Varying dietary EPA/DHA ratios influence growth and viral disease response in Atlantic Salmon (Salmo salar L.) fry

Abstract

Balancing dietary n-3 long-chain polyunsaturated fatty acids (LC-PUFA) is essential for optimizing growth and disease resilience in Atlantic salmon and will become an increasing challenge when implementing novel sources of EPA and DHA, such as GM oil and microalgae oils. To study this, Atlantic salmon fry (start weight 0.15 g) were pre-fed diets differing in EPA/DHA ratio (0.2E/D, 0.9E/D, 2.2E/D) while maintaining constant total EPA+DHA during four weeks. Following this, but kept on the same dietary treatments, the fry were challenged with wild-type or attenuated salmonid alphavirus (SAV-WT and SAV-Att, respectively). The duration of the disease challenge was up to 28 days, with final weights of the fish at the end of the trial at ~1.1g. There was a clear decrease in fatty acid stores of the SAV-WT infection, not observed under the SAV-Att infection. We demonstrate a preferential decrease in mono-unsaturated fatty acids during the SAV-WT infection, preserving the n-3 LC-PUFA content in the tissues. Increasing the dietary EPA/DHA ratio significantly reduced growth performance, indicating that dietary balance of EPA to DHA influences growth in start feeding salmon fry. Following infection, dietary effects were outcome dependent. Fish fed with the low EPA/DHA diet (0.2 E/D) showed overall higher overall viral loads, but improved histopathology at the end of the trial. In contrast, fish fed the high EPA/DHA diet (2.2 E/D) exhibited signs of improved histopathology at the end of the trial yet experienced the highest (non-significant) mortality after SAV-WT challenge. Fish receiving the balanced diet (0.9 E/D) showed poorer histopathological scores at the finalization of the trial, but did not display elevated mortality. These findings demonstrate that dietary EPA/DHA ratio differentially modulates growth and disease outcome, exerting distinct and sometimes opposing effects on viral load, tissue inflammation, and survival, highlighting the complexity of optimizing n-3 LC-PUFA balance to support both performance and antiviral resilience in Atlantic salmon.