Estimation of gastrointestinal evacuation rate in Atlantic salmon ( Salmo salar) using inert markers and collection of faeces by sieving: evacuation of diets with fish meal, soybean meal or bacterial meal

Gastrointestinal evacuation of Atlantic salmon was estimated using yttrium and ytterbium oxides (Y 2O 3 and Yb 2O 3) and collection of faeces from the outlet water of the tanks by sieving. The fish were fed three diets with different protein sources: fish meal (FM) (55% FM); soybean meal (SBM) (35% FM and 31% SBM); and bacterial meal (BM) (34% FM and 20% BM). The fish size was 0.15–0.20 kg and the water temperature was 9°C. Rate of passage was estimated by first feeding the fish the diets labelled with the inert marker Y 2O 3 and then changing to the same diets labelled with Yb 2O 3. Faeces were obtained by sieving of the outlet water and were pooled by tank every 3 h for 81 h after the switch in diets. The evacuation of the diets was expressed as the percent-wise ratio between Yb 2O 3 and the sum of inert markers (Y 2O 3+Yb 2O 3) in the faeces. For all three diets, ytterbium (Yb 2O 3) was first quantifiable in the faeces 12–15 h after the change of marker, and the gastrointestinal evacuation was almost complete 30 h after the switch, when yttrium (Y 2O 3) still accounted for up to 2% of the marker. The estimated rates of passage were described by s-shaped curves ( R 2≥0.98). The estimated time for Yb 2O 3 to reach 50% of the maximum marker concentration was approximately 18 h after the marker change in the fish fed the diets with FM and BM, while the corresponding value was approximately 20 h for the SBM diet. This was attributed to a slower onset of evacuation for the diet with SBM. The combination of two inert dietary markers and continuous collection of faeces permitted accurate estimation of gastrointestinal evacuation in salmon during feeding, without disturbing or stressing the fish.