Lepeophtheirus salmonis

From Fish
Female (large) and male (small) Lepeophtheirus salmonis sea lice
Sea louse on the basal part of the anal fin.
Adult female L. salmonis attached adjacent to the adipose fin of a small salmon (approx. 2 kg) caught from the River Tay estuary (July 1996). Two of the parasites bear egg strings.

Lepeophtheirus salmonis is a parasitic sea louse that infests both wild salmon (Salmo salar) and sea trout. L. salmonis and Caligus elongatus are the primary ectoparasites of farmed salmonids.

L. salmonis feed on the mucus, skin and blood of the host fish, and heavy infestations may be fatal.

Life cycle

L. salmonis has a direct life-cycle (i.e. a single host) that comprises of ten stages. Following hatching from paired egg-strings, two free-living nauplii stages are dispersed into the plankton. These stages are followed by a copepodid stage where attachment to the host takes place. The copepodid then moults through four attached chalimus stages before becoming a mobile pre-adult male or female. There are two pre-adult stages and this is followed by the fully mature adult phase. The adult female can produce a number of batches of paired egg-strings which in turn hatch into the water column to give rise to the next generation[1].

Clinical signs

L. salmonis can cause stress, pathological damage and death of the host fish, and the small post-smolt stage is perhaps the most vulnerable[2].

In wild and farmed salmonids, the sites of heavy infection by L. salmonis show abrasion and hemorrhage. The skin near the anal fin is usually heavily infected. Kimura (1970) suggested that the pathogenic bacteria, Aeromonas salmonicida masoucida, invade the fish through the skin damage induced by L. salmonis and cause a disease in masu and pink salmon kept in fresh waters for maturation[3].


L. salmonis is readily treated with ivermectin-based drugs applied to ponds. The pyrethroids deltamethrin and cypermethrin have been widely used by the aquaculture industry in the control of sea lice, and anecdotal reports of their decreased efficacy have ranged from Ireland and Scotland to Norway[4].

Populations of Atlantic L. salmonis have been shown to be genetically open: there is no indication that there is any genetic distinction between populations infesting wild or farmed salmon on either seaboard of the North Atlantic. This feature alone dictates that eradication of L. salmonis as a pest species on salmon farms is impossible[5].


  1. Heuch PA, et al (2005) A review of the Norwegian 'National Action Plan Against Salmon Lice on Salmonids': The effect on wild salmonids. Aquaculture 246:79-92
  2. Finstad, B., Bjørn P. A., Grimnes A. and Hvidsten, N. A. (2000) Laboratory and field investigations of salmon lice [Lepeophtheirus salmonis (Krøyer)] infestation on Atlantic salmon (Salmo salar L.) post-smolts. Aquac Res 31:795–803
  3. Kimura T. (1970) Studies on a bacterial disease occurred in the adult sakuramasu (Oncorhynchus masou) and pink salmon (O. gorbuscha) rearing for maturity. Sci Rep Hokkaido Salmon Hatchery 24:9-100
  4. Sevatdal, S., Copley, L., Wallace, C., Jackson, D. and Horsberg, T. E. (2005) Monitoring of the sensitivity of sea lice (Lepeophtheirus salmonis) to pyrethroids in Norway, Ireland and Scotland using bioassays and probit modelling. Aquaculture 244:19–27
  5. Todd, CD (2006) Salmonid-parasitic copepod interactions. J Plankton Res