| Summary: | The aquaculture feed industry continuously works to decrease the share of marine ingredients in the diets by searching for alternative feed ingredients. The increased use of such ingredients, on one hand expose the fish for new components, and on the other hand reduce the levels of components typical for diets rich in marine ingredients. Salmon diets have, over the last decades, also changed regarding lipid content. The level has increased, whereas the proportion of n-3 fatty acids has decreased. These changes may present challenges for the aquaculture industry. Fish farmers have, since 2000, reported symptoms of lipid malabsorption (LMS) in young as well as more mature salmon. The typically characteristics of LMS are swollen, pale and coarse appearance of the pyloric and mid intestine as a result of lipid accumulation in the tissue, seen histologically as large vacuoles. Previous studies have indicated that impaired lipoprotein formation, as a result of lack of phosphatidylcholine, could be the cause of the formation of excessive intestinal lipid vacuolation. Phosphatidylcholine is not considered as an essential nutrient for any fish species as it can be synthesized if sufficient choline is present. However, essentiality of choline has been established for many animal species, including some fish species in particularly for early stages, but not for any life stages of Atlantic salmon. In this thesis, three separate experiments were conducted to increase knowledge on the effects of choline and other key components involved in lipid and sterol metabolism on the development of LMS in Atlantic salmon in seawater. The experiments included firstly investigating whether choline chloride could prevent LMS. Secondly, whether other key components in lipid and sterol metabolism may modify the development of symptoms of LMS. The goal of the last experiment was to define the dietary requirement of choline for Atlantic salmon in seawater. The results from the first experiment showed that fish fed a diet with a choline level ...
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