Measurement of gastrointestinal passage rate in Atlantic salmon (Salmo salar) fed dry or soaked feed

A method for measurement of gastrointestinal passage rate was tried in Atlantic salmon, Salmo salar (body weight 1131 g, temperature 13.5 degrees C, salinity 32 parts per thousand). Salmon were force fed one single ration (10 g) extruded feed 'as is' (92% dry matter) or soaked 2 h in sea water (70% dry matter), in triplicate with three individuals per replicate. Content from stomach, small intestine and distal intestine was collected at 2, 6, 12, 18, 24 and 48 h after feeding. Two hours after feeding, significantly more feed was transferred from the stomach to the small intestine in salmon fed soaked feed than in those fed dry feed. After that, no significant differences in gastrointestinal passage rate were found. Numerically however, soaked feed seemed to pass through the gut faster than dry feed. The content in stomach declined gradually, and all stomachs were empty after 48 h. Salmon in both treatment groups used 6-12 h on average to empty the stomach 50%. The content in small intestine peaked at the sampling 12 h post feeding for salmon fed both feed types. After 48 h, the small intestines were empty in all three replicates of salmon fed soaked feed, and in one replicate fed dry feed. The largest amounts of dry matter in the distal intestine were found between 12 and 24 h for both feed groups. Some dry matter was still present in the hindgut after 48 h. Individual variation in the gastrointestinal passage rate was large and few significant differences were revealed. However, soaked feed gave a significantly higher gastric evacuation rate than dry feed shortly (2 h) after feeding. As the fish regurgitated some pellets, the method may be most suitable when feeding small rations. Besides, due to variation a larger number of replicates would increase the power of the test.