Effects of replacing fish oil with vegetable oils in feed for rainbow trout (Oncorhynchus mykiss) and Arctic charr (Salvelinus alpinus)

As global capture of fish has stagnated and fish consumption is increasing due to a growing human population, the demand can only be met by increased aquaculture production. Fish oil (FO), derived exclusively from wild pelagic fish, has traditionally been used as the primary lipid source in fish feeds. For a number of reasons, more sustainable development of aquaculture is necessary where FO needs to be replaced with a more sustainable lipid source. This thesis investigated the effects of FO replacement with two vegetable oils on growth, lipid content, feed preference and swimming performance of two salmonid species; rainbow trout and Arctic charr. In addition, a comparison of lipid content and composition with wild fish was performed to highlight the importance of natural food webs for successful production of specific fish species. The results obtained showed no negative effects on growth of fish fed vegetable oils. However, significant changes in fatty acid profiles were observed in fish tissues, with reduced levels of long-chain polyunsaturated fatty acids (LCPUFA), mainly EPA and DHA, and increased levels of 18:1n-9 and 18:2n-6. Comparisons of fatty acid profiles of wild and farmed Arctic charr showed significant differences in individual n-3 and n-6 fatty acids. The largest difference was found in arachidonic acid (20:4n-6) content with 7-fold higher levels in the phospholipid fraction in white muscle of wild Arctic charr compared with Arctic charr fed marine FO. Cholesterol-lowering effects were observed in fish fed rapeseed oil, possibly explained by the presence of phytosterols in the diet. Swimming performance at 4 ºC was significantly reduced in Arctic charr fed a blend of rapeseed oil and palm oil. This outcome is suggested to be an effect of the different levels of n-3 LCPUFA and saturated fatty acids due to their temperature influenced properties. The results in this thesis imply that an appropriate mix of vegetable oils and FO can replace the sole use of FO in fish feeds. However, researchers and feed manufacturers should be encouraged to continue to increase feed diversification in order to optimize the nutritional requirements of farmed fish