Digestibility in milkfish, Chanos chanos (Forsskal): Effects of protein source, fish size and salinity

The true digestibility of casein, gelatin, fish meal, defatted soybean meal and Leucaena leucocephala leaf meal was measured in 60- and 175-g milkfish (Chanos chanos Forsskal) in fresh- and seawater. The diets contained 45% of these feedstuffs and 1.3% of the indicator substance, chromic oxide. The intestinal dissection method was used to collect fecal material. Results showed that the length of time between initial feeding and fish sacrifice did not significantly affect digestibility. Gelatin was the most digestible (90-98%) protein, regardless of size. Casein, defatted soybean meal and fish meal were moderately digestible (50-90%) and digestibility coefficients tended to increase as a function of fish size. L. leucocephala was the least digestible (-10-40%). The digestibility of most of these feedstuffs was less in the anterior than in the posterior intestine, and tended to be lower in seawater than in freshwater. Rate of food movement was similar in both size groups, but was significantly faster when milkfish were in seawater rather than in freshwater. The effect of salinity on digestibility may in part be due to food motility changes necessitated by alterations in osmoregulatory processes when fish are in seawater.