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.
