Milkfish are produced in ponds, cages and pens. Culture practices are classified as extensive, modified extensive, semi-intensive, and intensive, depending on stocking density, feeding strategies, and water management (Baliao et al., 1999; Cruz, 1995; Fortes, 1996; Sumagaysay-Chavoso, 2003).
In extensive ponds, the fish depend primarily on natural food, and life-support systems such as aerators and pumps are not employed. Ponds are stocked at 1 000–3 000 juveniles/ha and production ranges from 0.5–1 tonne/ha/cycle.
Modified extensive systems are classified into either “modified straight run” or “modular” systems. In “modified straight-run” systems the fish depend on natural food for the first three months and in the last month of grow-out are provided with supplementary feed when natural food becomes limiting. Fertilization alone can only support a final biomass of ca. 0.6 tonne/ha/cycle (Sumagaysay et al., 1991), while this method allows for the production of 1 tonne/ha/cycle. The “modular” method of production comprises three stages of rearing. As the fish grow, they are moved from a smaller to larger ponds. Three ponds with areas increasing at a ratio of 1:2:4 or 1:3:9 form a module. The culture period in each pond lasts for 30 days, and once vacated, the pond is immediately prepared to receive the incoming stock. This is a continuous program of pond preparation, stocking, transfer, and harvest so that 6 to 8 cycles/year are possible, allowing for production of 2 tonnes/ha/yr.
Semi-intensive systems are adapted to increase yield over the traditional, extensive system, but with lower energy requirements compared to intensive systems. Fish are stocked at 8 000–12 000/ha. The nutrient requirement of the stock is supplied by both natural food and supplemental feed. Fish are dependent on natural food during the first month of culture when biomass is about 300–400 kg/ha; supplemental feed is provided from the second month onwards. Production in semi-intensive systems ranges from 1.5–3.5 tonnes/ha/cycle.
Intensive pond culture involves deepening of ponds and stocking at very high density (>20 000 fish/ha). Aeration, pumping, and feeding are employed to support high fish biomass and production levels in excess of 4 tonnes/ha/cycle are achieved.
Intensive culture of milkfish is also practiced in marine pens and cages. The stocking density in fish pens is 5–20 fish/m2. For floating, stationary cages, stocking density is maintained at 10–30 fish/ m3. In offshore cages stocking density is increased to 35–100 fish/m3 (de la Vega, 1998). Fish fed exclusively on commercial feeds in these systems
Successful production of milkfish in extensive, modified and semi-intensive pond systems is dependent on careful pond preparation to assure the necessary quantity and quality of natural food. Lab-lab and plankton are two types of natural food preferred by milkfish. Lab-lab is a biological complex of cyanobacteria, diatoms, filamentous algae, and associated invertebrates that occur initially on the pond bottom as a brownish, greenish or yellowish film. Some fish farmers promote lab-lab growth during the dry season when pond salinities are higher; and plankton during the rainy season. In areas where lab-lab grows poorly, filamentous algae may be grown although this is poorly digested by the fish.
Organic and inorganic fertilizers are applied in extensive, modified extensive, and semi-intensive ponds to stimulate growth of lab-lab and other natural food. Natural food production also varies according to soil condition and general pond management. Pond preparation is detailed below.
Pond preparation for lab-lab
- Level pond bottom for maximum use of production area and for more efficient water management.
- Drain pond completely and allow to dry for about one to two weeks until soil cracks. Eradicate unwanted species using organic pesticides such as tobacco dust, derris root, or combination of fertilizer and lime.
- When using tobacco dust, spread over moist bottom at 300–400 kg/ha, and allow to stand for a week. Apply tea seed powder at 15–30 ppm and derris root at 40 kg/ha, or apply a mixture of lime and ammonium sulfate fertilizer (21-0-0) at a ratio of 5:1 on wet areas of the pond bottom during sunny day. Reaction of lime and fertilizer releases heat and ammonia that effectively kills unwanted species in the pond.
- Apply additional lime, usually 1 tonne/ha CaCO3 for old ponds and more for newly excavated ponds depending on soil pH. Suggested lime requirements at different soil pH’s and texture is shown in Table 15. The cation exchange capacity increases with increasing clay content so that clay soil will normally have a higher lime requirement than loamy soil and loamy soils have higher requirement than sandy soil at similar pH.
- Install fine meshed screens at gates to prevent re-entry of wild species.
- Flood to a depth barely covering the pond bottom.
- Apply chicken manure at 1 000–2 000 kg/ha, then broadcast 50 kg/ha 16-20-0 fertilizer or 25 kg/ha 18-46-0 (N:P:K) two to three days later. Add urea (46-0-0) to speed up breakdown of chicken manure. Table 16 shows the NPK requirement using different inorganic fertilizers at recommended soil ph of 6.5–9. The suggested N:P2O5 ratio is 1:2 or 1:0.87 N:P in brackishwater ponds. List of organic and inorganic fertilizers that could be used for milkfish culture is shown in Table 17. The suggested fertilization schedules for milkfish farming under different stocking densities are shown in Table 18.
- Increase depth gradually over a period of 1 to 1 ½ months, 3 to 5 cm each time until stocking depth of 25 to 30 cm is reached. An abrupt increase in water depth causes lab-lab to detach and float.
- During the rearing period, broadcast 50 kg/ha 16-20-0 (or 25 kg/ha 18-46-0) + 15 kg/ha urea at two weeks interval to maintain lab-lab growth. Stop inorganic fertilization after a month in semi-intensive ponds when supplemental feeding starts but continue in extensive pond to maintain growth of natural food throughout the culture period.
Pond preparation for plankton
- Drain water completely to eradicate wild species and pests.
- Provide gates with fine mesh screens to prevent re-entry of wild species. Admit water to a depth of 65–100 cm.
- Apply 50 kg/ha 16-20-0 (or 25 kg/ha 18-46-0) on a platform. Plankton should bloom after few days, characterized by rich green pond water and visibility of about 15–40 cm. Re-apply fertilizer if plankton does not bloom. If visibility is less than 15 cm due to plankton bloom, stop fertilization and replace ¼ of pond water.
Pond preparation for filamentous algae
- Use the same fertilization program as described for lab-lab above.
- Seed when growth is sparse. Obtain seedstock from ponds where they survive or from special green-algae nurseries and select fibrous or silky filaments
- Plant by staking or sowing.
Stocking and fish transfer
The best time to stock milkfish is during the cooler part of the day - early morning. The container or transport bag is partially submerged and tilted to one side to allow pond water to flow in. This is to make sure that salinity and temperature levels in transport bags are close to those of the pond before fish are released.
Milkfish tend to swim against the current towards a fresh supply of water. When transferring fish from one pond to another, the pond is partially drained at low tide. Water is allowed to enter during high tide so that the fish will swim towards the inflowing water. A long drag net or seine is used to collect the fish near the gate. Fish are scooped into a counting net and finally transferred into another pond.
Water Management and aeration
In tide-fed ponds, water is exchanged every spring tide to maintain the desired salinity and water depth. Tidal exchange is done every two weeks in extensive systems and water depth is maintained at 40–60 cm throughout the rearing period.
Pumping is required for frequent water exchange in both semi-intensive and intensive systems. In semi-intensive ponds, water depth is gradually increased from 50 cm (first month of culture) to 100 cm (last month of culture) as the standing crop increases. In intensive ponds water depth is maintained at 1 m or more throughout the culture period. Aeration is needed when biomass reaches 800–1 000 kg/ha.
To prevent dissolved oxygen depletion fish biomass in un-aerated, semi-intensive ponds should not exceed 800 kg fish/ha. In intensive ponds biomass should not be higher than 5 000 kg fish/ha so as not to exceed acceptable water quality levels in effluent waters (Sumagaysay-Chavoso and San Diego-McGlone, 2003).
Culture period and harvesting
The culture period (cycle) in brackishwater pond is usually between 3–4 months (from fingerling to marketable size of 250–300 g/fish). The most common method of harvesting milkfish is the ‘pasulang’ method whereby fish are induced to swim against a water current (induced by tidal exchange) and are gathered in the catching pond or canal system and concentrated using a seine net. The fish are then scooped into chilling tanks or boxes which contain flaked ice. In the chilling tanks, a 1:1 ratio of fish and ice is enough to decrease the temperature of the fish to 4 oC in two hours.
When fish do not respond to the current, fish are harvested by draining the pond and leaving enough water to expose 1/3 of the dorsal portion of the fish as they congregate in deeper regions of the pond (trench). Fish are collected through seining and placed in chilling tanks.
In marine pens and cages, fish are grown for 6–8 months from fingerling (20 g) to marketable size (>500 g). Partial harvesting is done with seine nets or total harvest is achieved by lifting the entire net.