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Science & Nature • Marine Biology

Sea lice

Sea lice are marine ectoparasitic copepods in the family Caligidae that live on fish, especially salmonids.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 26, 2026|2 min read| Fact-checked

Sea lice are marine ectoparasitic copepods in the family Caligidae that live on fish, especially salmonids. They feed on mucus, skin, and sometimes blood, and are an important parasite in marine aquaculture and wild salmon ecology. The term can also be used informally for other marine biting or stinging organisms, so context matters.

Classification and biology

Sea lice are copepods in the family Caligidae within the order Siphonostomatoida. They are marine ectoparasites that use specialized mouthparts to feed on host tissue and fluids. Sea lice are crustacean parasites, not insects. The best-known species is the salmon louse, Lepeophtheirus salmonis.

Life cycle

Their life cycle includes free-swimming larval stages followed by attached juvenile stages on the host and mobile pre-adult and adult stages. This combination of planktonic and parasitic stages helps them spread in marine environments.

Hosts and distribution

Sea lice occur in marine waters worldwide and infect many fish species. The species most associated with salmon are Lepeophtheirus salmonis and several Caligus species, including C. elongatus and C. clemensi in some regions. They are parasites of wild and farmed salmon.

Economic and ecological impact

Sea lice are a major parasite problem in salmon farming because infestations can reduce growth and welfare and require treatment. They are also discussed in relation to possible impacts on wild salmonid populations. In Canada, sea lice are significant in salmon aquaculture, especially in Atlantic and Pacific coastal regions, where they are monitored and managed by fisheries and aquaculture authorities.

Key facts

  • Sea lice are crustacean parasites, not insects.
  • They are ectoparasites, meaning they live on the outside of the host.
  • The best-known species is the salmon louse, Lepeophtheirus salmonis.
  • Sea lice can damage fish skin and mucus and may contribute to secondary infections.
  • They are a major concern in salmon aquaculture and have been studied for effects on wild salmon populations.
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Canadian context

Sea lice are significant in Canadian salmon aquaculture, especially in Atlantic and Pacific coastal regions, where they are monitored and managed by fisheries and aquaculture authorities.

Frequently asked questions

What are sea lice?
Sea lice are small parasitic copepod crustaceans that live on marine fish, especially salmonids.
Are sea lice insects?
No. They are crustaceans, not insects.
Do sea lice affect humans?
The fish parasites called sea lice do not parasitize humans. Human skin irritation from 'sea lice' usually refers to other organisms, such as jellyfish larvae.
What do sea lice eat?
They feed on mucus, skin, and sometimes blood or other tissues from their fish hosts.
Why are sea lice important in aquaculture?
Infestations can harm fish health, reduce growth, and increase treatment costs in salmon farming.
What is the salmon louse?
The salmon louse is Lepeophtheirus salmonis, a sea louse that primarily parasitizes salmonids.

References

  1. Oxford Academic — https://academic.oup.com/icesjms/article/63/7/1304/758933
    Supports: Sea lice life cycle includes attached juveniles and mobile pre-adult and adult stages.
  2. Alaska Department of Fish and Game — https://www.adfg.alaska.gov/index.cfm?adfg=wildlifenews.view_article&articles_id=388
    Supports: Sea lice are tiny crustaceans found on salmon; mentions Caligus elongatus and Lepeophtheirus salmonis.
  3. Marine Institute — https://marine.ie/site-area/areas-activity/aquaculture/sea-lice/sea-lice
    Supports: Sea lice are ectoparasitic copepods in the family Caligidae; describes common species and effects on salmon.
  4. Fraser Institute — https://www.fraserinstitute.org/sites/default/files/SeaLiceScienceBehindHype.pdf
    Supports: Discusses sea lice life stages and temperature/salinity effects in British Columbia context.
  5. Government of Canada — https://publications.gc.ca/collections/collection_2015/mpo-dfo/Fs70-5-2014-019-eng.pdf
    Supports: Canadian report on sea lice biology and monitoring in western and eastern Canada.
  6. Royal Society — https://royalsocietypublishing.org/doi/10.1098/rspb.2009.0771
    Supports: Describes sea lice life cycle and discusses sources of outbreaks affecting salmonids.
  7. Natural History Museum — https://www.nhm.ac.uk/discover/the-problem-of-sea-lice-in-salmon-farms.html
    Supports: Explains sea lice as copepod crustaceans adapted to salmon and their significance in salmon farms.
  8. PubMed — https://pubmed.ncbi.nlm.nih.gov/16920027/
    Supports: Review of ecology and economic impact of sea lice on farmed and wild fish.