The effect of feeding level on apparent digestibility of dietary dry matter, crude protein and gross energy in the African catfish Clarias gariepinus (Burchell, 1822)

The effect of feeding level on apparent digestibility of dietary dry matter, crude protein and gross energy was investigated during a 21-day experiment with African catfish Clarias gariepinus (Burchell, 1822). Five feeding levels were tested in the range of 0.5 to 2.5% of fresh body weight. Mean fresh body weight at the start of the experiment was about 137 g. Water temperature was maintained at 25 ± 0.5°C. Digestibilities were determined with three different methods: (1) quantitative collection of faeces by sedimentation; (2) utilization of chromic oxide as indicator in feed and sedimented material; (3) utilization of chromic oxide in feed and samples obtained by dissection of the lower one-third of the intestine. The first two methods, which are based on sedimented material, were done at five different hydraulic loadings of the funnels where the sedimentation took place (range: 20 to 72 m 3·m −2·day −1). The results indicate that the sampling method determines to a large extent the value of the apparent digestibility coefficient. Quantitative collection of faeces gave high values, presumably due to loss of suspended solids and leaching of nutrients out of the faeces. Utilization of the indicator in the sedimented material gave lower values, but the loss due to leaching was as important as with the quantitative method. In the system described, samples taken out of the intestine will give the most reliable results. Apart from the resulting differences in digestibilities between methods, apparent digestibilities were negatively correlated with feeding level in all cases.