EPA alone cannot sustain the requirement for EPA + DHA in Atlantic salmon (Salmo Salar L.): Insights into the role of EPA:DHA ratio and varying EPA levels on growth and tissue fatty acid composition

Traditionally, the requirement for the long-chained n-3 fatty acids in Atlantic salmon has been based on the combined levels of EPA and DHA. However, it is now evident that EPA and DHA serve distinct, individual functions within the organism. Many potential fish oil replacements supplying EPA and DHA in salmon feeds have an imbalanced ratio of these fatty acids. The present study aimed to determine the extent to which EPA can replace dietary DHA as the source of essential n-3 fatty acids for Atlantic salmon. An EPA rich oil was used to replace fish oil or as an additional source of long-chained n-3 fatty acids, resulting in three diets with varying EPA:DHA ratios and EPA content. The results indicate that high levels of EPA alone are insufficient to meet the requirement for EPA + DHA in Atlantic salmon. This conclusion is supported by the significantly lower growth observed in fish fed a high EPA:DHA ratio compared to fish fed a balanced EPA:DHA (with the same total EPA + DHA) and similar feed intakes. Fish on the high-EPA/low-DHA diet exhibited increased endogenous DHA synthesis through elongation and desaturation but were unable to achieve the same level of DHA in the red blood cells and muscle tissue as observed in fish fed sufficient DHA. The increased energy expenditure due to higher DHA synthesis may explain the reduced growth. This finding is crucial for the industry, highlighting the importance of maintaining a balanced composition of EPA and DHA in feed formulations. • Results indicate that EPA and DHA are not directly interchangeable; both must be considered in diet formulation. • At a constant total EPA + DHA level, increasing the EPA:DHA ratio from 0.84 to 7.87, with low absolute DHA, significantly reduced growth in Atlantic salmon. • Increasing the dietary EPA:DHA ratio resulted in lower polar lipid DHA levels, indicating competition between EPA and DHA.