Introduction
Replacing fishmeal and fish oil with plant-based ingredients in Atlantic salmon diets has led to concerns regarding gut health and welfare, including excessive lipid accumulation (intestinal steatosis) and inflammatory reactions. Some of these issues are likely linked to altered nutrient profiles, including low choline levels in plant ingredients. Choline is critical for lipid transport, membrane structure, and methylation, and its deficiency is strongly associated with intestinal steatosis in salmon. Studies have shown that dietary choline, particularly as phosphatidylcholine or choline chloride, can prevent such symptoms [1]. Factors such as high dietary fat, warmer water temperatures, and rapid growth increase choline demand, especially in Arctic farming conditions where steatosis prevalence shows large seasonal variations. Data from the Arctic Salmon Research Centre (2016–2024) show a marked increase in steatosis during September–December, i.e. during the warmest period with high growth and feed intake. The current study aimed to assess the effect of krill meal - rich in phosphatidylcholine - on intestinal steatosis, mucosal health in gut and skin, flesh quality, and overall performance of Atlantic salmon farmed under Arctic conditions. It also sought to determine whether krill-derived choline differs from choline chloride in preventing steatosis, potentially supporting more targeted supplementation strategies for enhanced health and production outcomes.
Materials and methods
The trial was conducted at the Cermaq farming site at Storholmen (Bekkarfjorden , Finnmark, 70°20′09″N 23°19′36″E) using 9 open 120 m2 sea cages , each stocked with 200,000 smolts averaging 108 g in Aug 2023. The trial started when fish reached an average estimated body weight of 300 g in Oct 20 23. The fish were fed either a commercial diet with total choline supplied from 15% fishmeal only (Ctr) or a diet with high total choline from 10% krill meal + 5% of fishmeal (KM) or a diet with high total choline similar to KM diet from a combination of 15% fishmeal and choline chloride supplementation, respectively (ChCl), in triplicate cages each. Fish were fed Ctr and ChCl diets throughout the trial period (Oct 2023-Dec 2024) while fish were fed KM diet from Oct 2023-May 2024 and then continued to be fed the Ctr diet for rest of the trial period. All three diets contained similar gross composition, designed to meet the known nutritional requirements of Atlantic salmon. Pellet size and dietary composition were adjusted to fish size. Fish were initially sampled in Oct 2023 after being fed a commercial diet and subsequently at four sampling time points in Dec 2023, Apr, Sep, and Dec 2024, after feeding the experimental diets. Possible effects of the diets on performance, general morphology, fish welfare, gut- and skin health, nutritional status and sensory quality were evaluated by a comprehensive analytical package.
Results and discussion
The fish grew from an initial body weight of 0.3 kg to an overall average body weight of 3.8 kg during the feeding trial, corresponding with an overall thermal growth coefficient of 3.44 from Oct 2023-Jan 2024. The body weight of the fish was affected by both the diet and time. Fish fed the diet supplemented with choline from krill meal (KM) as main source or fish meal and choline chloride (ChCl ) in the diet showed generally lower growth compared to the fish fed the control diet with choline supplied only from fish meal. The final weights of Ctr , KM and ChCl groups were 4.1, 3.6 and 3.7 kg respectively in the last sampling point in Dec 2024. Feed conversion ratio and average mortalities for the trial period did not differ significantly among diet groups. Average mortalities were 3.8% in Ctr , 4.1% in KM and 3.5% in ChCl groups, mainly due to infections caused by Parvicapsula pseudobranchicola.
Occurrence of skin ulcers was low across the feeding groups, with no significant difference due to dietary treatment. On the contrary, supplementation of KM or ChCl showed indications of reduced welfare in terms of increased scale loss (ChCl ), eye bleeding and higher scores for red belly (both ChCl and KM) in Sept 2024. Skin health, as evaluated by blood plasma antibody levels, skin morphometrics and transcriptomics showed generally few clear effects of diet. KM supplementation seemed to enhance the integumentary health of Atlantic salmon; however, this improvement was not sustained after supplementation ended.
With regards to sensory quality, h igh inclusion level of choline from fish meal and choline chloride showed positive effects on fillet yield, however there was no effect on fillet color. Some negative effects with reference to melanin deposition, gaping, and texture, were observed. The fatty acid composition of the muscle generally reflected the type of feed treatment for each group, which was expected.
The prevalence and severity of intestinal steatosis symptoms were relatively low in all three diet groups during the period Oct 2023-Sept 2024, indicating that the choline requirement was met for most of the fish during this period. In contrast, moderate to marked intestinal steatosis was observed in most of the fish at the last sampling time point in Dec 2024. No statistically significant differences among diet groups were registered, but there was a trend towards ChCl-fed fish (having the highest dietary choline levels) displaying less severe steatosis symptoms. In this regard, it should be mentioned that analysed choline levels varied less than originally formulated between Ctr, KM, and ChCl fish groups. Dietary choline levels in the diets that were fed from April 2024 and throughout the remaining part of the trial, were 3.1 and 3.4 g/kg for the Ctr and ChCl diets, respectively. These levels fall within the range of the estimated choline requirement of 3.4 g/kg for post-smolt salmon of about 0.5 kg when fed a diet with 29% lipid [1]. However, the lipid level of the current diets was higher, about 34%. As increasing dietary lipid level has been shown to increase severity of steatosis, and thereby the choline requirement [2], it is possible that the choline requirement was not fully met in the latter part of the present trial. Further studies with greater contrasts in dietary choline levels are needed to better understand choline requirements in salmon under varying production conditions.
Overall, this study found that 11% inclusion of krill meal replacing fish meal from a diet containing 16% fish meal had no positive effect on growth, feed utilization, overall mortality and welfare, skin and gut mucosal health, or fillet quality.
References
1. Hansen AKG, Kortner TM, Denstadli V, Måsøval K, Björkhem I, Grav HJ, 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 2020, 123(10):1081-1093.
2. Siciliani D, Kortner TM, Berge GM, Hansen AK, Krogdahl Å: Effects of dietary lipid level and environmental temperature on lipid metabolism in the intestine and liver, and choline requirement in Atlantic salmon (Salmo salar L) parr. J Nutr Sci 2023, 12:e61.