Apparent digestibility of tuna oil for common carp, Cyprinus carpio - Effect of inclusion level and adaptation time

The effects of inclusion level and adaptation time on the apparent digestibility of tuna oil for common carp were determined by incorporation into a casein-based reference diet at 10% and 15% inclusion. Chromium oxide (1%) was used as an external indicator and faeces were collected via siphon. Analyses were performed on faeces pooled over 10 consecutive days (Days 8 to 17) and unpooled daily faecal samples. Apparent digestibility coefficients (ADCs) determined from pooled faeces indicated an effect of inclusion level on the digestibility of tuna oil with a significant decrease in digestibility from 10% inclusion (83.1 +/- 2.4%) to 15% inclusion (59.8 +/- 1.5). Digestibility of crude energy from tuna oil decreased significantly from 76.6 +/- 3.9% at 10% inclusion to 38.0 +/- 2.1% at 15% inclusion. The inclusion of tuna oil did not affect dietary protein digestibility. Analysis of unpooled daily faecal samples indicated a prolonged period of adaptation for common carp fed the 15% inclusion diet, with digestibility of tuna oil at 15% inclusion increasing markedly over the duration of the trial. Daily variability in digestibility was also observed. These results emphasise that ADCs for tuna oil should be determined over the range of inclusion levels to be used and that it is important to ensure the complete adaptation of the experimental animals to diets prior to the pooling of faeces for analysis.