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To measure the effect of physical feed quality on gut evacuation, Atlantic salmon with mean weight 1047 g (measured after finishing the sampling) was fed two extruded feeds with different physical quality to satiation. Each feed quality was produced in three batches to contain three different markers (La, Yb and Y). The measured hardness for Diet 1 was 128.5, 140.5 and 152.1 N for the batches added La, Yb and Y, respectively. For Diet 2, the corresponding figures were 148.2, 172.0 and 153.3 N, respectively. The mean water stability, given as % of dry matter remaining after 2 hours shaking in water bath, was 78.6, 76.9 and 77.9 % for Diet 1 added La, Yb and Y, respectively. For Diet 2, the corresponding figures were 84.9, 85.2 and 81.8, respectively. The durability, given as remaining intact pellets in the DORIS test, was 64.6, 65.9 and 81.6 for Diet 1, and 80.1, 81.4 and 78.3 for Diet 2, respectively, in the batches added La, Yb and Y. The pellet size was 10 mm. The salmon was fed one meal daily. The feeds added La was fed for a period of 26 days. On day 27, feeds containing Yb was given, and on day 28 and thereafter, the salmon was fed feeds with Y added. Faeces were collected from the outlet water during 30 minutes intervals at 8, 16, 24, 32, 40 and 48 hours after the feeding on day 27, and analyzed for La, Yb and Y. The ratio of the markers was calculated as concentration of each marker divided by concentration of sum of markers.
For both feed groups, some Yb appeared in faeces 8 h after feeding diets containing this marker, and amount of Yb peaked around 24 h and almost no Yb was left after 48 h. Sixteen hours after feeding, at the time when the change in marker concentrations had happened fastest, the relative concentration was lowest for La, and highest for Yb, in faeces from fish fed Diet 1, indicating a higher gastrointestinal transit rate for this feed in this period. The apparent digestibility of fat was approximately 2 % higher in Diet 1 than in Diet 2.
The data indicate that even small differences in the physical quality of feeds may have an impact on the gut evacuation rate in Atlantic salmon. This may affect how well the genetically inherent growth capacity of the fish is expressed (and utilized).