Effects of oilseed meals on pellet characteristics, faecal matter production, postprandial ammonia and phosphorus excretion rates in Nile tilapia, Oreochromis niloticus
This study assessed the physical characteristics, faecal, ammonia and phosphorus waste production rates of mixtures of groundnut, cottonseed, copra and soybean meals as a replacement for fishmeal in Nile tilapia diets. Six experimental diets were formulated such that the oilseed mixtures contributed 80% of total dietary protein. A fishmeal-based diet served as the control. The diets were randomly assigned in triplicates to the experimental tanks, containing 15 juvenile tilapia with a bulk weight of ∼1 kg. Ammonia and phosphorus excretion rates of the diets were monitored at 3-hour intervals over 24 h. The pellet characteristics, faecal and nutrient waste production were assessed. The control diet recorded a mean bulk density of 376.00 ± 5.19 g L⁻¹ which was lower than the oilseed-based diets which ranged from 550.67 ± 3.06 g L⁻¹ to 614.27 ± 2.08 g L⁻¹. There was a positive correlation (p < 0.0001; r = 0.93) between the sinking rates of pellets and their bulk densities. The oilseed-based diets recorded 4–45% higher faecal matter production compared to the control diet. Fish fed Diet 3 had the lowest Total Ammonia-nitrogen (TAN) excretion rate of 106.14 ± 1.95 mg kg⁻¹ while the control diet recorded the highest TAN excretion of 162.31 ± 4.39 mg kg⁻¹. Accumulated dissolved phosphorus was 64.52–74.09% lower in the oilseed-based diets compared to the control.
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