Genetic characterization of populations of the ectoparasitic caligid, Lepeophtheirus salmonis (Krøyer 1837) using randomly amplified polymorphic DNA

Genetic variability within salmon louse, Lepeophtheirus salmonis (Caligidae: Copepoda), populations parasitizing farmed and wild Scottish Atlantic salmon (Salmo salar) was investigated using analysis of randomly amplified polymorphic DNA (RAPD) fragments. Seven individual decamer primers were used to analyse samples of salmon lice collected from 15 different locations in Scotland. The polymerase chain reaction products were separated using agarose gel electrophoresis and the resulting band patterns were analysed using a semi‐automated analytical scoring system. Dendrograms were produced using the unweighted pair‐group average (UPGMA) method using Dice similarity values. The summary dendrogram of the analysis of all RAPD bands showed two separate clusters of salmon lice, the larger being sub‐divided into a further two sections. The collections of lice occupying each of these sub‐divisions, however, were a mix of sites, which did not exhibit a structured geographical pattern.     

An evaluation of three potential methods for preventing the spread of larval Lepeophtheirus salmonis (Krøyer, 1837)

Within the State of Maine, only a portion of the farm sites experience sea lice (Lepeophtheirus salmonis) infections on an annual basis. There has been concern about the further spread of lice by farming activities to unaffected areas despite routine disinfection of equipment with sodium hypochlorite or iodophors. We examined the effects of Povidone‐iodine, sodium hypochlorite and desiccation on egg strings of L. salmonis and the potential of these methods for preventing hatching of nauplii or development to the copepodid stage. L. salmonis egg strings were exposed to one of eight treatments: 200 ppm of sodium hypochlorite or Povidone‐iodine solutions for 1 min, 500 ppm of either disinfectant for one minute or 10 min, or desiccation for either 4 or 24 h. The egg strings were then incubated and the hatched copepods were kept for 6 days in filtered natural seawater at 32 ppt salinity at 12°C. Desiccation for 4 or 24 h were the only methods that prevented L. salmonis nauplii from hatching or developing to the infective copepodid stage. Common disinfection procedures against pathogenic bacteria and virus were not found to be effective against L. salmonis eggs.     

An assessment of salmon farms and wild salmonids as sources of Lepeophtheirus salmonis (Krøyer) copepodids in the water column in Loch Torridon, Scotland

Wild salmonids and farmed salmon can both be sources of Lepeophtheirus salmonis (Krøyer, 1838) larvae . Farmed salmon smolts free of L. salmonis infections are stocked in sea cages and may subsequently contract L. salmonis infections, probably from wild fish. The contribution of gravid L. salmonis at Atlantic salmon, Salmo salar L., farms to populations of L. salmonis larvae in the water column has in the past been based on estimated parameters, such as louse fecundity. This present study augments these calculations by combining empirical data on densities of infective L. salmonis copepodids in the field with estimates of the number of gravid L. salmonis on farmed and wild salmonids in Loch Torridon. Data collected between 2002 and 2007 show a significant correlation between mean densities of L. salmonis copepodids recovered in the water column and the numbers of gravid L. salmonis at the local salmon farms. Generally, the farms with greatest numbers of salmon were observed to have stronger correlations with densities of copepodids in the water than the farms with fewer fish. The study suggests that louse management approaches, e.g. treatment trigger levels, need to take account of individual farm biomass, or numbers of fish. This study highlights the importance of control of L. salmonis on salmon farms for the co-existence of both wild salmonid populations and the aquaculture industry.     

Resistance of sea lice, Lepeophtheirus salmonis (Krøyer), to hydrogen peroxide on farmed Atlantic salmon, Salmo salar L.

Hydrogen peroxide has been the only medicine used to treat salmon infected with sea lice [Lepeophtheirus salmonis (Krøyer)] on many farms in Scotland since 1992 and reports have suggested reduced treatment efficacy. The present study tests the sensitivity of sea lice to exposure to hydrogen peroxide under farm conditions and also in vivo by comparing lice from a farm with suspected resistance and lice from a farm that had not been pre‐exposed to hydrogen peroxide. In bin treatments on fish from a farm treated with hydrogen peroxide on 41 occasions, numbers of ovigerous lice declined by only 15% and 16% in two replicates and other mobile stages by 25%. Where hydrogen peroxide had not been used, ovigerous females declined by 90% and 87% and other mobile lice stages by 97% and 99%. These trials and observed poor efficacy of cage treatments, for example only 7.5% reduction in lice numbers when fish were treated with 2000 p.p.m. for 20 mins, indicated resistance of lice to hydrogen peroxide. The mechanisms involved in the development of resistance, possible genetic selection for lice with reduced carapace permeability or detoxifying enzymes such as catalase, or tolerance through induction by subtherapeutic exposure are reviewed. Implications for lice control strategies relying on hydrogen peroxide are discussed.     

Variation in sensitivity of sea lice [Lepeophtheirus salmonis (Krøyer)] to dichlorvos on Irish salmon farms in 1991–92

Variation in sensitivity of Lepeophtheirus salmonis (Krøyer) to dichlorvos in static toxicity tests was recorded on salmon farms in Ireland. Five hour LC50s ranged from 60 to 202 μg l ^?1 at 15 °C for five farms in Autumn 1991 and 76–151 μg l^?1 at 10 °C for four farms in Spring of 1992. Resistance ratios, calculated using the most sensitive sites as baseline, were low and ranged from 1.32 to 3.27 in 1991 and 1.28–1.98 in 1992. Repeat sampling on three farms in 1991 and 1992 also indicated short‐term changes in sensitivity to dichlorvos within farms.     

Egg production in the salmon louse [Lepeophtheirus salmonis (Krøyer)] in relation to origin and water temperature

Egg strings of salmon lice, Lepeophtheirus salmonis (Krøyer 1837), collected from farmed and wild Atlantic salmon had similar length and number of eggs string^?1. Egg production was investigated at water temperatures from 7.1 °C to 12.2 °C. A regression model indicated that at low temperatures egg strings were longer and had more eggs. Mean length of single eggs was significantly smaller and the percentage of non‐viable eggs in the strings was higher at 7.1 °C than at 12.2 °C. Adult females survived for up to 191 days at 7.2 °C, and during this period 11 pairs of egg strings were produced.     

A review of host finding behaviour in the parasitic sea louse, Lepeophtheirus salmonis (Caligidae: Copepoda)

Ectoparasitic sea lice are the most important parasite problem to date for the salmon farming industry in the northern and southern hemispheres. An understanding of host location in the specialist species, Lepeophtheirus salmonis , the most important louse species in the North Atlantic, is now being realized using behavioural in vitro and in vivo bioassays coupled with chemical analysis of fish conditioned waters. Both physical and chemical cues are important in host location. Responses of sea lice to physical cues such as light and salinity may enable them to gather in areas where host fish are likely to be found. Mechanoreception is an important sensory modality in host location and acts by switching on specific behaviours that enable landing on a fish. Chemoreception plays a defining role in host location and recognition. The detection of host kairomones switches on 'host search' behavioural patterns and also induces landing responses whereas non-host kairomones fail to induce attraction or significant landing behaviour. Semiochemicals derived from salmon and also non-host fish have been identified, and may prove useful for the development of integrated pest management strategies, by the introduction of odour traps for monitoring lice numbers, and by the use of stimulo-deterrent diversionary (push:pull) strategies in their control.     

The effect of light on the settlement of the salmon louse, Lepeophtheirus salmonis, on Atlantic salmon, Salmo salar L

The salmon louse, Lepeophtheirus salmonis, is an ectoparasitic copepod that infests both wild and farmed salmonid fish. Salmon lice are a major disease problem in the farming of Atlantic salmon, Salmo salar L., and the possibility of salmon lice playing a role in the decline of wild anadromous stocks has also been raised. Lepeophtheirus salmonis can detect a range of stimuli (pressure/moving water, chemicals and light) in the external environment. However, the response thresholds to various stimuli, and the spatial and temporal scales over which they operate in the context of host location, are largely unknown. In this context, we attempted to determine whether salmon lice copepodids settle onto hosts more effectively, or at different locations on the fish's body, under different qualities of light. Lice settlement trials were conducted under three lighting conditions; L1: unpolarized under ultraviolet A (UVA)-through visible; L2: unpolarized without UVA (control); L3: 100% linearly polarized without UVA. A dark control was also conducted. No statistically significant difference in lice settlement was found. While changes in light intensity are involved in host detection at spatial scales on the order of metres, the results presented here suggest that it is not the primary sensory modality underlying host location at smaller spatial scales (cm to mm).     

Distributions of planktonic sea lice larvae Lepeophtheirus salmonis in the inter-tidal zone in Loch Torridon, Western Scotland in relation to salmon farm production cycles

This study examines the relative density of sea lice, Lepeophtheirus salmonis (Krøyer), larvae in the inter‐tidal areas of Loch Torridon, a fjordic sea loch in Western Scotland and the site of several commercial salmon farms. Samples of planktonic sea lice were collected in each spring from 1999 to 2003 near the mouth of the River Shieldaig, where sea trout have been shown to congregate in the first few days after going to sea, and from 2001 to 2003 near the mouth of the River Balgy. No larval sea lice were found near the mouth of the River Shieldaig in spring 2000 or 2002, when gravid female sea lice were absent on local fish farms. The same finding was observed in 2002 at the mouth of the River Balgy. In springs of 1999, 2001 and 2003, peak densities of larval sea lice of 33, 94 and 143 m^?3, respectively, were observed at the mouth of the river Shieldaig. At the mouth of the River Balgy, peak densities of 74 and 78 lice m^?3 were observed in 2001 and 2003 respectively. At the time these samples were collected, the two local salmon farms were in the second year of their production cycle and gravid female sea lice were present. Samples of larval sea lice were also obtained year round, at approximately weekly intervals, from a 50‐m transect at the mouth of the River Shieldaig, from March 2001 to June 2003, and compared with frequencies of gravid female sea lice on the two local salmon farms. High levels were found in the winter. No planktonic sea lice were found in this transect when gravid females were not present on the local fish farms.     

Sea lice, Lepeophtheirus salmonis, transfer between wild sympatric adult and juvenile salmon on the north coast of British Columbia, Canada

We examine sea lice, Lepeophtheirus salmonis , on juvenile and adult salmon from the north coast of British Columbia between 2004 and 2006 in an area that does not at present contain salmon farms. There is a pronounced zonation in the abundance of L. salmonis on juvenile pink salmon, Oncorhynchus gorbuscha , in the Skeena and Nass estuaries. Abundances in the proximal and distal zones of these estuaries are 0.01 and 0.05 respectively. The outer zones serve as feeding and staging areas for the pink salmon smolts. Returning Chinook, Oncorhynchus tshawytscha , and coho salmon, Oncorhynchus kisutch , concentrate in these areas. We collected data in 2006 to examine whether L. salmonis on returning adult salmon are an important source of the sea lice that appear on juvenile pink salmon. Nearly all (99%) of the sea lice on returning Chinook and over 80% on coho salmon were L. salmonis. Most of the L. salmonis were motile stages including many ovigerous females. There was a sharp increase in the abundance of sea lice on juvenile pink salmon smolts between May and July 2006 near the sites of adult captures. As there are no salmon farms on the north coast, few sticklebacks, Gasterosteus aculeatus , and very few resident salmonids until later in the summer, it seems that the most important reservoir of L. salmonis under natural conditions is returning adult salmon. This natural source of sea lice results in levels of abundance that are one or two orders of magnitude lower than those observed on juvenile pink salmon in areas with salmon farms such as the Broughton Archipelago.     

Impact of co-infection with Lepeophtheirus salmonis and Moritella viscosa on inflammatory and immune responses of Atlantic salmon (Salmo salar)

This study was conducted to determine the effects of a co-infection with Moritella viscosa at different exposure levels of sea lice Lepeophtheirus salmonis in Atlantic salmon (Salmo salar). M. viscosa (1.14 × 10⁶ cfu/ml) was introduced to all experimental tanks at 10 days post-lice infection (dpLs). Mean lice counts decreased over time in both the medium lice co-infection (31.5 ± 19.0 at 7 dpLs; 16.9 ± 9.3 at 46 dpLs) and high lice co-infection (62.0 ± 10.8 at 7 dpLs; 37.6 ± 11.3 at 46 dpLs). There were significantly higher mortalities and more severe skin lesions in the high lice co-infected group compared to medium lice co-infected group or M. viscosa-only infection. Quantitative gene expression analysis detected a significant upregulation of genes in skin from the high lice co-infection group consistent with severe inflammation (il-8, mmp-9, hep, saa). Skin lesions retrieved throughout the study were positive for M. viscosa growth, but these were rarely located in regions associated with lice. These results suggest that while M. viscosa infection itself may induce skin lesion development in salmon, co-infection with high numbers of lice can enhance this impact and significantly reduce the ability of these lesions to resolve, resulting in increased mortality.     

The sea louse Lepeophtheirus acutus (Caligidae, Siphonostomatoida, Copepoda) as a pathogen of aquarium-held elasmobranchs

Lepeophtheirus acutus Heegaard, 1943 (Caligidae, Siphonostomatoida, Copepoda), was collected from or observed on four of six elasmobranch species held at Burgers’ Zoo (Arnhem, The Netherlands). Circumstantial evidence suggested that a zebra shark, Stegostoma fasciatum (Hermann), from the wild carried the infection into the facility, where copepods reproduced and colonized additional hosts. Copepods typically attached on and about the eyes, in the mouth and occasionally about the cloaca and on the claspers. Severe ocular lesions were associated with infections on zebra sharks, a grey reef shark, Carcharhinus amblyrhynchos (Bleeker), whitetip reef sharks, Triaenodon obesus (Rüppell), and giant shovelnose ray, Rhinobatos typus Bennett, while blacktip reef sharks, Carcharhinus melanopterus (Quoy & Gaimard), and blacktip sharks, Carcharhinus limbatus (Valenciennes), living in infested aquaria showed no sign of infection. Water treatments using trichlorfon were considered primarily responsible for the eradication of copepods from hosts and infested aquaria. This case is the first report of a copepod infection being closely associated with disease and death of an aquarium‐held elasmobranch. Given its ability to infect a wide variety of elasmobranchs and promote life‐threatening lesions on some hosts, L. acutus should be considered a dangerous pathogen of captive elasmobranchs.     

Modelling the effects of capture and sea lice [Lepeophtheirus salmonis (Krøyer)] infestation on the cortisol stress response in trout

Measuring baseline cortisol in wild salmonids is problematic because of the stress induced by most capture methods. The cortisol response to different periods of confinement stress was modelled in a laboratory population of rainbow trout [Oncorhynchus mykiss (Walbaum)]. A quadratic model was used to predict baseline (precapture) cortisol levels in these fish. A similar quadratic response pattern of cortisol was observed in wild sea trout (Salmo trutta L.). These were captured in timed sequences on four dates by seine netting in Clew Bay, west coast of Ireland. The estimated precapture cortisol for these fish indicated higher levels soon after migration to sea and a positive correlation with the level of infestation by Lepeophtheirus salmonis (Krøyer). Curvilinearity of the quadratic model was correlated with levels of lice infestation, indicating an enhanced cortisol response to confinement stress during capture when parasitic abundance was high. Estimated precapture cortisol for individual sea trout was positively correlated with numbers of lice although the linear regression had low predictive power. The method provides, for the first time, a means to estimate baseline cortisol levels in wild salmonids, and addresses one of the most difficult problems in working with wild fish. Keywords: Cortisol, trout, Lepeophtheirus salmonis (Krøyer), capture, quadratic model     

A review of the use of ivermectin as a treatment for sea lice [Lepeophtheirus salmonis (Krøyer) and Caligus elongatus Nordmann] infestation in farmed Atlantic salmon (Salmo salar L.)

Ectoparasitic sea lice, Lepeophtheirus salmonis (Krøyer) and Caligus elongatus (Nordmann) browse on the skin of farmed Atlantic salmon, Salmo salar L.. The resulting lesions cause stress and increase the susceptibility of the fish to secondary infections. In extreme infestations, fish can suffer from osmoregulatory failure and death. The most immediate treatment for the relief of sea lice infestations at fish farms is the use of chemotherapeutants, either by bath or oral administration. One compound which has been investigated for use as a chemotherapeutant is ivermectin. Ivermectin is a member of the avermectin group of macrocyclic lactones. The avermectins are neurotoxins, which have been used successfully in the treatment of helminthic parasitic infections in a number of terrestrial farm animals and also in the treatment of river blindness in humans. Owing to the low solubility of the compound, ivermectin has been administered as an oral treatment with the feed and has been found to be effective for the treatment of sea lice on Atlantic salmon. Ivermectin is poorly absorbed by fish with a high percentage of the administered dose being excreted in the faeces. The highest concentrations of the absorbed ivermectin were found in lipid‐rich organs. The ivermectin remained in the tissues of the treated fish for a prolonged period of time and was excreted mainly in the unchanged form. Ivermectin can reach the marine environment via excretion from the bile, unabsorbed via the fish faeces and by uneaten food pellets and has a strong affinity to lipid, soil and organic matter. Risk assessments have shown that ivermectin is likely to accumulate in the sediments and that the species therein would be more at risk than the species in the pelagic environment. Ivermectin has been shown to be toxic to some benthic infaunal species in single species tests, but there is no evidence that treatment of fish with ivermectin has affected multispecies benthic communities in the field situation. This review paper ends with details of the development and the status of the use of ivermectin in the treatment of sea lice infestations at Scottish fish farms.     

The settlement and reproductive success of Lepeophtheirus salmonis (Krøyer 1837; Copepoda: Caligidae) on atypical hosts

The reproductive success of Lepeophtheirus salmonis settled on host and non‐host fish has been compared. Triplicate single species tanks of Atlantic salmon, marine three‐spined sticklebacks, saithe and Atlantic cod were exposed to 10 adult female L. salmonis per tank (n=30 lice per species). Adult female L. salmonis settlement and egg string production occurred only on salmon and cod, with no egg production occurring on saithe and three‐spined sticklebacks. The number of eggs in egg strings, hatching success of eggs and the survival of all larval stages to the copepodid stage were severely affected by the species of fish on which female L. salmonis had settled. L. salmonis settled on cod produced significantly fewer eggs, lower hatching rates and lower survival rates of larvae than females on Atlantic salmon. The production of egg strings by L. salmonis females infecting cod, which successfully hatch and moult through to the infective copepodid stage, albeit in small numbers, is discussed in terms of the implications to aquaculture and salmon and cod farming scenarios.     

A comparison of epidemiological patterns of salmon lice, Lepeophtheirus salmonis, infections on farmed Atlantic salmon, Salmo salar L., in Norway and Scotland

This paper examines two large national data sets collected over several years and contrasts the patterns of sea lice, Lepeophtheirus salmonis (Krøyer, 1837), infections, the use of treatments and the occurrence of chalimus peaks between Norwegian and Scottish farms. Infection levels in Scotland were significantly higher in general over the period under study. For the chalimus stage group in the first quarter of the year, Norwegian mean abundance stayed below 10 lice per m² while Scottish means reached 45 lice per m² of fish skin per m³ of water. Both countries had more chalimus in summer than at other times of year, but in the last 3 months of the year Scottish fish had, on average, two to four times as many chalimus as Norwegian fish. Peaks of chalimus abundance were more frequent in Scotland, particularly in winter, but the most prominent peaks occurred in summer in both countries. In Scotland a marked mid-year build-up of mobile pre-adult and adult stages was seen, and both countries showed a tendency for mobile counts on the second year fish to increase towards the end of the year. Scottish fish carried, on average, three times as many mobile lice per m² of skin as Norwegian fish in the last 3 months of the year. The difference in lice loads was reflected in the greater use of veterinary medicines on Scottish farms. The higher infection levels in Scotland may be due to shallower and more enclosed water bodies used for farming, smaller and shallower pens, differences in sea water temperatures or in access to appropriate medication. The results highlight the importance of ensuring that effective veterinary medicines are available in the UK for the control of infection.