(Krøyer, 1837) 
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 lives off the mucus, skin and blood of the fish. They are natural marine parasites of fish such as adult salmon. 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 may adhere themselves to the skin, fins, the gills of the fish, and feeding off the mucus or skin. Sea lice only affect fish and are not harmful towards humans.
Salmon lice are natural ectoparasites of salmon, in the 1980s high levels of salmon lice were observed on Pink salmon smolts. Salmon lice are found in Pacific and Atlantic Oceans, they infect Pink salmon, Atlantic salmon, and Chum salmon.
There has been some research on the problems caused by this species in aquaculture, but little is known about the salmon louse's life in nature. It has been shown, however, that salmon louse infections in fish farming facilities can cause epizootics in wild fish. It is also common knowledge that when aquaculturalists place their post smolts into sea water they are obviously ectoparasite free and this can last for many months.
L. salmonis has a direct life cycle (i.e. a single host) with eight life stage. 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 0.5-0.6mm 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 host is not light dependent. They have been shown to be responsive to low frequency water accelerations, such as those produced by a swimming fish. Finding their migratory host 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 ca. 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 approximately 10 days for copepodid, ~10 days chalimus I, ~15 for chalimus 2, ~10 days for pre-adult 1 female and ~12 days for pre-adult 2 female at 10 °C (50 °F). Males develop faster, spending ~8 days as pre-adult 1 and ~9 days as pre-adult 2 at 10 °C (50 °F). Chalimus stages measure in length from c 1.1mm at stage 1 to c 2.3mm at stage 2.
Two pre-adult stages are followed by the fully mature adult phase. In the pre-adult stages the genital complex is under-developed and the mean length is about 3.6mm. Final moults to adult stages, both male and female, then take place. The female is larger than the male with males measuring 5-6mm and females 8-18mm. Female adult L. salmonis can produce ten to eleven pairs of egg strings over their life cycle. 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.
Effects on salmon farms
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 affect juvenile salmon while salmon from the rivers migrating to the ocean if on the way they pass by fish farms, the early stage and mature stages of sea lice may attaches onto them as well. These young salmon are smaller than a size of a key and still developing. When sea lice attach to the young salmon they can kill them.
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.
In small numbers, Sea 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 has 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 results 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 includes 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.
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- Ecological Genetics of Parasitic Sea Lice University of St Andrews Marine Ecology Research Group
- Fish farms drive wild salmon populations toward extinction Biology News Net – biologynews.net
- Wild Salmon in Trouble: The Link Between Farmed Salmon, Sea Lice and Wild Salmon Watershed Watch Salmon Society. Animated short video based on peer-reviewed scientific research, with subject background article Watching out for Wild Salmon.
- Aquacultural Revolution: The scientific case for changing salmon farming Watershed Watch Salmon Society. Short video documentary by filmmakers Damien Gillis and Stan Proboszcz. Prominent scientists and First Nation representatives speak their minds about the salmon farming industry and the effects of sea lice infestations on wild salmon populations.
- Sea Lice Coastal Alliance for Aquaculture Reform. An overview of farmed- to wild-salmon interactive effects.
- Salmon Farming Problems Coastal Alliance for Aquaculture Reform. An overview of environmental impacts of salmon farming.
- Sea Lice and Salmon: Elevating the dialogue on the farmed-wild salmon story Watershed Watch Salmon Society, 2004.