Optihealth - AGD challenge

Limited access of fish oil for use in salmon feed is a major challenge for the salmon industry. When the level of marine oils is reduced and replaced with plant oils in the fish feed, the fatty acid composition of salmon's tissues and organs changes. This research is aimed at determining how alterations in the ratios of n-6 and n-3 fatty acids affect stress responses in Atlantic salmon, particularly in response to challenges such as gill disease. Fatty acids play key roles in many biological functions, and altered the composition of the fat in the feed can thus affecting both growth, muscle quality and the health and robustness of the fish. Since the relative levels of EPA and DHA as well as n-6 and n-3 fatty acids are essential for fish health, the effects of these ratios (which are different in plant based oils compared with marine based oils) will be tested against amoebic gill disease (AGD) - a challenging parasitic disease facing the Norwegian salmon aquaculture industry. A total of 800 Atlantic salmon (Salmo salar) post-smolts that have been fed one of four different diets with different ratios of n-3 and n-6 fatty acids will be challenged with Neoparamoeba perurans 1000 cells /L in a standardized challenge model for 1 hour. The result will be development of AGD over the subsequent 4 weeks. The development of disease will be assessed by a modified gill scoring method, that increases the sensitivity of this method so producing more consistent results with fewer individuals (3Rs: reduction) and gill histology and qPCR of N. perurans. Although the disease at sub-clinical levels has minimal effect of the fish - long term exposure can cause respiratory and cardiovascular stress. The study will continue to gill score 2 - the clinical relevant limit at which fish are normally treated in the aquaculture industry and little effect of fish feeding response, nor physiological disturbances are seen. Exceeding this level has no clinical relevance and a refinement (3Rs: refinement) from previous studies. Increasing the robustness of salmon will impart a resilience against this disease. Optimization of the n-6 and n-3 ratios in providing resilience against AGD will be a significant way forward in managing production stressors in aquaculture and producing robust, healthy fish.