Effects of food containing betaine/amino acid additive on the osmotic adaptation of young Atlantic salmon, Salmo salar L.

Abstract

One-year-old salmon ( Salmo salar L.) were reared in freshwater (FW) and fed with either control feed or an experimental feed containing 1.5% of betaine/amino acid additive. After 1 month of rearing in freshwater, some of the fish in each group were gradually acclimated to 39 ppt seawater (SW) over a week, then reared in it for 4 weeks. The rest of the fish were maintained in FW. Physiological samples were taken at the start of the experiment, after 4 weeks' rearing in FW and five successive times during the SW stage. In addition, a group of the FW fish were challenged with SW three times during the experiment using an abrupt change from FW to SW and a further stay of 48 h in SW. The surviving fish were sampled for physiological analyses. The betaine/amino acid additive induced some physiological changes in FW fish. The most marked were a lowered muscle water content and delayed desmoltification of the experimental fish. The additive somewhat improved the capacity to maintain the ionic and osmotic balance in SW but, due to the low number of experimental animals, the difference in SW survival was insignificant. The seawater challenge tests indicated that betaine may also help the fish to maintain the ionic and osmotic balance after abrupt transfer to SW. There seem to be several mechanisms of action: the gill ion extrusion is somewhat enhanced as expressed by elevated gill Na,K-ATPase activity, but the effects on the water and magnesium balance are more pronounced, indicating changes in the intestinal and renal ion and water transport.