# Growth, Health and Physiological Responses of Freshwater-Reared Atlantic Salmon (Salmo salar) Fed Graded Dietary Lipid Levels

## TL;DR

This study finds that Atlantic salmon raised in freshwater don't need high-lipid diets, which can lower costs and improve aquaculture sustainability.

## Contribution

The study identifies 16% dietary lipid as optimal for freshwater Atlantic salmon, challenging the need for high-lipid diets in this rearing context.

## Key findings

- Salmon fed 16% lipid diets showed the best growth and feed efficiency.
- Lower lipid diets (14%) led to reduced antioxidant activity and higher immune-related gene expression.
- Transcriptomic analysis revealed 2117 differentially expressed genes between 14% and 16% lipid diets.

## Abstract

Atlantic salmon require large amounts of energy during smoltification and seawater transfer, processes that involve major physiological and metabolic adjustments. To meet these energy demands, high-lipid diets are widely used in Atlantic salmon aquaculture. However, excessive dietary lipid intake has been associated with negative effects, including metabolic stress, abnormal lipid accumulation, and impaired immune responses. In recent years, Atlantic salmon have been increasingly reared entirely in freshwater without undergoing seawater transfer, raising questions about whether high-lipid diets are still necessary under these conditions. In this study, we evaluated the optimal dietary lipid level for Atlantic salmon reared exclusively in freshwater by assessing growth performance, antioxidant capacity, metabolic responses, and liver gene expression. Salmon were fed diets containing graded lipid levels (14–20%) for 12 weeks to evaluate their effects on growth performance, antioxidant enzyme activity, hematological parameters, and gene expression profiles. These findings demonstrate that high dietary lipid levels are not required for freshwater-reared Atlantic salmon. Optimizing lipid inclusion in freshwater salmon feeds may reduce feed costs and support more sustainable salmon aquaculture.

This study evaluated the optimal dietary lipid level for Atlantic salmon (Salmo salar) reared in freshwater, aiming to provide foundational knowledge for the development of cost-effective and nutritionally balanced aquafeeds. Four experimental diets were formulated to contain comparable crude protein levels (47%) but graded lipid levels of 14% (L14), 16% (L16), 18% (L18), and 20% (L20), and were fed to salmon with an initial mean body weight of 241.5 ± 9.7 g during a 12-week feeding trial. Fish in the L16 group exhibited the highest weight gain (WG) and feed efficiency (FE), whereas those in the L14 group showed significantly reduced growth performance. Antioxidant analysis revealed that glutathione peroxidase (GPx) activity was lowest in the L14 group (p < 0.05), while plasma glucose concentration was minimized in the L16 group (p < 0.05). Transcriptomic profiling of liver tissue from the L14 and L16 groups identified 2117 differentially expressed genes (DEGs). Genes associated with lipid metabolism were more highly expressed in the L16 group, whereas immune- and inflammation-related genes were upregulated in the L14 group. These findings suggest that a dietary lipid level of approximately 16% is most favorable for promoting growth, metabolic stability, and overall health in freshwater-reared Atlantic salmon, thereby providing practical guidance for optimizing feed formulation and improving the economic efficiency of freshwater salmon aquaculture.