| Summary: | The rapid growth of the aquaculture industry and the sharp decline of capture fisheries necessitates finding alternative sources for fish oil and fish meal in aquafeeds. However, the inclusion of VOs in aquafeed alters the dietary fatty acid composition, significantly reducing the amount of essential n-3 LC-PUFA, inducing the n-3/n-6 ratio, and increasing the MUFA contents. In the last few decades, numerous studies have been conducted to demonstrate the possibility of partial or complete replacement of FO with vegetable oils (VOs) without any adverse effects on the growth and welfare of the fish, provided sufficient n-3 LC-PUFA from other dietary sources. However, most feeding trials are run in controlled, stable environmental conditions, where there is minimal stress on fish. In contrast, under demanding environmental conditions in sea cages, fish are exposed to various stressors, including fluctuating water temperatures, handling, parasitic pressure, delousing, etc. Therefore, we need increased knowledge on how optimal FA nutrition can be used to maintain a healthy and robust fish that can cope with stressful situations, such as fluctuating environmental conditions and disease pressure. Besides the change in dietary FA profile, increased inclusion of plant ingredients also reduces the supply and availability of dietary minerals to fish. Further, little is known about how this change in the FA profile affects the intracellular fate of these fatty acids in intestinal cells. Therefore, this Ph.D. project investigates how stressful conditions combined with the change in dietary FA level affect the absorption and intracellular fate of dietary fatty acids, stress and immune responses, and the utilization of minerals in the fish. In the present thesis, the three trials were conducted i) short-term challenge experiment, ii) long-term seawater trial, and iii) in vitro trial in RTgutGC cells. In the short-term challenge trial (Paper I), Atlantic salmon were fed diets containing different ratios of n-6/n-3 FA (at 1.3, ...
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