Milkfish Culture in Brackishwater Ponds

Milkfish or popularly known as “bangus” is the Philippines national fish. The demand for milkfish is increasing yearly as the consumer population increases. The popularity of milkfish as a food fish may be traced to its abundance in the local market as well as to its good and tasty white meat. However, with the growing environmental constraint over the country’s dwindling mangrove resources, expansion of production areas is not anymore seen as an option. The direction of milkfish farming will inevitably be towards improvements on the existing production areas.

Semi-intensive Milkfish Culture

Existing brackish water fishponds are modified to improve water management and stocks manipulation. Milkfish yield can be increased through increased stocking rate and giving supplemental feed when natural food become scarce. Stocking density is 9,000 pcs. fingerling per hectare.

Site Selection

The suitable site for milkfish culture are brackish water fishponds with 0.8 – 1 m water depth. Ideal water salinity is between 10 – 30 ppt. provided it is stable during the duration of the culture period (no abrupt changes during rainy day). Ideal water temperature for growth is between 26 – 30ºC. pH is 7.5 – 8.5 and dissolved level is 3 – 5 ppm. Easy access to inputs (e.g. fry and feeds) and market outlet is preferable. Pond bottom must be sandy clay or clay loam.

Pond Lay-out and Design

Grow-out operation must have a nursery pond, transition pond/formation pond and a rearing pond. All compartments must be provided with gate. This set-up is used if the fish seed to be used are still at fry stage. If fingerling size are to be used, the nursery and transition pond should be disregarded. If only one pond is available, it must be subdivided meantime using fine mesh nets, until such time that the fry will reach fingerling stage. A diagonal canal inside the pond shall be constructed to facilitate more efficient water exchange and harvesting of stocks. The pond bottom must be leveled flat but inclined towards the gate for convenient water management and easy draining.

Pond Preparation and Natural Food Production

Milkfish thrive on natural food such as lab-lab, lumot and plankton. Comparatively, lab-lab excel over other food types when it comes to growth of milkfish. Lab-lab is a local term of benthic algal community which consists of yellowish-greenish minute plants and animals that form a mat on the pond bottom. They are sometimes detached and float in clumps or patches.

1. Drain the pond completely and allow it to dry for about 1 – 2 weeks until the pond bottom cracks to facilitate release of toxic gases. Prolonged drying is not advisable as it makes the soil hard and powdery.
2. Eradicate unwanted species using organic pesticides such as tobacco dust and / or combination of ammonium sulfate fertilizer and agricultural lime. When using tobacco dust and / or combination of ammonium sulfate fertilizer and agricultural lime, spread it over the moist pond bottom at a rate of 300 – 400kg/ha. and allow it to stand for about a week. Prepare a mixture of hydrated lime and ammonium sulfate fertilizer (21-0-0) at a ratio of 3:1 at a rate of 100-grams/1000 sqm. and broadcast it in wet pond bottom during a sunny day. The mixture releases heat and ammonia, which effectively kills unwanted species in the pond.
3. Apply chicken manure at 2 tons per hectare. Admit water barely covering the pond bottom and broadcast urea (45-0-0) at 15kg/ha. 2 – 3 days later to speed up the breakdown of chicken manure.
4. Increase the water depth gradually over a period of 1/2 – 1 month at 3 – 5 cm from time to time until the stocking depth of 0.8 – 1.0 meter. An abrupt increase in water depth will cause the lab-lab to detach and float. Install fine-mesh screens (bastidor or lumpot) at the water gates to prevent re-entry of unwanted species and the possible escape of cultured species.

Eradication of Snails

The most common snail pest are called suso and bagungon. These pests destroy lab-lab mat and compete lab-lab consumption with bangus. Use alternative molluscide to eradicate the snails like tobacco dust, applied at the rate of 300 – 400kg./ha. The common practice to get rid of the snails is by collecting them by sweeping or hand picking and burn them.

GROW-OUT OPERATION

Freshen the pond water at about 25% a day before stocking. Stocks should have an initial average body weight of 50 grams post bangus fingerlings at 9,000 pcs. per hectare. Best time for stocking is early morning, nighttime or any colder part of the day. Release the fingerlings in different parts of the pond.

The transition pond will serve as a stunting compartment for fingerlings that will be used in future production. The grow-out process is divided into 2 phases, namely: the formation and rearing phases. The formation phases is an intermediary rearing period that will allow the fingerlings to grow from 5g to 50g using the natural food as their primary food. The post fingerlings are finally transferred to the rearing pond to grow up to marketable size for a period of 4 months.

MANAGEMENT

1. Supplemental fertilizer application to the pond will enhance the growth of lab-lab or lumot. Use 16-20-0 fertilizer at a rate of 15kg./ha. Application is done weekly but not closer than 3 days before pond water freshening or in stocking the pond with fish.
2. After fish are stocked no dressing (fertilization) should be done in nursery ponds, but in all other ponds, 15kg./ha. is to be applied every 7 – 10 days to maintain good growth of natural food.
3. The pond water should not be freshened for at least 3 days after fertilization unless fish becomes stressed and freshening is needed to save the stocks.
4. Fertilization is stopped 20 days before the fish are removed from the pond.
5. If at any time during the fish growing phase the natural food is depleted and cannot be replenished by fertilization or water management, provide supplemental feeding with commercial feeds at the rate of 4% of the average body weight daily until harvest and rationed at 2% in the morning and afternoon. In designated are, broadcast or use feeding tray to condition the fish to eat pellets.
6. Regularly change water by least 25 – 30% specially when new spring tides occur. Admit new water even at night if there is the occurrence of tide. During rainy months, drain the uppermost water (freshwater) to prevent salinity fluctuations. Change pond water immediately after heavy rainfall. Use water pump when tide action is not possible.
7. Maintain a water depth of at least 0.8 – 1 meter.

Mitigating measures during stressful conditions:

1. Do not feed when fish are under stress.
2. Reduce feed ration when temperature is low and when salinity is too high.
3. Unusual fish behavior may sometimes be experienced when the stocks are exposed to stress. This phenomena is characterized by the presence of fish at the water surface gasping or swimming in circles. These are indications of low dissolve oxygen (DO) concentration.
4. Replenish water at the first time stress associated behavior is observed in the fish. Water may be splashed on a piece of wood to increase oxygen concentration.
5. Anticipate adverse weather conditions. Sudden rain on or after a hot day may cause abrupt change in water temperature which can result to severe stress or fish kill.

HARVESTING

To attain the highest possible profit, culture period should be about 4 months for cost efficiency. Yield range from 1.2 – 1.5 tons/ha./crop which is equivalent to 3.6 – 4.5 tons/ha./year for 3 cropping.

The most common and undisputed technique of harvesting is the “pasulang method”. This technique relies on the instinctive habit of milkfish to swim against the current thereby leading them into the catching area. Fish are harvested by seining or scooping.

Total Harvesting

Is done by seining the whole stocks and collect the remaining fish by picking them manually from the pond bottom. Sort and pre-chill the harvested fish so as to maintain and prolong their freshness and quality.

Partial Harvesting

Is done by seining using bigger meshed nets so that small fishes could pass through the meshes of the net, trapping only the bigger and harvestable stocks.

Sources: BFAR,trc.dost.gov.ph; Photo: bar.gov.ph