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Sea lice
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Sea lice

Sea lice are the most common parasite on farmed salmon, and the biggest health issue for the industry. For a number of years, oral and bath treatments have been used to combat sea lice. Our monitoring of sea lice shows that their numbers are increasing noticeably, and that in some cases they are developing resistance to the favoured treatment.

Sea lice belong to the copepod family, and are found naturally throughout the northern hemisphere.

Sea lice have affected salmon fishing for a long time, being first mentioned in the 17th century. The zoologist Henrik Nikolai Krøyer described the species and gave it the Latin name Lepeophtheirus salmonis in 1837.

Sea lice are host-specific, and depend on salmonids to complete their life cycle. The saltier the water, the more they thrive, and they fall off the salmon when the fish head up river. When lice are found on a salmon caught in a river, it is commonly seen as a sign that the fish recently entered the river, but laboratory experiments show that sea lice can remain on salmon for up to 14 days after entering fresh water.

Sea lice cause damage to salmonids by eating their mucus, skin tissue and blood. This paves the way for other problems such as bacterial or fungal infections, and also affects the osmotic balance of the fish.

Sea lice are today one of the most important causes of mortality in farmed salmonids. At a rough estimate, each year the Norwegian aquaculture industry loses around NOK 500 million as a result of direct losses, the cost of chemicals, extra work associated with delousing, weight loss due to stress, loss of fish, etc. These days there are limits on the number of sea lice permitted on salmon in fish farms, and the Norwegian Food Safety Authority has clear guidelines on how to combat the parasite.
Increases in sea lice numbers pose a threat to wild salmon. Although the main impact of sea lice is reduced growth, in the worst case scenario, reduced growth in vulnerable wild populations can harm their reproductive potential.

Facts about sea lice

Latin name: Lepeophtheirus salmonis
Distribution: occur naturally in Norwegian waters. Their numbers have risen significantly in parallel with the growth of the aquaculture industry.
Biology: sea lice are parasites with eight life stages, three of which are free-swimming, two of which are stationary and three of which are mobile. They attach themselves to salmon in the third life stage.
Size: adult female: 12 mm (approx. 29 mm including egg strings); adult male: 6 mm.
Diet: the skin and blood of salmonids. The lice only start feeding when they have attached themselves to a host fish (stationary and mobile stages).
Reproduction: all year round, but reproduce increasingly quickly as temperatures rise in spring.
Dispersal: free-swimming stages spread on currents in fjords and coastal waters.
Treatment: biological methods (wrasse) or chemicals (medication).