Introduction
Sea lice is one of the major problems for salmon farming operations, resulting in 2024 in over 3000 mechanical treatments that have the potential to negatively impact fish performance, fish welfare and farm benefits (Moldal et al., 2025). Industry efforts to decrease parasite pressure combine technical, medical and genetic solutions (Santigosa, 2025). Moreover, the use of nutritional approaches to optimize sea lice management is often considered as part of the toolbox. To demonstrate the benefits of a feed additive on sea lice attachment, in this trial we tested a control feed versus three experimental feeds containing a matrix-encapsulated phytogenic that is based on essential oils of oregano, thyme, and citrus (Digestarom®) at three inclusion levels (400, 800 and 1200 g/ton). The aim of the trial was to validate the following hypothesis: this feed additive can reduce the attachment of sea lice at adult stage whilst maintaining zootechnical performance.
Methods
Atlantic salmon post smolt (186g) were acclimated for 7 days in seawater tanks and then fed one of the dietary trial treatments for 45 days, using triplicate tanks per treatment. After this pre-challenge period, animals were challenged with sea lice (Lepeophtheirus salmonis ) using a flush. The trial continued for a further 53 days. At the end of the trial (day 105), sea lice counts were performed, in addition to zootechnical performance and welfare scoring. Samples were taken for the determination of blood biochemistry .
Results
Results show a significant decrease (70%) in the total number of sea lice with the two highest inclusion doses (800 and 1200 g/t). In addition, in these two treatments there was a concomitant increase in the percentage of fish without any lice in those specific treatments. Feed intake was not negatively impacted by the inclusion of the feed additive. Final body weight, FCR and SGR welfare scores were similar across the four treatments.
Conclusions
The role of feed additives in sea lice management has long been considered as part of the toolbox to reduce sea lice infestation in salmon farming operations. The results obtained in this trial show that the in-feed inclusion of Digesterom® at 800 and 1200g/ton had a significant effect on sea lice attachment, reducing the count by 70%. The findings from the present R&D work support the need for large-scale commercial testing of this feed additive as the next step toward full validation.
References
Moldal, T., Wiik-Nielsen, J., Oliveira, V.H.S., Svendsen, J.C., and Sommerset, I. Fiskehelserapporten 2024, Veterinærinstituttets rapportserie nr. 1a/2025, utgitt av Veterinærinstituttet 2025.
Santigosa, E. Skin, gill and gut robustness for protecting fish health and welfare. North Atlantic Seafood Forum, Bergen, 2025.