(Forsskål, 1775)
1. Distinctive characters of early developmental stages (After Bensam, 1983; Watanabe, 1986; Buri et al., 1981; Goran and Hong, 1981; Delsman, 1928; Vanstone et al., 1977; Chaudhuri et al., 1978; Liao et al., 1979)
(a) Eggs (Not from mangroves)
Chanos chanos 1
Eggs are pelagic spherical and large in size with 1.1 to 1.25 mm diameter. Eggs initially float in seas with clear water where the depth ranges from 20 to 50 m; they soon start sinking slowly. Eggs take about 24 hours for hatching at depths down to several metres. Yolk is segmented; segmentation finer than clupeioid eggs. The colour of the yolk is slightly yellowish. Oil globule is absent.
(b) Prolarvae (Not from mangroves)
Chanos chanos 2; Chanos chanos 3
The transparent prolarvae measure between 3.2 and 5.3 mm in length; with large yolk sac of 2.2 mm long and 0.28 mm width; eyes are not pigmented and there is no fin buds. Mouth and anus net yet formed. Pre-anal myomeres vary between 33 and 34 and 8 to 10 post-anal myomeres could be traceable. Two days old larva possesses anal opening. Anus is situated far backward and hence with short tail. More number of black branching pigment cells are present especially in the dorsal and anal fin folds. Head does not project in front of the yolk in newly hatched larva and the head projects beyond the yolk the next day as the yolk is resolved. Yolk sac larvae are slightly negatively buoyant.
(c) Early post larvae (Not from mangroves)
Chanos chanos 4; Chanos chanos 5
On the 3rd day yolk is completely absorbed and the prolarval stage becomes early post larval stage measuring 6 to 8 mm in total length. At the end of 3rd day the larvae possess a functional mouth with opening. Eyes are pigmented. Muscle fibre in the myomeres are parallelly arranged to the longitudinal axis of the body. Tail and rudiment of dorsal fin are developing in this stage. Post larvae measuring 8 to 10 mm are almost unknown from nature (Goran and Hong, 1981).
(d) Late larval stage (or) fry
Chanos chanos 6
Late larval stages of 10 mm - 16 mm total length are referred as fry by milk fish culturists (Venkataraman, 1956). Larvae of 18-20 days old measure between 10.3 and 14.9 mm in total length. The environmental variables. Pelvic fins start developing at this stage. When the larvae measure 12.5 - 13 mm in length, they approach the coast and enter brackish waters, estuaries and lagoons including the mangrove waterways. The postlarva possesses 33 pre anal myomeres as in the prolarval stage. Two rows of black pigment spots are present of which one row is seen in the upper edge of the myomeres and the other one ventrally. Black pigment spots in the ventral region do not exclusively line the edge of the myomeres but partially located below the gut. Pigmentation patterns are usually constant but number and size of melanophores often vary among individuals, parental stock, geographic location and environmental conditions (Thangaraja, 1982 and Snyder et al., 1977). At 14 mm length the rudiments of ventral fins appear under the 20th myomere.
(e) Post metamorphotic juveniles
Chanos chanos 7
At 16-17 mm total length, the larvae have 30 pre-anal myomeres and fin rays appear in the ventral fin. In this stage, they appear in large number in estuaries and mangroves. During metamorphosis the height of the larva increases gradually as it approaches the adult-like form. Fine black dots are spread all over surface of the body, with more density in the dorsal region. The length reduces from 17 mm to 14 mm when the larval form passes the transition stage and becomes fully metamorphosed fish.
2. Distinguishing characters of early developmental stages in similar species occurring in the mangroves
(a) Larvae of clupeioids
Anus not situated far backward. Mass larval ingress has not been noticed in any clupeioids. Muscle fibres are crossly arranged in the myomeres. (In milkfish larvae, the muscle fibres are arranged parallel to the longitudinal axis of the body).
3. Salient biological characteristics
(a) Maximum size and Age
It grows up to 1.8 metres total length. Milk fish lived up to 12 years in captivity (Goran and Hong, 1981).
(b) Growth of life history stages in natural waters
1) Size of fertilized egg: 1.1 - 1.25 mm diameter.
2) Period of embryonic development: 24 hours.
3) Embryo to yolk. sac larvae: 1.2 mm to 5 mm in 3 days
4) Larval size and period: 5 mm to 11 mm in 2 to 3 weeks
5) Postlarval size and period: 11 mm to 15 mm in 1 week.
6) Juvenile size and period: 15 mm to 200 mm in 8 to 9 months.
7) Subadult size and period: 200 mm to 500 mm in 3 to 5 years.
8) Adult size and period: 500 mm to 1800 mm - plus 2 to 6 years.
(c) Food and feeding habits of juveniles and adults
The fry or the larvae feed on algae (Bacillariophyceae, Chlorophyceae and Myxophyceae). Fingerlings feed on diatoms, filamentous algae, bivalve veligers and ichthyoplankton. In culture systems, it feeds on lab-lab, a microscopic plant and animal complete with rich decaying organic matter. Juveniles of this species (20 to 200 mm) feed on detritus, algal weeds, benthic diatoms, fish eggs, planktonic diatoms and dinoflagellates.
(d) Reproductive biology
Females mature for the first time when reaching a size of 94 cm total length and 5-7 years of age. Fecundity ranged between 200,000 and 570,000 for female specimen of 110 cm to 157 cm total length. This species feels shy to breed in confined waters. It is found to spawn during February - March and October - November in Indian waters and during April - May and September - November in Indonesian waters. Spawning occurs at moderate distances from the shore in clear waters of 20 to 50 m depth, often over sandy or coral bottoms, where the salinity of the water is around 32 to 33 ppt .
4. Salient ecological information
(a) Habitat
The larvae and juveniles of milk fish occur frequently in mangroves and coastal waters. The adults of this species inhabit fairly deepwaters of the sea coastal waters, estuaries and other brackishwater systems including mangroves. It is a tropical fish with tolerance towards changes in temperature and salinity in natural waters and culture system. It thrives in waters with temperature as low as 15°C. The upper limits of water temperature for the survival of young and adult fishes are 39°C and 43°c respectively. The fry are acclimatized and grown ever in freshwater. This species readily grows well in brackish water. It survives even in salt pans with upper limit of 87.25 ppt water salinity.
(b) Geographic distribution
Tropical and subtropical regions of Indian and Pacific oceans including Hawaii and northern part of Australia.
(c) Behaviour
The floating eggs (fertilized) start sinking slowly . The eggs hatch out at depths down to several matters. Larvae of 6-10 mm total length are active swimmers and feed vigorously under laboratory condition. In natural waters, these actively swimming larvae undertake first major migration from offshore to the fry aggregation area in the shallow littoral waters close to sandy shore. Fry are non-schooling, feeding on plankton, while juveniles form school and feed on benthic algae. In Indian waters, during summer months (February -April), fry of around 12.5 - 13 mm length ingress into shallow coastal lagoons and mangroves in millions.
5. Evolutionary significance (After Goran and Hong, 1981; Greenwood et al., 1966; Rosen and Greenwood, 1970)
The milk fish is unique and it is the only species of the genus Chanos and family Chanidae. The family Chanidae is one among the six families of unusual fishes of the order Gonorynchiformes is considered to be one among the 5 under the suborder Chanoidei. The order Gonorynchiformes is considered to be closely related to Cypriniformes. (The common name in Tamil will vouch for such evolutionary relationship, meaning equivalent to ‘Milk carp’ - thus proving the indigenously inborn skill of traditional fisher folk). Fossil fishes considered to be closely related to milk fish were reported to be dated back to middle eocene (about 50 million years ago).
6. Note on aquaculture (After Liao et al., 1979; Lam et al., 1985; Liao and Chen, 1986; Baguilat, 1979; Pamplona and Mateo, 1985; Chen, 1981; Tamse, 1983; Apud and Pudadera, 1983)
In Indonesia, leaves and twigs of mangrove vegetation (Avicennia spp.) are used to manure milk fish culture ponds. Laboratory reared 21 days old larvae are robust enough to stock in nursery ponds. Immersion of postyolk-sac larvae in 5 PPM L-thyroxine-sodium solution accelerated the growth and development of larvae to juveniles in a shorter period. Pen culture of milk fish in the Philippines resulted in 5 tonnes/ha/year yield when stocked at a rate of 30,000 te 40,000 fingerlings/ha. In Taiwan the annual production of milk fish to the tune of 2 to 2.5 tonnes/ha was obtained in shallow water pond culture system, whereas, 8 to 11.76 tonnes/ha. annual production was achieved in 'deepwater pond' culture system. Tilapia hybrid, mullets and cyprinids are some of the fishes cultured along with milkfish in fresh water polyculture systems for quite a long time. Due to declining profit within milkfish industry, economically important shell fish, fin fish and algae (including Penaeus monodon , Scylla serrata , Lates calcarifer and Gracillaria sp.) are being cultured along with milk fish according to fish farmers' manipulation, preference and market demand. Milk fish and mangrove prawn (P. monodon ) were stocked at a ratio of 1 : 25 in polyculture ponds in certain South East Asian countries. Polyculture of milkfish and Tilapia integrated with pig rearing and polyculture of milkfish and while Indian shrimp (P. indicus ) integrated with poultry have been attempted in brackish water aquaculture systems in the Philippines.
7. Utilization
Fishes obtained out of capture and culture fisheries (mostly less than one year age) are marketed fresh, smoke cured, canned or in frozen form.