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 μg 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 × 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.