Comparison of estimates of digestibility of two diets for rainbow trout, Oncorhynchus mykiss (Walbiaum), using two markers and two methods of faeces collection

Abstract

Estimates of apparent digestibility using an indirect method were made for two rainbow trout, Oncorhynchus mykiss (Walbaum), diets (NF and LT) containing 450 g kg−1 fish meal. Two inert markers, titanium dioxide and chromic oxide, were used as digestibility markers. Faeces collections were made using two methods: stripping and the Choubert (tank) method. Faecal oil (g kg−1) and gross energy (GE; MJ kg−1) were higher for the tank system, but differences were greater for NF (P < 0.001 and P < 0.05 for method X diet interaction, respectively). Faecal titanium and chromium contents were also higher for the tank system, but the differences were greater for LT (P < 0.01 and P < 0.05, respectively). The main method effects were significant for crude protein (CP) and carbohydrate (CHO) content in that values were higher (P < 0.001) for the stripping method. The main diet effects were significant for ash and CHO contents (P < 0.001), with values being highest for LT and NF, respectively. The only significant method X diet interaction for apparent digestibility was with oil (P < 0.01) in that tank values were higher than stripping for NF but not for LT. Marker X method interactions were significant with dry matter (DM), ash, CP, GE, CHO (P < 0.05) and oil (P < 0.01) in that titanium values were lower than chromium values for the tank method. Marker X diet interactions were also significant with DM, ash, CP, GE, CHO (P < 0.05) and oil (P < 0.01) in that titanium values were lower than chromium values for NF. Both methods of faeces collection have inherent weaknesses, but it is suggested that the Choubert method gives more accurate and repeatable estimates of apparent digestibility of diets for fish.