Impact of Novel Omega-3-Rich Oils on Atlantic Salmon (Salmo salar L.) Health

Abstract

This thesis evaluated novel omega-3 (n-3)-rich alternative oils as replacements for traditional fish oil (FO) in aquafeeds including genetically modified (GM) camelina oil rich in eicosapentaenoic (EPA, 20:5n-3) and/or docosahexaenoic (DHA, 22:6n-3) acids (EPA-rich, ECO; and EPA+DHA-rich, EDCO); GM-rapeseed oil (DHA-rich, DRO); and Microbial oil (MO), alongside traditional fish oils (Northern and Southern Hemisphere, NHFO and SHFO, respectively), krill oil (KO) and a terrestrial control oil (sunflower; SO), all within a unified experimental design spanning critical phases of the Atlantic salmon (Salmo salar) life cycle. The first experimental chapter showed that gill fatty acid (FA) profiles were strongly diet-imprinted, with selective retention of long-chain polyunsaturated fatty acids (LC-PUFA) in low supply groups evident across pre- smoltification, the smoltification window, and through the seawater (SW) challenge. In contrast, smoltification and SW transition markedly induced lipid-class-level remodelling (a neutral-to-polar shift). The smoltification index showed a transient diet effect at mid-smoltification (EDCO, NHFO, DRO highest; KO lowest) that largely converged by late smoltification. Novel n-3 LC-PUFA-rich oils—particularly EDCO and MO—supported smoltification outcomes comparable to FO and superior to SO, without compromising gill lipid homeostasis or osmoregulatory performance. The second experimental chapter investigated diet effects on head-kidney lipid profiles, circulating lipid inflammatory mediators (LIM) dynamics in response to SW transition, and transcriptomic profiling of the head kidney at late smoltification. Head kidney FA profiles mirrored diets. Despite low dietary DHA and polar lipids (PL), ECO and SO salmon showed relative enrichment of head kidney PL, and enhanced DHA retention. LIM profiles were dictated by diet, whereas the SW challenge had modest effects. Transcriptomic data showed that in SO (vs. MO), acyl-CoA synthetase long-chain family member genes (acsl3 and acsl4) were upregulated, consistent with a LC-PUFA retention/partitioning mechanism. Overall, appropriately formulated feeds with novel n-3 oils maintained lipid homeostasis and transcriptomic stability at late smoltification. The third experimental chapter explored how novel oils impacted Atlantic salmon response to sea-lice (Lepeophtheirus salmonis) challenge at the grow-out stage. MO- and ECO-fed salmon had significantly lower lice burdens than DRO at 150 degree-days (dd) post challenge. Lice challenge drove a LIM plasticity signature, characterized by increased EPA:DHA-derived LIM in all treatments except DRO-fed salmon. Skin gene expression differences at 150 dd were modest. Diet shaped the skin-mucus microbiome: MO tended toward higher alpha diversity; SO showed predicted enrichment for DNA repair, motility, oxidative phosphorylation functions, whereas MO and ECO were enriched for butyrate metabolism. Results suggest that novel n-3 oils such as MO and ECO may enhance resistance to parasitic infestation, likely through coordinated effects on skin FA composition, EPA-biased LIM profiles, and a supportive skin microbiome. Together these findings show that diet shapes membrane FA composition and circulating LIM profiles; SW transition had limited impact, whereas lice challenge induced EPA-biased mediator plasticity. Overall, novel oils (particularly EDCO and MO) can replace FO in aquafeeds without compromising smoltification readiness, membrane homeostasis, or parasitic resistance, provided EPA+DHA requirements are met and balance is maintained.