Digestibility in goldfish fed diets with and without chromic oxide and exposed to sublethal concentrations of cadmium

Effects of three sublethal levels of cadmium (40, 120 and 260 mu g L-1) on apparent total dry matter (TDM) and apparent protein (PD) digestibility in goldfish, Carassius auratus (L.), over an 8-week experimental period were investigated using diets with and without chromic oxide (0.5%) marker. The % TDM and % PD were estimated based on crude fibre as the marker for the diet without Cr2O3, and Cr2O3 and crude fibre as markers for the diet with Cr2O3. Generally, digestibility estimations were lower in goldfish fed the diet with the Cr2O3 marker. Similarly, for the diet with Cr2O3 marker, digestibility estimates based on crude fibre were higher than those based on Cr2O3. In all treatments (2 diets 3 control and 3 cadmium levels), the weekly estimations of % TDM and % PD did not differ significantly (P > 0.05). The effect of cadmium on weekly % TDM and % PD was not that obvious in all treatments. However, the overall mean % TDM and % PD differed significantly (P < 0.05) between diets and between markers, at all cadmium concentrations. The present study raises doubts regarding the validity of the use of Cr2O3 as a marker in digestibility studies in fish, as had been recently suggested by other authors but for different reasons.