Adult salmon lice look like small, flattened, oval crustaceans attached to a fish’s skin. Adult females are usually the easiest to recognize: they are about 10–18 mm long and may have two trailing egg strings. Early larval stages can be nearly translucent and far smaller, so they may look like tiny pale specks—or be difficult to see at all.
What adult sea lice look like
“Sea lice” is a common name for parasitic copepods that includes species in the genera Lepeophtheirus and Caligus. Salmon lice, Lepeophtheirus salmonis, are a well-described example, but not every sea-louse species has exactly the same appearance. The Scottish Government describes adult sea-lice stages as highly visible on infested fish.
An adult salmon louse has a broad, rounded, flattened front section and is attached to the fish’s skin. Adult females are generally larger than males and have a more prominent rear segment. A female may carry a pair of egg strings trailing behind her body. The Scottish Government Marine Directorate gives adult female salmon lice as 10–18 mm long; their egg strings can reach 2 cm and contain up to 1,200 eggs. These measurements apply to L. salmonis, not every species called a sea louse.
How appearance changes by life stage
| Stage | What it may look like | How easy it is to see |
|---|---|---|
| Nauplius larva | Almost translucent and tiny; the Marine Institute gives a length of about 0.5–0.6 mm. | Hard to spot without close inspection. |
| Copepodid and chalimus | Small early stages attached to the fish. | Can be difficult to see; close inspection and good light help. |
| Pre-adult and adult | Larger, more developed lice; adults have a broad, flattened body. Adult females may have paired egg strings. | Adult stages are more visible. Male and female lice cannot be distinguished by naked eye in nauplius, copepodid, and chalimus stages; visual sex determination becomes possible from the first pre-adult stage. |
The stage descriptions and visibility guidance are based on the Marine Institute’s sea-lice information, a Scottish Government technical guide, and the Scottish Government’s 2025 cleaner-fish report. A quick glance can miss small stages even when adult lice would be conspicuous.
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Where to look on a salmon
Inspect the head and neck, the upper or dorsal region, and the skin around fin bases. The Scottish Government factsheet notes whitish spots on dorsal areas, particularly the head and neck, and identifies the bases of dorsal fins as affected areas. A technical guide also notes that mobile adult females are often found along the midline behind the dorsal, adipose, and anal fins. Use close inspection and good light when looking for small stages.
Do spots or sores prove that a fish has sea lice?
No. Whitish spots, skin erosion, or open wounds may be consistent with sea-lice feeding, especially when parasite numbers are high, but a mark on the skin alone does not confirm the cause. Look for the parasite itself; the Scottish Government says precise identification can be achieved by examining parasites under a microscope.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Can you identify the exact kind of sea louse by sight?
Appearance can suggest that a visible parasite is a sea louse, and larger adult stages are easier to inspect than tiny larvae. However, visual observation alone may not establish the exact species. Microscopic examination may be needed for precise identification, particularly when distinguishing among species or confirming a suspected parasite.
This description concerns parasites of fish, especially salmonids. It does not identify a particular fish from a photograph or establish the severity of an infestation.
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