Milkfish fry are either caught from the wild or obtained from a hatchery. Growth is fast in the first year as shown in the linear growth of fry to fingerling (0.005–6.3 g in 6 weeks) and from fingerling to marketable size (3–550 g) in 5 months from stocking (Figure 3). Specific growth rate (SGR) of fry and juveniles is high (4.5–13.4 and 2.4–2.7 %/day, respectively) and influenced by food availability, feed quality and stocking density.
Fish growth in the wild
Shore-caught fry with a total length of 10–13.5 mm are estimated to be 2–3 weeks old (Bagarinao, 1991). It was estimated that fry in inshore waters grow at a rate of 0.5 mm/day (Kumagai and Bagarinao, 1981; Kumagai, 1990; Bagarinao, 1991). In terms of weight, the average growth rate of milkfish fry (between 0.3 and 4.7g) in small mangrove lagoons in central Philippines was 0.14 to 0.27 g/day and is best described by log Weight (g) = -5.2991 + 3.2388 log Length (mm).
There is no reliable accurate data on longevity of milkfish, but based on the rings on the vertebrae, the estimated age of adult wild milkfish from the Philippines and Taiwan ranged from 3 to 7 years (Bagarinao, 1991), although in captivity they may live for longer than 12 years (Aquaculture Department of Southeast Asian Fisheries Development Center (SEAFDEC/AQD), Central Philippines and as reported by Schuster, 1960).
Fish growth under culture conditions
There is no difference between growth and survival of hatchery-bred and wild milkfish fry in the Philippines (Baliao et al., 1980, Sumagaysay et al., 1999), but Lin (1985) reported that hatchery-reared fry in Taiwan had better survival and growth as well as a higher tolerance to low temperature.
Generally, growth is faster at lower stocking densities during the nursery period (Figure 3-a). Similarly, growth rate in semi-intensive pond culture with supplementary feeding (8 000 fish/ha) is higher in comparison to intensive pond culture (20 000 fish/ha). Growth in marine cages is also faster at lower densities (0.5–1 fish/m3) than at higher densities (1.5–2 fish/m3) (Figure 3-b).
The length-weight relationship of milkfish reared under extensive pond culture conditions in the Philippines is expressed by the equation: log W = -5.0463 + 2.98895 log L, where W = weight in g and L=total length in mm (Grover and Juliano, 1976). With the use of artificial feed, this relationship changes depending on the quality of the diet. A high fat and protein diet may result in wider fish and a more rounded shape.
Figures 3. Growth of different life stages of milkfish, Chanos chanos under natural and farming conditions
Sexes are separate in milkfish. There are two openings in the anal region of mature males and three in mature females (Chaudhuri et al., 1976). During the peak of the breeding season the ovary is about 10 percent to 25 percent of body weight. Induction of spawning using hormones is effective when oocyte diameter is 0.67 mm or greater (Tamaru et al., 1988; Marte et al., 1988). Milkfish mature and spawn in captivity (Lacanilao and Marte, 1980; Marte and Lacanilao, 1986). They are able to spawn more than once a year and up to three times in floating cages and concrete tanks (Emata and Marte, 1994). Spawning usually occurs at midnight, signaled by increased swimming activity, chasing, occasional leaping and slapping of water late in the afternoon and evening (Marte et al., 1986) and a decrease in feeding activity three days before spawning (Lin, 1985). The spawning season can be observed through the occurrence of wild fry in inshore waters, with two peaks in abundance near the equator and a single peak at higher latitudes. Because of the high demand for fingerlings, milkfish hatcheries have been developed to lessen the dependence on wild caught fry. Newly hatched milkfish larvae are stocked in circular fibre glass tanks at 24 larvae/L. Tanks are filled with filtered seawater (30–32 ppt). Tank bottoms are cleaned daily by siphoning waste materials and by replacement of approximately one third of the water volume (Borlongan et al., 2000). The feeding schedule shown below was developed to reduce the requirement for natural food in hatcheries.
