The development of the digestive tract of milkfish occurs in parallel with ontogenetic shifts in diet and habitat (from coastal areas to inland waters) (Ferraris et al., 1987). The digestive tract of the newly hatched larvae is a simple undifferentiated tube. Three days after hatching, differentiation of the oesophagus begins with the development of mucous secreting cells; mucosal folds develop after 14 days. In 21-day old larvae, the stomach differentiates into the cardiac and pyloric regions while goblet cells start to develop in the intestine. In fish undergoing metamorphosis (>42 days old), the mucosal cells of the cardiac stomach develop into two distinct cell types: the columnar cells which make up the folds nearest the lumen, and the cuboidal cells which constitute the gastric glands. The cardiac stomach is the only region in the digestive tract where mucus secretion is not acidic. Alkaline phosphatase is localized in the brush border of the intestinal epithelial cells in 3-day old larvae. Aminopeptidase is found only in the brush border of the enterocytes of 21-day old larvae and older fish. Intestinal esterases are found in the brush border and in cystoplasm of enterocyes of 3-day old larvae and older fish. Esterase is found in both columnar and gland cells of the cardiac stomach only in postmetamorphic fish (at 60-days or older).

Based on the morphology and osteological development of the oral cavity early stage larvae from first feeding to 200 hours after hatching feed by “straining” (Kohno et al., 1996). Fundamental elements of the oral cavity have developed by the time of initial mouth opening (54 hours after hatching). The oral cavity is long and cylindrical, with a short robust Meckel’s cartilage, and robust quadrate and symplectic-hyomandibular cartilages. Feeding ability improves between 120–146 hours after initial mouth opening when existing elements undergo initial ossification and by addition of new elements. Mouth width is about 189–270 μm at opening and 306 μm when the larvae begin to feed on rotifers, 125 hours after hatching (Duray and Kohno, 1990).

The morphology of the digestive system of juvenile and adult milkfish suggests that it is an herbivore with generalist tendencies (Bagarinao, 1991). It has a small toothless mouth, fine closely spaced gill rakers, and a pair of muscular raker-like epibranchial organs. The oesophagus is long and thick walled, with 20–22 spiral folds and many mucus cells. The stomach is large, the cardiac region characteristically bent or doubled over, and the pyloric region has a spherical gizzard with very thick walls and a mucus membrane. The cardiac stomach has gastric glands; the gizzard has none and seems to function in the trituration of food materials. Numerous pyloric caeca cluster behind the gizzard. The intestine is convoluted and extremely long. The entire digestive tract is highly coiled and forms a compact mass in the abdominal cavity.

Figure 2. General morphology of the digestive tract of milkfish (E–oesophagus, CS–cardiac stomach, PS–pyloric stomach, I–intestine, R–rectum) (courtesy of N.S. Sumagaysay-Chavoso).

The major sites of lipase and protease activity in the digestive tract are the intestine, pancreas, and pyloric caeca and also in the liver for amylase activity (Chiu and Benitez, 1981; Benitez and Tiro 1982; Borlongan, 1990). The relative activity of the digestive enzymes are correlated with the nature and composition of food consumed.

The feeding mode shifts from straining (early larval stage) to particulate visual feeding (late larval stage). Milkfish take copepods and suction feed on benthic diatoms and epiphytic algae in their natural environment. In the hatchery, larvae are fed rotifers (Brachionus), water flea (Moina), copepods, and brine shrimp (Artemia). Milkfish fry can be weaned on to artificial feed 2–8 days after hatching.

Milkfish fry, juveniles, and adults are substratum–feeders in shallow-water environments and also considered as iliophagous, ingesting the top layer of bottom sediments with the associated micro- and meiofauna (Whitfield and Blaber, 1978; Blaber, 1980). In culture systems, they ingest artificial feed whole and their reaction to feed is fast by feeding immediately on the water surface. Feeding also occurs in the water column and on the bottom. They can be trained to feed on artificial feed by making a sound or by feeding them at a fixed place and time.

Gut passage rate is 10–15 min in 20 g milkfish and 27–50 min in 60 g juveniles (Ferraris et al., 1986). The food passage rate is higher in seawater than in freshwater in juveniles, which could partially explain the low digestibility of fishmeal and soybean meal and high feed conversion ratios in seawater. Feeding activity of milkfish fry varies with temperature. Satiation (full stomach) is attained within 15 min after introduction of food at high temperatures (26–35 °C) to 60 min at lower temperature (18–23 °C) (Villaluz and Unggui, 1983). Undigested food is expelled from the anus one hour to two hours at high and ambient temperatures (24–29 °C) and three hours at low temperature. In juvenile milkfish, over 50 percent of ingested food is evacuated after two hours suggesting an optimum feeding frequency of every two hours (Sumagaysay, 1993). Frequent feeding over a wider spread of time is an efficient strategy for feeding milkfish in culture systems (Chui et al., 1987).

Hatchery-reared larvae exhibit diurnal feeding behavior with lower gut content or an empty gut in the evening (Hara et al., 1983). Fry fed on rotifers continue eating and defecate throughout the day provided there is enough light and dissolved oxygen level is above 1 ppm. Juveniles consume food in natural habitat and in ponds during the day, and stomachs are empty or contain little food at night (Kumagai et al., 1985; Bagarinao and Thayaparan, 1986; Luckstädt, 2004). Peak feeding occurs around noon when digestive enzyme activity, temperature and dissolved oxygen are highest. Feeding continues in the dark if dissolved oxygen levels are more than 3 ppm (Chiu et al., 1986). Since milkfish are generally daytime feeders feeding with artificial feed begins soon after sunrise and is terminated at sunset. Ration size is decreased when dissolved oxygen and temperatures are low or during winter months.

Milkfish are considered to be opportunistic generalists, feeding on anything present in the environment from detritus, phytoplankton, zooplankton, filamentous algae to artificial feed (Table 1). The food of juveniles in their natural habitat consists of algae, detritus, diatoms, animal elements, plant debris, and sand particles. When supplemental feed is provided to milkfish juvenile they preferentially feed on pellets followed by detritus, diatoms, and filamentous algae. Adult milkfish feed on diatoms, zooplankton (including fish eggs and larvae), algae, detritus, and small amounts of sand particles.