Scientific name: Chanos chanos (Forsskal, 1775)
Common names: English - milkfish ; Hawaiian - pua awa, awa ; Japanese - sabahii ; Indonesian - bandeng, golu ; Tagalog (Philippines) - bangus , sabalo (for adult fish);
History of use: Native to the tropical (and parts of the Northern sub-tropical) Indo-Pacific; used for centuries as a foodfish in Southeast Asia and Oceania; farmed in ponds and coastal enclosures for at least 700 years in Indonesia, 400 in the Philippines and Taiwan and 300 in Hawaii; farming has been based, until recently, entirely on the capture of wild fry.
Production statistics: According to FAO (1997) the leading countries in production of farmed milkfish are Indonesia, the Philippines and Taiwan which produced about 157,000, 151,000 and 63,000 t respectively in 1995. Production of farmed milkfish occurs on a much smaller scale in some Pacific islands; for example in 1995, Guam 25 t, Kiribati 50 t. Milkfish production increased continuously until the late 1980's but declined during early 1990's mainly because of detoriation of spawning grounds (reefs), nursery grounds (mangrove areas) and fry collecting grounds (sandy coast lines) (SEAFDEC 1995). Moreover, many pond owners then changed to shrimp farming.
Where farmed: Regions - mainly Southeast Asia, FAO Area 04-, Asia, Inland; also to a minor extent Oceania, FAO Areas - 71, 77.
Countries: Federated States of Micronesia (Guam), Indonesia, Kiribati, Philippines, Taiwan, Thailand.
Climate and environmental tolerance: Milkfish, except for spawning adults and the early larval stages, have very wide ranges of environmental tolerance (Schuster 1960, Gordon and Hong 1986, Bagarinao 1991). Most juvenile stages tolerate low temperatures of 14 to 18 °C and high temperatures of 38 to 41 °C. In the Philippines wet season (May to October), milkfish pond temperatures range from 25 to 34°C, salinities from 15 to 25 ppt and dissolved oxygen from 4 to 11 ppm. In the dry season (November to April) there are similar fluctuations, but temperatures stay below 30°C from November to December and peak in February and March, sometimes exceeding 35°C and with concurrent salinities up to 40 ppt (Juliano 1985; Sumagaysay 1994). Milkfish, except for spawning adults and early land stages, are remarkably euryhaline and tolerate all salinities from freshwater to hypersaline lagoons. The pH preferred by milkfish is around pH 8.0 and more acidic waters such as ponds that are affected by acid sulphate sorts can depress growth and survival.
Current farming methods: Milkfish fry are still mainly caught from shallow inshore waters. Historically, catches of wild fry have been huge: reaching over 1 billion annually in the Philippines and 700 to 800 million in Indonesia. However, artificial propagation and hatchery operations are being developed to avoid the unpredictable nature of supplies of fry from the wild (Duray 1996).
During the 1980's, the first spontaneous maturation of hatchery-bred milkfish in captivity was observed after 3.5 to 5.5 years of rearing spawners in net cages (Marte and Lacanilao 1986). Subsequent efforts have been made to induce spawning using gonadotropin-releasing hormone analogues (GnRH-A). In the Philippines, hatchery-bred fish have spawned spontaneously in net cages and concrete tanks, producing 50,000 to 956,000 eggs per spawning (Emata 1995). Milkfish farming in Taiwan is now based almost entirely on hatchery-raised fry. Hatchery production stabilises the supply of fry and can promote increased production of milkfish. A milkfish hatchery needs larval rearing tanks, culture tanks for rotifers (Brachionus) and green algae (Chlorella), and hatching tanks for brine shrimp (Artemia). Milkfish eggs are collected from broodstock cages and hatching occurs after 14-16 hours (at 28-29°C) after collection. Hatching tanks have about 300 eggs per liter for maximum hatching success.
To obtain an optimum survival of 30-40%, larval rearing tanks are stocked at a density of 30 larvae per liter, with moderate aeration (23-33°C, 30-40 ppt). A 'green water' technique is used, with Chlorella added in the morning, before feeding the larvae. Starting on day 2 until day 21 larvae are fed with rotifers. From day 15 to day 21, additional Artemia nauplii are added. Water exchange in the rearing tanks is 30% from day 2 until 14, 50 - 70% from day 15-21.
Rearing milkfish larvae to fry also conducted in ponds. Nursery ponds are dried, limed and fertilised with1 to 3 t.ha-1 of chicken manure or 0.5 to 1.0 t.ha-1 of rice bran to develop natural foods. The water depth is gradually increased to 30 cm before stocking fry at 30 to 50 per m2 (Baliao 1984). Postlarvae begin to feed shortly after the eyes have become fully pigmented and before the yolk is completely resorbed. Unfed larvae all die about 150h from hatching at rearing temperatures of 25 to 27 °C. Postlarvae and fry are particulate, visual feeders on small live prey such as rotifers (Brachionus), cladocerans (Moina), harpacticoid copepods (Tisibintra) and brine shrimp (Artemia). When milkfish larvae are about 2 weeks old, they can take artificial feed. Milkfish fry from artificial rearing on day 21 are at about the same developmental stages as wild caught fry.
Fry grow to 5 - 8 cm fingerlings in 4 to 6 weeks. Milkfish fingerlings are day-time feeders and take food mainly from the bottom. The kinds of food ingested vary with location and fish size. Juveniles from natural habitats feed mainly on cyanobacteria, diatoms and detritus along with filamentous green algae and invertebrates such as small crustaceans and worms. Milkfish tend to ingest the top layer of bottom sediments, with its associated micro - and meiofauna, as mullet do. Much of this material is detritus, rich in bacterial protein, fungi, and protozoans.
Adult milkfish graze on surfaces and on floating algae and, like the juveniles, are opportunistic omnivores. Growout of milkfish is evolving from traditional low intensity systems in shallow ponds based on natural food supply to more intensive systems in pens and deeper ponds with supplemental feeding (Fortes 1986, Lee 1995, Kühlmann 1998). The former include the "lab-lab" culture system of the Philippines and the shallow water culture ponds of Taiwan. "Lab-lab" is a term used in the Philippines for algal mats and their associated biota. Traditional shallow ponds yield around 800 kg ha-1 yr-1 from three crops, whereas more intensive modular ponds can yield up to 200 kg ha-1 yr-1 from six to eight crops. Using semi-intensive ponds, fertilised with chicken manure (2 to 3 t ha-1) and inorganic fertilisers (100 kg ha-1 ammonia phosphate, 10 kg ha-1 urea) and stocking densities up to 7,000 fish ha-1 in combination with supplemental feeding at 4 % body weight have yielded 1,160 kg ha-1 per crop (Sumagaysay and Borlongan 1995). In the Philippines, growth rates are much higher in the wet season than in the dry season (Kühlmann 1998). Adult milkfish spawners can be kept on a diet of commercial pellets with about 42 % protein, given at 1.5 to 2 % of body weight twice daily, presented on a feeding tray.
Milkfish are normally farmed in monoculture but there is some polyculture to utilise ecological food niches more efficiently e.g. with crustaceans (Penaeus indicus, Penaeus monodon, Scylla serrata) or fish (Oreochromis mossambicus; Megalops cyprinoides) (Eldani and Primavera 1981; Fortes 1984).
Processing and Marketing: The preferred market size for milkfish in Asia is about 300 - 400 g, meaning fish of less than one year old. Milkfish do not reach sexual maturity until 5 - 7 years of age. Milkfish is mainly sold whole, fresh or frozen. It has very limited market acceptability outside the main countries for production, because of its many fine bones. Producers of milkfish do not usually sell fish directly to consumers, but instead deliver fish through co-operatives, brokers, dealers, collectors or wholesalers, and retailers. The majority are sold at auctions, through dealers, brokers, wholesalers or co-operatives to other dealers, and then to retailers. Milkfish are also processed by marinating, smoking, or canning for export to Asian minorities; for example, to the USA.
Likely future trends: The introduction of milkfish hatchery technology will continue to have a positive net economic impact on farmed milkfish production, employment, and income, while removing the livelihoods of some wild fry collectors. Moreover, if the production efficiency of intensive and semi-intensive (Focken and Becker 1996) systems can be improved, farmed milkfish production is likely to increase. There is also scope for wider consumption of value-added products, such as canned milkfish.
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K.-J. Kühlmann and B. Ueberschar