The shift from a marine-based to plant-based diet in farmed Atlantic salmon has revealed new challenges concerning the dietary requirements necessary to ensure fish health. Among the nutrients discovered to be lacking in plant feeds, choline has been identified as essential to guarantee efficient lipid transport and metabolism1. The distinctive sign of choline deficiency is an excessive presence of fat within the enterocytes, a condition known as steatosis. A recent study estimated a choline requirement of 3.4 g/kg in Atlantic salmon weighing 200-400g raised in freshwater2. However, it is not unlikely that choline requirement is influenced by production conditions such as dietary lipid level, environmental temperature, life stage, growth rate, etc. In a previous experiment we investigated to which extent dietary lipid level and water temperature influence steatosis symptoms. The findings confirmed the influence of lipid level and water temperature. The results to be presented are part of a following screening study investigating the possible effects of dietary lipid quality and fish size on choline requirement in Atlantic salmon. To perform the statistical analysis, a Bayesian approach was used. This approach provides careful and robust reasoning as well as accurate uncertainty handling for resource demanding studies aiming to describe dose-response relationships.
Materials and methods
Six experimental diets were formulated to contain 32 % fat, varying in ratios of rapeseed oil to fish oil, from 0/32 to 24/8. The diets were fed to two groups of Atlantic salmon raised in sea water: the smaller fish had an average initial weight of 1500g, while the larger fish weighted 4500g. At the end of the 8-weeks feeding trial, twelve fish from each tank were sacrificed and their body measures taken. The fish were then opened ventrally, and the intestinal package removed. The intestine was sectioned into pyloric, mid, and distal portion and weighed. The pyloric section was cleaned of mesenteric fat and weighed again. The liver was removed and weighed. Tissue samples from all the organs and sections were taken and processed for analyses of fat content and fatty acid profile, gene expression and histological characteristics. Enterocyte vacuolization was scored according to their morphological appearance and graded as normal, mild, moderate, marked and severe. Feed and feces were analyzed for dry matter, ash, crude protein, fatty acids and starch content. The digestibility of the macronutrients was assessed by using yttrium oxide as internal marker. Plasma was collected and stored for evaluation of biomarkers of nutritional status: glucose, free fatty acids, cholesterol and total triacylglycerides.
Growth performance, identified as Thermal Growth Coefficient (TGC), was higher in the large fish, whereas the increasing rapeseed oil level had no effect. A similar picture was observed for the organosomatic index of the pyloric intestine (OSI PI), which was higher in the large fish and not clearly influenced by lipid quality (Figure 1). Likewise, the histological assessment showed a higher vacuolization degree in the pyloric caeca of the larger fish. The vacuolization increased with the increasing rapeseed oil level, suggesting a dose-response effect (Figure 2), which was confirmed by analyses on fatty acids content. Neither fish size nor lipid quality affected the expression of the targeted biomarker genes.
Other data are under evaluation and further results will be presented at the conference.
The results obtained so far showed that steatosis symptoms and therefore choline requirement is mainly affected by fish size, whereas lipid quality showed less clear effects.
1. Hansen, A. K. G. et al. Choline supplementation prevents diet induced gut mucosa lipid accumulation in post-smolt Atlantic salmon (Salmo salar L.). BMC Vet. Res. 16, 1–15 (2020
2. Hansen, A., Kortner, T., Denstadli, V., Måsøval, K., Björkhem, I., Grav, H., & Krogdahl, Å. Dose–response relationship between dietary choline and lipid accumulation in pyloric enterocytes of Atlantic salmon (Salmo salar L.) in seawater. British Journal of Nutrition, 123(10), 1081-1093
The experiment is one of a series conducted under the GutMatters project funded by The Norwegian Seafood Fund (FHF, project 901435).