniques are done: The nursery pond must be drained and dried until the bottom soil cracks to release toxic gases, oxidize mineralized nutrients, eradicate some pests and predators. In cases where the pond cannot be completely drained, derris root (rotinone) may be applied at the rate of 20 kg/ha toeradicate unwanted species. Derris root is prepared by cutting them into small pieces, crushing and soaking in water overnight. Only the solution is applied to the pond. If derris root is not available, a mixture of 50 kg/ha of ammonium sulfate (21-0-0) with lime at a ratio of 1:50 will be sufficient to weed out unwanted species. The mixture is applied to the portions of pond with water. The use of any chemicals or inorganic pesticides is not recommended because the residual effect remains for many years and can reduce the pond production. If pond soil is acidic, the pond bottom should be neutralized with lime before letting the water in.
Production techniques of juvenile in nursery ponds have been improved recently at Satul Fishery Station, Thailand. The improved technique is based on the live food production in the pond supplemented with chopped or grounded trash fish. After neutralizing pond bottom by liming, organic fertilizer (chicken manure) is applied at the rate of 500 kg/ha. Then water depth is gradually increased for the propagation of natural food. Two to three weeks prior to stocking, newly-hatched Artemia nauplii are inoculated into the pond (1 kg of dry cyst/ha). Artemia will utilize the natural food as feed for growth and will reach adult stage within 10–14 days. The fry are immediately stocked at the rate of 20–50 individual per square meter.
Another approach to the improved technique is to stock Artemia nauplii in the separate pond and grow them into adult. Adults could be harvested daily to feed the fry.
Although seabass can be cultured in either freshwater or saltwater, fry must be acclimatized to the salinity and temperature prevailing in the pond on stocking to prevent loss.
Acclimatization is done in the following manner: transfer the fry to a tank, then gradually add nursery pond water. This can be completed within one day or more depending on the salinity difference. If the temperature and salinity in transport bag does not differ by more than 5°C and 5 ppt with the pond water, acclimation can be done by floating the bag in the pond for sometime to even out temperature difference. Pond water is then added gradually until both salinity become equal and the fry can be released.
Seabass fry are stocked in the nursery pond at a density of 20–50 fry/m2. Stocking is usually done in the erly morning (0600–0900 hours) or early evening (2000–2200 hours) when the temperature is cooler.
Water replenishment is needed to prevent deterioration of pond water quality due to the decomposition of uneaten feed or excess growth of natural food. Normally, 30% of pond water is changed daily.
Supplementary feed is given daily. The feed used for nursing seabass is chopped and grounded (4–6 mm3) trash fish, normally at the rate of 100% of biomass given twice daily in the first week (at 0900–1700 hours), gradually reduced to 60% for the second week and 40% in the third week. This has been found to be most effective feeding strategy for ponds without artemia inoculation.
The application of supplementary feed is a vital operational activity that should be done properly, if not, contamination of culture water and wastage of feeds result. Although the seabass in nature prefer live food, the fish can be trained to feed on dead animal. Prior to feeding, the fish should be attracted by sound (such as tapping a bamboo pole in the water) to induce them to form a school. Feeding time and place should be fixed. After the fish have formed a school, small amounts of feed are introduced by spreading into the water within the school of fish fry. It must be remembered that seabass never eat the feed when it sinks to the pond bottom. Therefore, feeding should be slow. When the fish are filled to satiation, they disappear thus feeding should be stopped. The same procedure should be followed at every feeding time. The first few days after stocking, feeding should be 5 to 6 times a day to teach them to accept dead feed. Once the fish is accustomed to it which takes about 5–7 days, feeding frequency is reduced to twice daily. In nurseries where Artemia is the main diet, once the Artemia population has thinned down, chopped or grounded trash fish can be supplemented using above described practice.
The nursing period lasts about 30–45 days until fingerling stage (size 5–10 cm). At this stage, they are ready for transfer to grow-out ponds.
Nursing of seabass fry (1–2.5 cm to 8–10 cm) in cages is an approach to the nursery phase. The method has been successful since conducive environmental conditions such as flow through water, necessary for good health and growth of fish are used. It is likewise easy to maintain and require very little capital investment.
The most convenient cage design is a rectangular cage made of synthetic netting attached to wooden frames. It is either (a) kept afloat by styrofoam, plastic or metal drum, or (b) stationary by fastening to a bamboo or wooden pole at each corner. The size of cages vary from 3 cubic meter (3 × 1 × 1m) to 10 cubic meters (5 × 2 × 1m). The mesh size of the net used for nursery cages is 1.0 mm. The cages may be installed in the river, coastal area or in a pond. Suitable sites for net cages should be free from biofoulers since the mesh size of a nursery cage is very small. Cages are easily damaged in strong currents and clogging by biofoulers. (Fig. 20)
Seabass fry (1–2.5 cm in size) are stocked in the nursery cage at the rate of 80–100 per square meter.
Stocking and feeding activity are the same as in nursery pond culture practice.
The net cages should be checked daily to ensure that the cages are not damaged by animals such as crabs or clogged with fouling organisms. Cleaning of the cages should be done every other day by brushing. This will allow water to pass through the cages naturally.
Fig. 20 Floating nursery cage for seabass.
After the nursing period of 30–45 days (in pond or cages) or when the fry have reached 5–10 gm, these are ready for transfer to grow-out ponds. Prior to stocking in grow-out ponds, grading procedure should be applied. Fish are graded into several sizes. It will give maximum advantage if the various sizes are stocked in separate ponds to prevent cannibalism.
The grow-out phase involves the rearing of the seabass from juvenile to marketable size. Marketable size requirement of seabass vary country to country e.g. Malaysia, Thailand, Hong Kong and Singapore. The normally accepted marketable size of seabass among these countries and region is between 700–1200 g while in the Philippines, marketable size is between 300–400 g. The culture period in grow-out phase also vary from 3–4 months (to produce 300–400) to 8–12 months.
Cage culture of seabass is quite well developed in Thailand, Malaysia, Indonesia, Hong Kong and Singapore. The success of marine cage culture of seabass and its economical viability have contributed significantly to large scale development of this aquaculture system
2.1 Suitable site for cage culture
Criteria for selecting a suitable site for cage culture of seabass include:
Protection from strong wind and waves. The cage culture site should preferably be located in protected bays, lagoons, sheltered coves or inland sea.
Water circulation. The site should preferably be located in an area where influenc of tidal fluctuation is not pronounced. Avoid installing cages where the current velocity is strong.
Salinity. Suitable site for seabass culture should have a salinity ranging from 13–30 ppt.
Biofouling. The site should be far from the area where biofoulers abound.
Water quality. The site should be far from the sources of domestic, industrial and agricultural pollution and other environmental hazards.
In general, square and rectangular cages with size varying from 20 to 100 m3 are preferable because they are easy to construct, manage and maintain. Seabass cages usually are made of polyethelene netting with the mesh size ranging from 2 to 8 cm. The choice of mesh size depends on the size of the fish (Table 8).
There are two types of cages used in seabass culture:
The net cages are attached to wooden, GI pipe or bamboo frames. The cage is kept afloat by floating material such as metal, plastic, styrofoam drum or bamboo. The shape of the cage is maintained with the use of concrete weights attached to the corners of the cage bottom (Fig. 22). The most manageable size for a floating cage is 50 m3 (5 × 5 × 2m). This cage dimension is easy to change when clogged with fouling organisms.
The cage is fastened to the bamboo or wooden poles installed at its four corners (Fig. 21). Stationary cages are popularly used in shallow bays since they are easy to install.
Prior to stocking seabass juvenile in cages, fish should be acclimatized to the ambient temperature and salinity prevailing in the cages. The fish should be graded into several size groups and stocked in separate cages. The stocking time should be done in the early mornings (0600–0800 hours) or late in the evening (2000–2200 hours) when the temperature is cooler.
Stocking density in cages is usually between 40–50 fish per cubic meter. Two to three months thereafter, when the fish have attained a weight between 150–200 g, the stocking density should be reduced to 10–20 fish per cubic meter. Table 9 shows the growth of seabass under varying densities in cages. There should be spare cages as these are necessary for transfer of stock and to effect immediate change of net in the previously stocked cage once it has become clogged with fouling organisms. Changing cages allows for grading and controlling stock density.
Feed is the major constraint confronting the seabass culture industry. At present, trash fish is the only known feed stuff used in seabass culture. Chopped trash fish are given twice daily in the morning at 0800 hours and afternoon at 1700 hours at the overall rate of 10% of total biomass in the first two months of culture. After two months, feeding is reduced to once daily and given in the afternoon at the rate of 5% of the total biomass. Food should be given only when the fish swim near the surface to eat.
|mesh size||size of fish|
|0.5 cm||1–2 cm|
|1 cm||5–10 cm|
|2 cm||20–30 cm|
|4 cm||bigger than 25 cm|
Table 8. The choice of netting mesh size of fish.
FIG. 21 STATIONARY CAGES
FIG. 22 FLOATING CAGES
Table 9. Monthly growth of seabass at different stocking densities in cages.
|Culture period (month)||Stocking density|
|0||67.80 g||67.80 g||67.80 g|
|1||132.33 g||137.53 g||139.20 g|
|2||225.20 g||229.10 g||225.50 g|
|3||262.88 g||267.50 g||264.11 g|
|4||326.15 g||331.97 g||311.50 g|
|5||381.08 g||384.87 g||358.77 g|
|6||498.55 g||487.06 g||455.40 g|
Since the supply of trash fish is insufficient and expensive in some countries, its use is minimized by mixing rice bran or broken rice to the trash fish (Table 10). However, even with these cost cutting measures, feed cost remain quite high.
A very recent development on improving the dietary intake of seabass is the introduction of moist feed. So far, the use is still on experimental stage. The feed composition recommended is presented at Table 11.
Regular observation of cages is required. Since fish cages are immersed under water all the time, they are vulnerable to destruction by aquatic animals such as crabs, otter, etc. If damaged, they should be repaired immediately or replaced with a new one.
In addition to biofouling, the net walls of cages are subjected to siltation and clogging. Biofouling is unavoidable since the net walls usually represent a convenient surface for attachment by organisms such as amphipod, polycheate, barnacles, molluscan spats, etc. These could lead to clogging and reduce exchange of water and may result in unnecessary stress to the cultured fish due to low oxygen and accumulation of wastes. Feeding and growth would likewise be affected.
To date, mechanical cleaning of fouled nets is still the most efficient and cheap method. In areas where fouling organisms are abundant, rotational usage of net cage is highly recommended.
Although methods of pond culture of seabass have been practiced for over 20 years in Southeast Asia and Australia, not much has been done on the commercial scale. At present, culture of seabass in brackishwater pond has been identified in some countries as having tremendous market potential and high profitability. These, however, can be achieved if conditions are met such as adequate fry supply, availability of suitable site and properly designed fish farm. Supply of fry from the wild is very limited. As with cage culture, it is one of the constraints in the intensification of seabass culture in ponds. However, with the success in artificial propagation of seabass, fry supply may largely come from this source in the future. A comparison of hatchery bred and wild fry cultured in ponds did not show very significant difference in growth rate (Table 12).
There are two culture systems employed in pond culture of seabass:
Monoculture is that type of culture where a single species of animal is produced, e.g. seabass. This culture system has a disadvantage. It is entirely dependent on supplementary feeding. The use of supplementary feed reduces profit to the minimal, especially where the supply of fresh fish is limited and high priced.
Table 10. Combination of feed stuff.
|Rice bran or broken rice||30%|
Table 11. Combination of moist diet
|Soy bean meal||15%|
|Squid Oil (or fish oil)||7%|
Table 12 Comparison of growth rate of seabass (Lates calcarifer) culture in pond between wild fry and hatchery bred fry at stocking density of 3/m2.
|Stocking||10.5 cm||40.44 g||5.2 cm||5 g|
This type of culture approach shows great promise in reducing if not totally eliminating the farmers' dependence on trash fish as food source. The method is achieved by simply incorporating a species of forage fish with the main species in the pond. The choince of forage fish will depend on its ability to reproduce continuously in quantity sufficient to sustain the growth of seabass throughout the culture period. The forage fish must be such a species that could make use of natural food produced in the pond and does not compete with the main species in terms of feeding habit such as Oreochromis mossambicus, Oreochromis niloticus, etc.
The site should have enough good water quality supply all year round. Water quality includes all physico-chemical and microbiological characteristics of water being used for culture of seabass. The following are the parameters normally considered as suitable water supply:
|Dissolved oxygen||4–9 ppm|
|NH3||less than 1 ppm|
|H2S||less than 0.3 ppm|
|Turbidity||less than 10 ppm|
Area best suited for seabass should have moderate tide fluctuation range between 2–3 meters. With this tidal characteristic even for ponds as deep as 1.5 meters, complete drainage during low tide can be done. In addition, the pond can readily admit water during spring tide.
It is advantageous if the selected site is mapped topographically. This would reduce development and operational costs such as for water pumping.
Ideally, the soil at the proposed site should have enough clay content to ensure that the pond can hold water. Area with acid sulphate soil should be avoided.
Accessibility is an important consideration in site selection for logical reasons. Overhead cost and delay in the transport of material and product may be minimized with good site accessibility.
Other factors in the selection of site that should be considered include availability of seed, labour, technical assistance, market demand and suitable social condition.
Seabass ponds are generally rectangular in shape with size ranging from 2000 m2 to 2 hectares and depth of 1.2 to 1.5 meters. Each pond has separate inlet and outlet gate to facilitate water exchange. The pond bottom is entirely flat levelling toward the drainage gate (Fig. 23).
Preparation of grow-out ponds is similar to the procedure followed in pond system. In monoculture, the fish are stocked immediately after neutralizing the pond soil with lime. Ponds are filled immediately after pond preparation.
In polyculture, after the pond soil is neutralized, organic fertilizer (chicken manure) is applied at the rate of 1 ton per hectare. Then water depth is gradually increased for propagation of natural food. When abundance of natural food are observed, selected tilapia broodstocks are released to the pond at the rate of 5,000–10,000 per hectare. Sex ratio of male to female is 1:3. The tilapia are reared in pond for 1 to 2 months or until tilapia fry appear in sufficient number. Seabass juveniles are then stocked.
Seabass juveniles (8–10 cm in size) from nursery are stocked in the grow-out pond at the rate of 10,000–20,000 per hectare in monoculture and 3,000–5,000 per hectare in polyculture system. Prior to stocking, juveniles are acclimatized to pond culture and salinity conditions. Stocking the fish in uniform sizes will be most ideal and should be done at cooler times of the day.
Fig. 23 Pond lay-out for seabass culture.
Due to the need of maintaining natural food in ponds, water replenishment in polyculture system should be minimized. Water change should be done once in three days for about 50% of capacity.
However, in monoculture where supplemental feed is given daily, there are chances that excess feed may pollute the water. Hence, daily water replenishment is necessary.
Supplementary feed is not required in the polyculture system, but in monoculture, daily feeding is a normal practice. The method of supplying feed in ponds follows often the practice employed in cage culture.