Sea louse
Sea lice | |
---|---|
Male and female Lepeophtheirus salmonis | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Class: | Copepoda |
Order: | Siphonostomatoida |
Family: | Caligidae Burmeister, 1834 [1] |
Genera [2] | |
| |
Synonyms | |
Euryphoridae |
Sea lice (singular: sea louse) are
The genera Lepeophtheirus and Caligus parasitize marine fish, in particular those species that have been recorded on
Caligus rogercresseyi has become a major parasite of concern on salmon farms in countries including Chile[3] and Scotland.[4] Studies are under way to gain a better understanding of the parasite and the host-parasite interactions. Recent evidence is also emerging that L. salmonis in the Atlantic has sufficient genetic differences from L. salmonis from the Pacific to suggest that Atlantic and Pacific L. salmonis may have independently co-evolved with Atlantic and Pacific salmonids respectively.[5]
Diversity
The family Caligidae is estimated to contain around 559 species in 37 genera.[1] The largest of these are Caligus, with around 268 species,[6] and Lepeophtheirus with around 162 species.[7]
Wild fish
Most understanding of the biology of sea lice, other than the early morphological studies, is based on laboratory studies designed to understand issues associated with sea lice infecting fish on salmon farms. Information on sea lice biology and interactions with
Many sea louse species are specific with regard to host genera, for example L. salmonis, which has high specificity for
How
The source of L. salmonis infections when salmon return from fresh water has always been a mystery. Sea lice die and fall off
Sea lice possibly survive on fish that remain in the estuaries or they transfer to an as yet unknown alternate host to spend the winter. Smolt get infected with sea lice larvae, or even possibly adults, when they enter the estuaries in the spring. How sea lice distribute between fish in the wild also is not known. Adult stages of Lepeophtheirus spp. can transfer under laboratory conditions, but the frequency is low. Caligus spp. transfer quite readily and between different species of fish, and are regularly found in the plankton.[12]
Morphology
L. salmonis tends to be about twice the size of most Caligus spp. (e.g. C. elongatus, C. clemensi, etc.). The body consists of four regions:
Development
Sea lice have both free-swimming (planktonic) and parasitic life stages, all separated by moults.[17][18][20][21] The development rate for L. salmonis from egg to adult varies from 17 to 72 days depending on temperature. The lifecycle of L. salmonis is shown in the figure; the sketches of the stages are from Schram.[20]
Eggs hatch into
Feeding habits
Until they locate a host, the naupliar and copepodid stages are non-feeding and live on endogenous food stores. Once attached to the host, the copepodid stage begins feeding and begins to develop into the first chalimus stage. Copepods and chalimus stages have a developed gastrointestinal tract and feed on host mucus and tissues within range of their attachment. Pre-adult and adult sea lice, especially pregnant females, are aggressive feeders, in some cases feeding on blood in addition to tissue and mucus. Blood is often seen in the digestive tract, especially of adult females. L. salmonis is known to secrete large amounts of trypsin into its host's mucus, which may assist in feeding and digestion.[8][24] Other compounds such as, prostaglandin E2, have also been identified in L. salmonis secretions and may assist in feeding and/or serve the parasite in avoiding the immune response of the host by regulating it at the feeding site.[8][25] Whether sea lice are vectors of disease is unknown, but they can be carriers of bacteria and viruses likely obtained from their attachment to and feeding on tissues of contaminated fish.[26]
Disease
Pathology
Sea lice cause physical and enzymatic damage at their sites of attachment and feeding, which results in abrasion-like lesions that vary in their nature and severity depending upon a number of factors, including host species, age, and general health of the fish. Whether stressed fish are particularly prone to infestation is unclear. Sea-lice infection causes a generalized chronic stress response in fish since feeding and attachment cause changes in the mucus consistency and damage the epithelium resulting in loss of blood and fluids, electrolyte changes, and cortisol release. This can decrease salmon immune responses and make them susceptible to other diseases and reduce growth and performance.[27][28]
The degree of damage is also dependent on the species of sea lice, the developmental stages that are present, and the number of sea lice on a fish. Little evidence exists of host tissue responses in Atlantic salmon at the sites of feeding and attachment, regardless of the development stage. In contrast, coho and pink salmon show strong tissue responses to L. salmonis characterized by epithelial hyperplasia and inflammation. This results in rejection of the parasite within the first week of infection in these species of salmonids.[8] Heavy infections of farmed Atlantic salmon and wild sockeye salmon (Oncorhynchus nerka) by L. salmonis can lead to deep lesions, particularly on the head region, even exposing the skull.
Interactions between wild and farmed fish
Some evidence indicates that sea lice flourishing on salmon farms can spread to nearby wild juvenile salmon and devastate these populations.
Several scientific studies have suggested that
Fish farming
Control on salmon farms
This has been reviewed by Pike & Wadsworth,[21] McVicar,[42] and Costello.[12] Integrated pest management programs for sea lice are instituted or recommended in a number of countries, including Canada,[43][44] Norway,[40] Scotland,[45] and Ireland.[46] Identification of epidemiological factors as potential risk factors for sea lice abundance[47] with effective sea lice monitoring programs have been shown to effectively reduce sea lice levels on salmon farms.[48]
Natural predators
Husbandry
Good husbandry techniques include fallowing, removal of dead and sick fish, prevention of net fouling, etc. Bay management plans are in place in most fish farming regions to keep sea lice below a level that could lead to health concerns on the farm or affect wild fish in surrounding waters. These include separation of year classes, counting and recording of sea lice on a prescribed basis, use of parasiticides when sea lice counts increase, and monitoring for resistance to parasiticides.
Salmon breeding
Early findings suggested genetic variation in the susceptibility of Atlantic salmon to Caligus elongatus.
In October 2012, the grocery chain Sobeys pulled whole Atlantic salmon from 84 store locations in the Canadian Maritimes after concerns were raised over sea lice. [53]
In 2017, salmon prices in Norway increased by 15% over a 3-month period because of a sea lice outbreak. [54]
Treatments
Freshwater
Freshwater is sometimes adequate to kill the sea lice and as salmon eventually swim in fresh water, they are not harmed.[55]
Drugs and vaccines
The range of therapeutants for farmed fish was limited, often due to regulatory processing limitations. All drugs used have been assessed for environmental impact and risks.[56][57] The parasiticides are classified into bath and in-feed treatments as follows:
Bath treatments
There are both advantages and disadvantages to using bath treatments. Bath treatments are more difficult and need more manpower to administer, requiring skirts or tarpaulins to be placed around the cages to contain the drug. Prevention of reinfection is a challenge since it is practically impossible to treat an entire bay in a short time period. Since the volume of water is imprecise, the required concentration is not guaranteed. Crowding of fish to reduce the volume of drug can also stress the fish. Recent use of well-boats containing the drugs has reduced both the concentration and environmental concerns, although transferring fish to the well boat and back to the cage can be stressful. The major advantage to bath treatments is that all the fish will be treated equally, in contrast to in-feed treatments where amount of drug ingested can vary due to a number of reasons.
Organophosphates
Pyrethroids
Topical disinfectants
Bathing fish with hydrogen peroxide (350–500 mg/L for 20 min) will remove mobile sea lice from fish. It is environmentally friendly since H2O2 dissociates to water and oxygen, but can be toxic to fish, depending on water temperature, as well as to operators.[61] It appears to knock the sea lice off the fish, leaving them capable of reattaching to other fish and reinitiating an infection.
In-feed treatments
In-feed treatments are easier to administer and pose less environmental risk than bath treatments. Feed is usually coated with the drug and drug distribution to the parasite is dependent on the pharmacokinetics of the drug getting in sufficient quantity to the parasite. The drugs have high selective toxicity for the parasite, are quite lipid-soluble so that there is sufficient drug to act for approximately 2 months, and any unmetabolized drug is excreted so slowly that there are little to no environmental concerns.
Avermectins
Avermectins belong to the family of macrocyclic lactones and are the major drugs used as in-feed treatments to kill sea lice. The first avermectin used was ivermectin at doses close to the therapeutic level and was not submitted for legal approval for use on fish by its manufacturer. Ivermectin was toxic to some fish, causing sedation and central nervous system depression due to the drug's ability to cross the blood–brain barrier. Emamectin benzoate, which is the active agent in the formulation SLICE,[62] has been used since 1999 and has a greater safety margin on fish. It is administered at 50 µg/kg/day for 7 days and is effective for two months, killing both chalimus and mobile stages. Withdrawal times vary with jurisdiction from 68 days in Canada[63] to 175 degree days in Norway. Avermectins act by opening glutamate-gated chloride channels in arthropod neuromuscular tissues, causing hyperpolarization and flaccid paralysis leading to death. Resistance has been noted in Chalimus rogercresseyi in Chile and L. salmonis on North Atlantic fish farms. The resistance is likely due to prolonged use of the drug leading to up-regulation of P-glycoprotein,[64] similar to what has been seen in nematode resistance to macrocyclic lactones.[65]
Growth regulators
Teflubenzuron, the active agent in the formulation Calicide,[66] is a chitin synthesis inhibitor and prevents moulting. It thus prevents further development of larval stages of sea lice, but has no effect on adults. It has been used only sparingly in sea lice control, largely due to concerns that it may affect the moult cycle of non-target crustaceans, although this has not been shown at the concentrations recommended.[56]
Vaccines
A number of studies are underway to examine various antigens, particularly from the gastrointestinal tract and reproductive endocrine pathways, as vaccine targets, but no vaccine against sea lice has been reported to date. Two published studies have tested vaccine candidate antigens against salmon lice, which resulted in a reduced infection rate.[67][68]
Optical methods
A more recent advance in the delousing strategy is to use pulsed lasers operating at the wavelength of 550 nm to delouse.[69]
Other points of interest
See also
- Aquaculture of salmon
- Fish diseases and parasites
- Salmon louse
References
- ^ a b Shane T. Ahyong; James K. Lowry; Miguel Alonso; Roger N. Bamber; Geoffrey A. Boxshall; Peter Castro; Sarah Gerken; Gordan S. Karaman; Joseph W. Goy; Diana S. Jones; Kenneth Meland; D. Christopher Rogers; Jörundur Svavarsson (2011). "Subphylum Crustacea Brünnich, 1772" (PDF). In Z.-Q. Zhang (ed.). Animal biodiversity: an outline of higher-level classification and survey of taxonomic richness. Vol. 3148. pp. 165–191.
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ignored (help) - ^ T. Chad Walter & Geoff Boxshall (2011). Walter TC, Boxshall G (eds.). "Caligidae". World of Copepods database. World Register of Marine Species. Retrieved January 12, 2012.
- ^ a b S. Bravo (2003). "Sea lice in Chilean salmon farms". Bulletin of the European Association of Fish Pathologists. 23 (4): 197–200.
- ^ Ungoed-Thomas, Jon (16 September 2023). "'Monstrous' sea lice and jellyfish invasions blighting Scottish salmon farms". The Guardian.
- PMID 18574633.
- ^ Geoff Boxshall (2011). Walter TC, Boxshall G (eds.). "Caligus O.F. Müller, 1785". World of Copepods database. World Register of Marine Species. Retrieved January 12, 2012.
- ^ T. Chad Walter & Geoff Boxshall (2011). Walter TC, Boxshall G (eds.). "Lepeophtheirus von Nordmann, 1832". World of Copepods database. World Register of Marine Species. Retrieved January 12, 2012.
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- ^ "Grocery chain pulls whole salmon following Facebook posts - Your Community".
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- ^ "Ship that can wash sea lice from farmed salmon now on Vancouver Island". CTV News. The Canadian Press. 23 April 2019. Retrieved 25 April 2019.
- ^ a b L. E. Burridge (2003). "Chemical use in marine finfish aquaculture in Canada: A review of current practices and possible environmental effects". Canadian Technical Reports of Fisheries and Aquatic Sciences. 2450: 97–131.
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- ^ "Depletion of Emamectin Benzoate (SLICE®) from Skeletal Muscle (and Skin) of Atlantic Salmon (Salmo salar) following a Multiple Oral (Dietary) 50 µg/kg Dose Regimen, in Seawater at 10 ±1°C; One Laboratory and Two Field-based trials". MG-06-04-004. Fisheries and Oceans Canada. 31 March 2007. Retrieved 4 March 2011.
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External links
- DFO Canada information on sea-lice [1]
- Pacific Salmon Forum Interim Results [2]
- Watershed Watch Salmon Society British Columbia advocacy group for wild salmon.
- Wild Salmon in Trouble: The Link Between Farmed Salmon, Sea Lice and Wild Salmon - Watershed Watch Salmon Society. Animated short film based on peer-reviewed scientific research.
- 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 effect of sea lice infestations on wild salmon populations.
- Sea Lice - Coastal Alliance for Aquaculture Reform. Overview of farmed to wild salmon interactive effects of sea lice.
- Salmon Farming Problems - Coastal Alliance for Aquaculture Reform. Overview of environmental impacts of salmon farming.
- Fish farms drive wild salmon populations toward extinction Biology News Net. December 13, 2007.
- Ecological Genetics of Parasitic Sea Lice University of St Andrews Marine Ecology Research Group.
- Sea Lice expert reviewed and published by WikiVet