The salmon louse (Lepeophtheirus salmonis) is a species of copepod in the genus Lepeophtheirus. It is a sea louse, a parasite living mostly on salmon, particularly on Pacific and Atlantic salmon and sea trout, but is also sometimes found on the three-spined stickleback. It feeds on the mucus, skin and blood of the fish. Once detached, they can be blown by wind across the surface of the sea, like plankton. When they encounter a suitable marine fish host, they adhere themselves to the skin, fins, or gills of the fish, and feed on the mucus or skin. Sea lice only affect fish and are not harmful to humans.
Salmon lice are ectoparasites of salmon. In the 1980s, high levels of salmon lice were observed on pink salmon smolts. Salmon lice are found in the Pacific and Atlantic Oceans; they infect pink salmon, Atlantic salmon, and chum salmon.
Some research has occurred on the problems caused by this species in aquaculture, but little is known about the salmon louse's life in nature. Salmon louse infections in fish farming facilities, though, can cause epizootics in wild fish. When aquaculturalists place their post smolts into sea water, they are commonly known to be ectoparasite free, and this can last for many months.
L. salmonis has a direct lifecycle (i.e. a single host) with eight life stages  with ecdysis in between. These planktonic nauplii cannot swim directionally against the water current, but drift passively, and have the ability to adjust their depth in the water column. They are almost translucent in colour and are about 0.5-0.6 mm long.
At 5 °C (41 °F), the nauplius 1 stage lasts about 52 hours, and about 9 hours at 15 °C (59 °F). Nauplius 2 takes 170 hours and 36 hours at these temperatures, respectively. They are responsive to light and salinity. Low salinities appear to have a greater effect on the planktonic stages than on the parasitic stages. Newly hatched larvae do not survive below salinities of 15‰ and poor development to the infective copepodid occurs between 20 and 25‰. Nauplii and copepodids are positively phototactic and exhibit a daily vertical migration, rising during the day and sinking at night. The ability to find their hosts is not light dependent. They are responsive to low-frequency water accelerations, such as those produced by a swimming fish. Finding their migratory hosts in the vastness of the ocean is still a mystery for scientists to solve, but the species has managed to do this effectively for millennia.
The third stage is the copepodid stage, in which the length is about 0.7 mm and could take 2 to 14 days depending on water temperature, and the salmon louse attaches itself to the fish.
Stages four and five are the chalimus stages. The salmon louse becomes mobile and can move around the surface of fish and can also swim in the water column, and grows to a length of 5 mm for the males, 10 mm for the females.
Duration times are roughly 10 days for copepodid, 10 days chalimus I, 15 for chalimus 2, 10 days for preadult 1 females, and 12 days for preadult 2 females at 10 °C (50 °F). Males develop faster, spending around 8 days as preadult 1 and 9 days as preadult 2 at 10 °C (50 °F). Chalimus stages measure in length from 1.1 mm at stage 1 to 2.3 mm at stage 2.
Two preadult stages are followed by the fully mature adult phase. In the preadult stages, the genital complex is underdeveloped and the mean length is about 3.6 mm. Final moults to adult stages, both male and female, then take place. The female is larger than the male, with males measuring 5–6 mm and females 8–18 mm. Female adults can produce 10-11 pairs of egg strings over their lifecycle. Mean egg numbers per string (fecundity) have been recorded as 152 (+16) with a range from 123 to 183 at 7.2 °C (45.0 °F).
The development to sexual maturity following attachment to the host fish depends on water temperature and the generation time, from egg to mature adult, and ranges from 32 days at 15 °C (59 °F) to 106 days at 7.5 °C (45.5 °F). Egg strings tend to be longer with higher fecundity at lower temperatures, but factors affecting egg production are poorly understood.
The sea louse generation time is around 8–9 weeks at 6 °C (43 °F), 6 weeks at 9 °C (48 °F), and 4 weeks at 18 °C (64 °F). The lifespan of the adult under natural conditions has not been determined, but under laboratory conditions, females have lived for up to 210 days.
The thorax is broad and shield shaped. The abdomen is narrower, and in the females, filled with eggs. The females also have two long egg strings attached to the abdomen. The salmon louse uses its feet to move around on the host or to swim from one host to another.
This parasite is one of the major threats to salmon farmers. Salmon are stocked usually for a 14 - 18-month cycle. Salmon farms are an unusual, but ideal environment for the sea lice to breed. The infestations of sea lice in salmon farms increases the number of lice in the rest of the surrounding water dramatically if the eggs from the gravid louse are allowed to disperse. Sea lice can also attach to juvenile salmon migrating from rivers to the ocean if they pass by fish farms.
The Salmon louse currently infests nearly half of Scotland's salmon farms. In 2016 Guardian news stated that the lice killed thousands of tonnes of farmed fish, caused skin lesions and secondary infections in millions more, and cost the Scottish salmon industry around £300m in control efforts.
Farmers recently started using lasers with machine vision to fight lice:on YouTube.
In small numbers, salmon lice cause little damage to a fish although if populations increase on a fish, this can lead to death. The parasites can cause physical damage to the fish's fins, skin erosion, constant bleeding, and open wounds creating pathways for other pathogens. The sea lice may also act as a vector for diseases between wild and farmed salmon. These copepod vectors have caused infectious salmon anemia (ISA) along the Atlantic coast. An outbreak of ISA occurred in Chile during 2007 where it spread quickly from one farm to another, destroying the salmon farms.
Salmon lice infection in pink salmon weakens ionic homeostasis in pink salmon smolts. Homeostasis is needed for the internal regulation of body temperature and pH levels; the process allows fish to travel from fresh water to sea water. Disruption of ionic homeostasis in pre-mature smolt stages can result in reductions in growth rate, limit swimming capabilities, and even death. Disturbances in hydro mineral balance can result in negative consequences at the cellular, tissue, and organism levels. High levels of salmon lice infections result in a weaker ion regulation system.
The ability to activate an inflammatory response is a way to combat salmon lice infection. The intensity of inflammatory response controls how fast the parasites are rejected from the body. Intensity is determined by recognition of and regulation by salmon lice secretory/excretory products (SEP), which include proteases and prostaglandin E2. The marine parasite secretes SEP into the damaged skin of the salmon which inhibits proteolytic activity. Proteolytic activity increases the amount of host peptides and amino acids that can be used as a source of nutrition and lowers the intensity of inflammatory responses.