Introduction
This study was conducted within a broader project aimed at improving the nutritional quality of marine and freshwater fish fillets by enhancing nutrient transfer from raw materials to the final product. The ultimate goal is to deliver fish products with optimal nutritional value that meet consumer expectations.
Omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFAs), especially eicosapentaenoic and docosahexaenoic fatty acids (EPA and DHA), are essential nutrients in the human diet and are mainly obtained from the consumption of fish products. In the present study, a microbial oil rich in EPA and DHA was investigated as a feed additive for rainbow trout (Oncorhynchus mykiss). In addition to assessing the accumulation of these fatty acids in the fillet, we evaluated their effects on fish physiology and health. .
Methods
Two different inclusion levels of microbial oil were investigated in a 90-day feeding trial. At the beginning of the trial, 810 juvenile rainbow trout were randomly distributed into nine 500 L fiberglass tanks. Each tank was randomly assigned to either the control diet or one of the microbial oil diets (3 tanks per group). At the end of the feeding trial, fillet samples were collected to assess their fatty acid profile. Gas chromatography coupled with mass spectrometry was employed for the purpose. Moreover, the composition and functionality of the gut microbiota were analyzed using high-throughput sequencing techniques previously described in publications by our group (Hasan et al., 2024; Rimoldi et al., 2025; Kalemi et al., 2025).
Results
Dietary microbial oil at higher inclusion levels led to an enrichment of n-3 LC-PUFAs, especially EPA and DHA, in the fillets of rainbow trout. Consistent with this increase, the ratio of omega-3 to omega-6 PUFAs was higher in the rainbow trout fillets fed the experimental diets. Overall, the fatty acid profile in the fillets matched that of the diets, as previously observed in a similar study (Terova et al., 2021). As for the gut microbiota status, subtle qualitative shifts were detected at the highest inclusion level, but the overall microbial diversity, dominant taxa and predicted functional pathways remained largely unchanged between the feeding groups.
Conclusions
This study highlights the importance of aquafeed not only for fish growth and health, but also for the nutritional quality of the final product. In particular, it emphasizes the role of feed formulations in shaping the nutritional quality of farmed fish for human consumption. Our results showed that the addition of microbial oil to rainbow trout feed increases the content of omega-3 PUFAs in the fillet without any negative effect on the health of the fish, particularly on the status of the microbiota. Most importantly, these results provide a basis for the next phases of the I-FISH project, where different additives will be included in fish feed to improve the nutritional value of the final product, i.e., the fish fillet. Understanding how different nutritional strategies affect gut microbiota, metabolic outcomes and fillet composition is essential for the development of more sustainable and nutritionally valuable aquaculture products.
Acknowledgments
Research was supported by the project: I-FISH. Protocollo nr: 414352—del 07/12/2023—AOO_IAI—AOO_Incentivi Fondo per la Crescita Sostenibile-Accordi per l'innovazione di cui al D.M. 31 Dicembre 2021 e D.D. 14 Novembre 2022.
References
Hasan, I., Rimoldi, S., Chiofalo, B., Oteri, M., Antonini, M., Armone, R., Kalemi, V., Gasco, L., & Terova, G. (2024). Effects of poultry by-product meal and complete replacement of fish oil with alternative oils on growth performance and gut health of rainbow trout (Oncorhynchus mykiss): a FEEDNETICSTM validation study. BMC Veterinary Research, 20(1). https://doi.org/10.1186/s12917-024-04324-0
Kalemi, V., Rimoldi, S., Costa, R. S., Basto, A., Monteiro, M., Terova, G., Valente, L. M. P. (2025). Replacing fishmeal with an insect meal blend: Implications for intestinal microbiota in European seabass. Aquaculture Reports, 43. https://doi.org/10.1016/j.aqrep.2025.102939
Rimoldi, S., Quiroz, K. F., Kalemi, V., McMillan, S., Stubhaug, I., Martinez-Rubio, L., Betancor, M. B., & Terova, G. (2025). Interactions between nutritional programming, genotype, and gut microbiota in Atlantic salmon: Long-term effects on gut microbiota, fish growth and feed efficiency. Aquaculture, 596. https://doi.org/10.1016/j.aquaculture.2024.741813
Terova, G., Moroni, F., Antonini, M., Bertacchi, S., Pesciaroli, C., Branduardi, P., Labra, M., Porro, D., Ceccotti, C., & Rimoldi, S. (2021). Using Glycerol to Produce European Sea Bass Feed With Oleaginous Microbial Biomass: Effects on Growth Performance, Filet Fatty Acid Profile, and FADS2 Gene Expression. Frontiers in Marine Science, 8. https://doi.org/10.3389/fmars.2021.715078