Sea lice

Sea lice are a significant problem in open seawater salmonid aquaculture. Sea lice attach to fish skin and feed on mucus, skin tissue, and blood, causing significant welfare problems. Different sea lice species pose varying levels of risk to fish. Heavy infestations cause physical damage, including skin lesions, scale loss, and open wounds, which create gateways for infection.

Regulations have different maximum levels of sea lice. These levels are not set based on farmed fish welfare, but on impacts to wild salmonid populations. Examples:

  • Scotland: 2.0 adult female lice per fish on average results in enhanced regulatory oversight.22
  • Norway: maximum of 0.5 adult female lice per fish, or below 0.2 in the period of salmonid out-migration.23
  • Ireland: must reduce sea lice levels when they reach 0.5 ovigerous female lice per fish during the sensitive period, or 2.0 adult female lice per fish.24
  • Canada: must take delousing actions when reaching an average of 3.0 motile sea lice per fish.25

It is difficult to provide a scoring method appropriate for rainbow trout production, as the welfare impact of sea lice depends on a number of variables, including:

  • The absolute lice count (number of lice per fish)
  • Relative intensity of the infestation (e.g., the number of lice per gram of body weight, or the number of lice per cm2 of body surface area)
  • The lice developmental stage (e.g., mobile preadult/adult stages cause more damage than the sessile chalimus stage)

Given the multi-factorial nature of the welfare impact of sea lice, paired with the practical difficulties of counting sea lice in the field (especially for earlier life stages), the thresholds for this indicator are based on damage attributable to sea lice only.

Level 0 Level 1 Level 2 Level 3

No skin damage

Associated small areas of dryness

Associated moderate skin damage (lesions, sores, scale loss, skin loss, patches of dryness)

Associated severe skin damage (lesions, sores, scale loss, skin loss, patches of dryness)
Protocol application

Assessment requires handling individual fish for reliable counting.

This indicator captures damage caused by sea lice infestation (e.g., damage located under attachment sites). Such damage may also appear under ‘wounds’ or ‘skin hemorrhaging’ indicators; this overlap is not a problem as the indicators will be interpreted together.

Calculate the distribution of scores across the thresholds within the sample.

Scoring notes
  • Damage should be interpreted together with recorded lice numbers, recent delousing, and rising damage/lice count prevalence.
  • Validation: The damage thresholds are based on expert opinion.
Key evidence
Source Summary of source Key result
Dalvin et al., 2020 Experimental study investigating histopathological changes associated with initial infestation of 80 copepodids in 112 ongrowing rainbow trout (330 g) in seawater laboratory conditions for up to 41 days post-infestation.

Fish were sampled at multiple time points post-infestation.		 At the early stages of infection, only minor epidermal erosions were observed. At 16 days post infection, at a mean of 32 preadult lice per fish, there was more evident tissue damage.

At 41 days post infection, a mean of 11 adult lice per fish was recorded. At this time, the adult female lice were fully mature and carrying their first set of egg strings. Up to this point, shallow ulcerations were detected where the louse mouth tube had contacted the skin. Other changes observed were a local depletion of mucus-producing goblet cells and immune cell infiltration, as well as moderate epidermal hyperplasia and edema.

No gross lesions with scale loss or hemorrhages were observed.
Wagner et al., 2008 Review article on the welfare impacts of sea lice infection on salmonids. Even low subclinical infection levels (e.g., 0.04 lice/g or 10 preadult/adult lice of both sexes per fish (citing Nolan et al., 1999)) were found to induce a stress response in Atlantic salmon. Higher levels, such as 0.2 lice/g, can lead to changes in host mucus, physical damage, and compromised immune function.

Infection levels of 0.08 lice/g were also found to show altered cardiac and swimming performance.

It is well recognized that high mobile sea lice loads (> 0.75 lice/g) can cause morbidity and death.
Supporting evidence
  • Barisic, Josip, Stuart Cannon, and Brian Quinn. “Cumulative Impact of Anti-Sea Lice Treatment (Azamethiphos) on Health Status of Rainbow Trout (Oncorhynchus Mykiss, Walbaum 1792) in Aquaculture.” Scientific Reports 9, no. 1 (November 7, 2019). https://doi.org/10.1038/s41598-019-52636-1.
  • Bui, S., T. Dempster, M. Remen, and F. Oppedal. “Effect of Ectoparasite Infestation Density and Life-History Stages on the Swimming Performance of Atlantic Salmon Salmo Salar.” Aquaculture Environment Interactions 8 (June 14, 2016): 387–95. https://doi.org/10.3354/aei00184.
  • Costello, Mark J. “Ecology of Sea Lice Parasitic on Farmed and Wild Fish.” Trends in Parasitology 22, no. 10 (October 2006): 475–83. https://doi.org/10.1016/j.pt.2006.08.006.
  • Dalvin, Sussie, Louise v. G. Jørgensen, Per W. Kania, Sindre Grotmol, Kurt Buchmann, and Aina-Cathrine Øvergård. “Rainbow Trout Oncorhynchus Mykiss Skin Responses to Salmon Louse Lepeophtheirus Salmonis: From Copepodid to Adult Stage.” Fish & Shellfish Immunology 103 (August 2020): 200–210. https://doi.org/10.1016/j.fsi.2020.05.014.
  • Fast, M. D., N. W. Ross, A. Mustafa, D. E. Sims, S. C. Johnson, G. A. Conboy, D. J. Speare, G. Johnson, and J. F. Burka. “Susceptibility of Rainbow Trout Oncorhynchus Mykiss, Atlantic Salmon Salmo Salar and Coho Salmon Oncorhynchus Kisutch to Experimental Infection with Sea Lice Lepeophtheirus Salmonis.” Diseases of Aquatic Organisms 52, no. 1 (2002): 57–68. https://doi.org/10.3354/dao052057.
  • Fisheries and Oceans Canada. “Regulating and Monitoring British Columbia’s Marine Finfish Aquaculture Facilities,” 2020. https://waves-vagues.dfo-mpo.gc.ca/library-bibliotheque/41055147.pdf.
  • Fjelldal, Per Gunnar, Tom J. Hansen, and Ørjan Karlsen. “Effects of Laboratory Salmon Louse Infection on Osmoregulation, Growth and Survival in Atlantic Salmon.” Edited by Steven Cooke. Conservation Physiology 8, no. 1 (January 1, 2020). https://doi.org/10.1093/conphys/coaa023.
  • Gjelland, Karl Øystein, Rosa Maria Serra-Llinares, Richard David Hedger, Pablo Arechavala-Lopez, Rune Nilsen, Bengt Finstad, Ingebrigt Uglem, Ove Tommy Skilbrei, and Pål Arne Bjørn. “Effects of Salmon Lice Infection on the Behaviour of Sea Trout in the Marine Phase.” Aquaculture Environment Interactions 5, no. 3 (July 9, 2014): 221–33. https://doi.org/10.3354/aei00105.
  • González, Margarita P., Sandra L. Marín, Melinka Mancilla, Hernán Cañon-Jones, and Luis Vargas-Chacoff. “Fin Erosion of Salmo Salar (Linnaeus 1758) Infested with the Parasite Caligus Rogercresseyi (Boxshall & Bravo 2000).” Animals 10, no. 7 (July 9, 2020): 1166. https://doi.org/10.3390/ani10071166.
  • Grimnes, A., and P. J. Jakobsen. “The Physiological Effects of Salmon Lice Infection on Post-Smolt of Atlantic Salmon.” Journal of Fish Biology 48, no. 6 (June 1996): 1179–94. https://doi.org/10.1111/j.1095-8649.1996.tb01813.x.
  • Ives, Stephen C., J. D. Armstrong, Catherine Collins, Meadhbh Moriarty, and Alexander G. Murray. “Salmon Lice Loads on Atlantic Salmon Smolts Associated with Reduced Welfare and Increased Population Mortalities.” Aquaculture Environment Interactions 15 (March 23, 2023): 73–83. https://doi.org/10.3354/aei00453.
  • Marine Scotland. “The Regulation of Sea Lice in Scotland.” Marine Scotland, 2021. https://www.gov.scot/binaries/content/documents/govscot/publications/factsheet/2019/11/marine-scotland-topic-sheets-aquaculture/documents/the-regulation-of-sea-lice-in-scotland-updated-june-2019/the-regulation-of-sea-lice-in-scotland-updated-june-2019/govscot%3Adocument/71%2BThe%2BRegulation%2Bof%2BSea%2BLice%2Bin%2BScotland%2B2021.pdf.
  • Mustafa, Ahmed, Chris MacWilliams, Nicole Fernandez, Kelly Matchett, Gary A Conboy, and John F Burka. “Effects of Sea Lice (Lepeophtheirus Salmonis Kröyer, 1837) Infestation on Macrophage Functions in Atlantic Salmon (Salmo Salar L.).” Fish & Shellfish Immunology 10, no. 1 (2000): 47–59. https://doi.org/10.1006/fsim.1999.0229.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. “Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare.” Nofima. Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. “Welfare Indicators for Farmed Atlantic Salmon: Tools for Assessing Fish Welfare.” Nofima, 2018.
  • Nolan, D. T., P. Reilly, and S. E. Wendelaar Bonga. “Infection with Low Numbers of the Sea Louse Lepeophtheirus Salmonis Induces Stress-Related Effects in Postsmolt Atlantic Salmon (Salmo Salar).” Canadian Journal of Fisheries and Aquatic Sciences 56, no. 6 (June 1, 1999): 947–59. https://doi.org/10.1139/f99-021.
  • Øverli, Øyvind, Janicke Nordgreen, Cecilie M. Mejdell, Andrew M. Janczak, Silje Kittilsen, Ida B. Johansen, and Tor E. Horsberg. “Ectoparasitic Sea Lice (Lepeophtheirus Salmonis) Affect Behavior and Brain Serotonergic Activity in Atlantic Salmon (Salmo Salar L.): Perspectives on Animal Welfare.” Physiology & Behavior 132 (June 2014): 44–50. https://doi.org/10.1016/j.physbeh.2014.04.031.
  • Poole, W. R., D. Nolan, and O. Tully. “Modelling the Effects of Capture and Sea Lice [Lepeophtheirus Salmonis (Krøyer)] Infestation on the Cortisol Stress Response in Trout.” Aquaculture Research 31, no. 11 (November 2000): 835–41. https://doi.org/10.1046/j.1365-2109.2000.00516.x.
  • Ruane, N. M, D. T Nolan, J Rotllant, J Costelloe, and S. E Wendelaar Bonga. “Experimental Exposure of Rainbow Trout Oncorhynchus Mykiss (Walbaum) to the Infective Stages of the Sea Louse Lepeophtheirus Salmonis (Krøyer) Influences the Physiological Response to an Acute Stressor.” Fish & Shellfish Immunology 10, no. 5 (July 2000): 451–63. https://doi.org/10.1006/fsim.1999.0260.
  • Thorstad, E. B., C. D. Todd, I. Uglem, P. A. Bjørn, P. G. Gargan, K. W. Vollset, E. Halttunen, S. Kålås, M. Berg, and B. Finstad. “Effects of Salmon Lice Lepeophtheirus Salmonis on Wild Sea Trout Salmo Trutta—a Literature Review.” Aquaculture Environment Interactions 7, no. 2 (August 20, 2015): 91–113. https://doi.org/10.3354/aei00142.
  • Wagner, G. N., and R. S. McKinley. “Anaemia and Salmonid Swimming Performance: The Potential Effects of Sub-Lethal Sea Lice Infection.” Journal of Fish Biology 64, no. 4 (March 26, 2004): 1027–38. https://doi.org/10.1111/j.1095-8649.2004.0368.x.
  • Wagner, Glenn N., Mark D. Fast, and Stewart C. Johnson. “Physiology and Immunology of Lepeophtheirus Salmonis Infections of Salmonids.” Trends in Parasitology 24, no. 4 (April 2008): 176–83. https://doi.org/10.1016/j.pt.2007.12.010.
  • Wells, Alan, Christal E. Grierson, Monique MacKenzie, Iain J. Russon, Helena Reinardy, Claire Middlemiss, Pål A. Bjørn, et al. “Physiological Effects of Simultaneous, Abrupt Seawater Entry and Sea Lice (Lepeophtheirus Salmonis) Infestation of Wild, Sea-Run Brown Trout (Salmo Trutta) Smolts.” Canadian Journal of Fisheries and Aquatic Sciences 63, no. 12 (December 1, 2006): 2809–21. https://doi.org/10.1139/f06-160.

  1. Marine Scotland. “The Regulation of Sea Lice in Scotland.” Marine Scotland, 2021. https://www.gov.scot/binaries/content/documents/govscot/publications/factsheet/2019/11/marine-scotland-topic-sheets-aquaculture/documents/the-regulation-of-sea-lice-in-scotland-updated-june-2019/the-regulation-of-sea-lice-in-scotland-updated-june-2019/govscot%3Adocument/71%2BThe%2BRegulation%2Bof%2BSea%2BLice%2Bin%2BScotland%2B2021.pdf.

  2. Nærings- og fiskeridepartementet [Ministry of Trade, Industry and Fisheries]. “Forskrift Om Bekjempelse Av Lakselus I Akvakulturanlegg [Regulation on the Control of Salmon Lice in Aquaculture Facilities],” 2012. https://lovdata.no/dokument/SF/forskrift/2012-12-05-1140.

  3. Department of the Marine and Natural Resources. “Monitoring Protocol No. 3: Offshore Finfish Farms - Sea Lice Monitoring and Control,” 2000.

  4. Fisheries and Oceans Canada. “Regulating and Monitoring British Columbia’s Marine Finfish Aquaculture Facilities,” 2020. https://waves-vagues.dfo-mpo.gc.ca/library-bibliotheque/41055147.pdf.