A basal purified diet was formulated to contain approximately 35% protein and 3.4 kcal DE/g based on the feedstuff values reported in NRC for channel catfish (Table 1).
The basal diet was supplemented with three levels of lipid (combination of equal levels of corn oil and menhaden oil at 6, 10, 14% of diet) and three levels of vitamin E (50, 100, 200 mg/kg diet) as α-tocopheryl acetate (250 mg vitamin E activity/g) at each level of lipid (3×3 factorial experiment).
Levels of dextrin and celufil were adjusted to maintain diets isocaloric. Dry ingredients were thoroughly mixed for 10 min in a Hobart mixer before the oil was added.
After the oil was diffused into the mix, approximately 250 mL of deionized water/kg of diet was added. The moist mixture was extruded through a 3-mm diameter die in a Hobart meat grinder.
The resulting moist pellets were air-dried at room temperature to a moisture content of about 10%. Pellets were ground into small pieces, sieved to obtain approximate sizes and stored frozen in plastic bags at −20 °C until fed.
The analyzed levels of lipid and vitamin E of the experimental diets are presented in Table 2.
Fish in three randomly assigned aquaria were fed one of the nine experimental diets twice daily (between 0730–0830 h and 1500–1600 h) to apparent satiation for 12 weeks. During each feeding, feed was offered by hand four to five times until satiation was reached.
The amount of diet consumed was recorded daily by calculating the differences in weight of diets prior to the first and after the last feeding.
Once a week, aquaria were scrubbed and accumulated waste was siphoned.
On cleaning days, fish were fed only in the afternoon.
Fish in each aquarium were group-weighted and counted at 3-week intervals.
Feed was not offered on sampling days.