by Richard Liu, Global Project Manager, Kemin AquaScience
Atlantic salmon are highly efficient at using dietary proteins and lipids for energy production. Historically, salmon feeds included protein well beyond the animals’ amino acid requirements, with surplus protein catabolised to meet energy demands. However, in recent decades, feed formulations have moved towards reduced protein levels. This shift aims to reduce reliance on expensive marine-derived proteins, such as fishmeal, and instead take advantage of the protein-sparing effect of lipids.
At the same time, lipid inclusion has increased to offset the reduction in protein-derived energy. Plant oils are being used more often in place of marine lipids, thanks to economic and sustainability advantages. As seen in Figure 1, in 2020 Norwegian Atlantic salmon feeds included 40.5 percent plant proteins and 20.1 percent plant oils, compared with 12.1 percent marine proteins and 10.3 percent marine lipids.
However, high inclusion of plant oils has been linked with hepatic lipidosis in Atlantic salmon, caused by disruptions in lipid metabolism. Rapeseed oil—rich in unsaturated fatty acids but also containing phytosterols—may interfere with cholesterol homeostasis by competitively inhibiting intestinal cholesterol absorption. Phytosterols, which have a structure similar to cholesterol, may reduce the bioavailability of endogenous cholesterol. This impairs lipoprotein synthesis and promotes abnormal lipid deposition in liver cells. The accumulation of lipids in the liver harms liver health, increasing metabolic stress and reducing growth performance.
As plant oils will remain essential for meeting energy needs in intensive aquaculture, it is critical to address the issue of lipidosis. Lysophospholipids (LPLs), amphiphilic molecules that enhance lipid emulsification and absorption, may offer a potential solution. LPLs promote micelle formation in the intestinal lumen, improving the transport of fatty acids across enterocytes and enhancing the efficiency of lipid use.
Kemin AquaScience, a division of Kemin Industries dedicated to sustainable aquaculture solutions, is focused on developing eco-friendly innovations for aquafeed and farming. Its new product, Aquatria™, is based on research into lipid metabolism and liver health across various species. This formulation contains four bioactive ingredients specifically designed to enhance nutrient digestion, absorption and utilisation in key aquaculture species, including fish and crustaceans. In vivo trials have shown that the product improves growth performance and reduces lipid accumulation in the liver, supporting more sustainable aquaculture practices.
Trial results in Chile
An 81-day trial at Temuco Catholic University in Chile evaluated the efficacy of Aquatria in Atlantic salmon (Salmo salar) pre-smolt juveniles (initial weight: 19.8g ± 0.2g). The study assessed growth performance, feed use and liver health under varying dietary lipid and energy levels. Three isoproteic diets (49 percent crude protein) were formulated: a positive control (PC; 26.1 percent lipids, 24.0 MJ/kg gross energy), a negative control (NC; 20.3 percent lipids, 22.9 MJ/kg), and an experimental diet (NC + 1000ppm Aquatria).
Despite the reduced lipid and energy content, this supplementation maintained growth performance comparable to the PC group. Final weights in the supplemented group were similar to those of PC (no significant difference), while the NC group had numerically lower weight gain. Specific growth rate (SGR) was highest in the PC group (0.947), closely followed by the supplimentedgroup (0.945), with NC lower at 0.903. Feed conversion ratio (FCR) significantly improved in the Aquatria group (1.51), compared with 1.55 in PC and 1.63 in NC, indicating improved metabolic efficiency of lipid use with Aquatria supplementation.
Improved visceral fat index
The hepatosomatic and viscerosomatic index represent the weight of the liver and viscera respectively, relative to the total body weight of the fish. The viscerosomatic index (VSI) is a useful measure of adipose tissue deposition in the viscera. VSI has been shown to increase with high levels of vegetable oil and protein in the diet. Salmon farmers aim for a lower VSI, as this usually corresponds with a higher fillet yield.
Results from the present study indicated no difference in hepatosomatic index (HSI) among the groups. However, VSI in the positive control group was significantly higher than in the Aquatria group. In other words, using this supliment in a diet with reduced energy and lipid content reduced visceral fat accumulation without negatively affecting liver health.
The results demonstrate that this supplementation can overcome the growth-limiting effects of low-energy diets, achieving FCR levels equivalent to high-energy diets. The product’s bioactive components likely improve lipid emulsification and absorption in the intestines, leading to better energy utilisation. Furthermore, the reduced VSI in the Aquatria group suggests more efficient use of lipids, rather than deposition in visceral tissue.
Histopathology results
Liver histopathology and liver condition scoring were carried out in this study. The following grading system was used for hepatic lipidosis:
- Grade 0: No vacuoles with defined borders in the cytoplasm, and no extension.
- Grade 1: Few microvacuoles, mostly with defined edges, maintaining hepatocyte architecture. Multifocal extension with up to 25 percent fat degeneration.
- Grade 2: Some micro- and macro-vacuoles with defined borders, greater hepatocyte involvement. Multifocal extension with 25–50 percent fat degeneration.
- Grade 3: Moderate presence of macro- and microvacuoles with defined borders. Nucleus slightly displaced to the cell periphery. Diffuse extension with 50–75 percent fat degeneration.
- Grade 4: Severe presence of macro- and microvacuoles. Nucleus flattened and displaced to the cell periphery, with loss of cell structure. Diffuse extension with 75–100 percent fat degeneration.
In the present study, salmon fed a high-rapeseed oil diet (positive control, PC) showed severe hepatic lipidosis. Fifty-six percent of fish were scored at grade 4 (severe), and 22 percent were scored at grade two or three. Reducing rapeseed oil levels (negative control, NC) improved liver condition scores but negatively affected growth (FCR: 1.63). Notably, supplementation in the energy- and lipid-reduced diet-maintained growth performance equal to PC and improved FCR (1.51 vs. 1.55), while also enhancing liver health. In the Aquatria group, 67 percent of fish were scored at grade 1 (mild), and 33 percent at grade 0 (normal).
A sustainable solution
Plant oils are being used more frequently to address the limited availability and rising cost of fish oil. However, they introduce phytosterols that disrupt lipid metabolism, contributing to hepatic lipid accumulation and steatosis. The product addresses this challenge by improving lipid emulsification, micelle formation and absorption in the intestines, enhancing energy use from plant oils.
These findings demonstrate Aquatria’s dual functionality: first, as a nutrient efficiency enhancer, it supports growth even under energy-restricted conditions; second, as a liver health promoter, it reduces abnormal lipid deposition. In conclusion, this product enables the formulation of energy-reduced feeds without compromising growth, offering a sustainable strategy to reduce dietary lipid levels while limiting visceral fat accumulation and supporting liver health in farmed Atlantic salmon.
Published in the May 2025 issue of International Aquafeed & Fish Farming Technology
