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 (Y2O3 and Yb2O3) and collection of faeces from the out let water of the tanks by sieving. The fish were fed three diets with diffe rent protein sources: fish meal (FM) (55% FM), soybean meal (SBM) (35% FM a nd 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 degrees C, Rate of passage wa s estimated by first feeding the fish the diets labelled with the inert mar ker Y2O3 and then changing to the same diets labelled with Yb2O3, Faeces we re 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 expre ssed as the percent-wise ratio between Yb2O3 and the sum of inert markers ( Y2O3 + Yb2O3) in the faeces. For all three diets, ytterbium (Yb2O3) was fir st quantifiable in the faeces 12-15 h after the change of marker, and the g astrointestinal evacuation was almost complete 30 h after the switch, when yttrium (Y2O3) still accounted for up to 2% of the marker. The estimated ra tes of passage were described by s-shaped curves (R-2 greater than or equal to 0.98). The estimated time for Yb2O3 to reach 50% of the maximum marker concentration was approximately 18 h after the marker change in the fish fe d 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 co ntinuous collection of faeces permitted accurate estimation of gastrointest inal evacuation in salmon during feeding, without disturbing or stressing t he fish. (C) 1999 Elsevier Science B.V. All rights reserved.