© Nofima
New scientific findings highlight the early immune response as the key to natural sea lice resistance in salmon.
Sea lice remain one of the most persistent challenges in salmon farming. Despite their small size and harmless appearance, they feed on the skin and mucus of fish, causing health and welfare issues and requiring repeated treatment. Mechanical and freshwater treatments for removal are commonly used, but they are stressful for the fish and costly for producers. Not all salmon are equally affected by lice. Pacific Ocean species such as coho and pink salmon often carry fewer lice than Atlantic salmon. This difference has been observed for years, but the biological reasons behind it have not been fully understood.
In the project named CrispResist, we are a team of scientists who aim to better understand what makes coho salmon naturally more effective at resisting lice and how we can use this knowledge to improve resistance in Atlantic salmon.
Mucous layer thickness matters in four salmon species
As part of a controlled trial in Canada, we exposed four salmon species (Atlantic, coho, chum, and pink) to salmon lice and observed the early stages of infestation. When we measured mucous cell density, we found that chum and pink salmon had more mucous cells than Atlantic salmon. Although mucus may seem like a small detail, it plays an important role in protecting fish from parasites. In fact, during the trial, lice rarely attached to chum and pink salmon unless the fish were sedated. This supports the idea that a thicker mucous layer helps prevent lice from settling.
Read the full publication here.
Quick response
Coho salmon presented a different pattern. Lice attached readily, as in Atlantic salmon, but then “disappeared” within a few days. To understand why, we examined the lice attachment site, which is the point where the parasite and host first interact. Here we found that coho salmon mount a rapid and localised immune response. Within 24 to 48 hours, neutrophils (a type of immune cell) accumulated at the site of attachment, triggering inflammation that likely helps to remove the lice. To verify the neutrophil response, we used a combination of methods: histology, proteomics, and spatial transcriptomics. These complementary tools allowed us to study the attachment site from different angles: cell structure, protein content, and gene expression. The results consistently pointed to a fast, localised immune defence against the lice at the point of attachment in coho salmon.
This study builds on earlier work from our CrispResist project team, including a research paper published in 2024 by Salisbury et al., which identified keratinocyte hyperplasia, a thickening of the outer skin layer, in coho salmon following lice exposure. That study mapped out the changes in gene expression in each of the coho cell types present at the point of lice attachment and how this leads to a structural response, while the current findings highlight earlier immune components that may help initiate those changes. The latest discoveries complement and add to our earlier findings. All methods have their limitations, but by combining them, and drawing on the diverse knowledge and skills of the collaborators in the CrispResist project we have now gained a more complete understanding of how lice resistance works.
© Lene Sveen
Impact for future battle against sea lice
By identifying the early immune markers and protective skin traits that give coho salmon their resistance, we have generated knowledge that could be used to improve selective breeding in Atlantic salmon. It may also inform the development of new treatments, or vaccines that support the fish’s natural defences. While more work is needed, this study is a step toward a more targeted, biology-based approach to sea lice management, one that builds on what we can learn from naturally resistant species.
Improving the early immune response of Atlantic salmon so that lice are rapidly rejected or killed after infection would provide a more natural, ethical and likely more effective way of suppressing the numbers of lice carried by fish than current treatments used by salmon industries around the world. By preventing lice from maturing and reproducing on fish it could also significantly reduce the number and spread of sea lice in our coastal environments. It seems likely that lice at early stages of development when they first attach to fish have little effect on the health and welfare of salmon, but this is an area that needs further ethical and biological research. Our hope is that the knowledge from this project can help improve the immune response, and that together with other preventative measures, we can significantly suppress lice burdens and improve fish welfare.
The CrispResist project is funded by the Norwegian Seafood Research Fund (FHF). This is a collaboration between Nofima, The University of Edinburgh, University of Prince Edward Island, University of Stirling, Bigelow Laboratory of Ocean Sciences, University of Bergen, University of Gothenburg, Rothamsted Research, Institute of Marine Research, Deakin University, Benchmark Genetics, Mowi and Salmar.
