The diversity of sea lice (Copepoda: Caligidae) parasitic on threespine stickleback (Gasterosteus aculeatus) in coastal British Columbia

The prevalence, intensity, and abundance of sea lice belonging to Lepeophtheirus or Caligus clemensi are reported from threespine stickleback (Gasterosteus aculeatus) collected from the Broughton Archipelago region of coastal British Columbia, Canada, between 2005 and 2008. In total, 25,130 sea lice were collected from 7,684 sticklebacks. The prevalence of Lepeophtheirus ranged from 51% in 2005 to 11% in 2008 and that of C. clemensi from 56% in 2007 to 24% in 2008. Chalimus stages accounted for approximately 69% of all Lepeophtheirus and 88% of Caligus specimens. Cytochrome c oxidase subunit 1 (COI) gene sequences, useful in distinguishing reference specimens belonging to 8 species of Lepeophtheirus, Caligus, and Bomolochus, were used to identify the Lepeophtheirus specimens from stickleback as L. salmonis (71%) and L. cuneifer (29%). A COI phylogenetic analysis confirmed a monophylogenetic origin of Lepeophtheirus but not of Caligus. Two genotypes were resolved in L. cuneifer, i.e., genotype A occurred twice as often as genotype B. Virtually all immature Lepeophtheirus specimens from juvenile salmon were L. salmonis. The results emphasized the need to accurately identify immature sea lice as a prerequisite to understanding sea lice ecology. The threespine stickleback may be a useful sentinel species for the abundance and diversity of the sea lice that are also parasites of wild and farmed salmon in coastal ecosystems in British Columbia.     

Effects of host migration, diversity and aquaculture on sea lice threats to Pacific salmon populations

Animal migrations can affect disease dynamics. One consequence of migration common to marine fish and invertebrates is migratory allopatry-a period of spatial separation between adult and juvenile hosts, which is caused by host migration and which prevents parasite transmission from adult to juvenile hosts. We studied this characteristic for sea lice (Lepeophtheirus salmonis and Caligus clemensi) and pink salmon (Oncorhynchus gorbuscha) from one of the Canada's largest salmon stocks. Migratory allopatry protects juvenile salmon from L. salmonis for two to three months of early marine life (2-3% prevalence). In contrast, host diversity facilitates access for C. clemensi to juvenile salmon (8-20% prevalence) but infections appear ephemeral. Aquaculture can augment host abundance and diversity and increase parasite exposure of wild juvenile fish. An empirically parametrized model shows high sensitivity of salmon populations to increased L. salmonis exposure, predicting population collapse at one to five motile L. salmonis per juvenile pink salmon. These results characterize parasite threats of salmon aquaculture to wild salmon populations and show how host migration and diversity are important factors affecting parasite transmission in the oceans.     

Evaluating the effect of clustering when monitoring the abundance of sea lice populations on farmed Atlantic salmon

Using cluster random sampling theory and empirical estimates of the intra‐class correlations for sea lice Lepeophtheirus salmonis abundances, methods on how best to sample Atlantic salmon Salmo salar from cages on farms were derived. Estimates of intra‐class correlations for the abundance of the chalimus and mobile sea lice stages on Atlantic salmon in Scottish farms are given. These correlations were higher for mobile stages than for chalimus, and they had a substantive effect on increasing the number of cages and fish to be sampled for all sea lice stages. An important finding is that it is better to have a procedure that randomly samples a large number of cages using a small number of fish from each. This finding and the cluster random sampling approach have relevance to the monitoring of all marine species being farmed in cages or tanks.     

Clinical efficacy of teflubenzuron (Calicide) for the treatment of Lepeophtheirus salmonis infestations of farmed Atlantic salmon Salmo salar at low water temperatures

The efficacy of teflubenzuron (Calicide) for the treatment of farmed Atlantic salmon Salmo salar L. infested with sea lice Lepeophtheirus salmonis (Kr?yer, 1838), was investigated at low water temperatures in 2 commercial salmon farms. Calicide, coated on commercial feed pellets, was administered orally at 10 mg kg(-1) d(-1) for 7 consecutive days. Fish were randomly sampled and lice numbers recorded from both treated and control groups on 3 or 4 sampling occasions post-medication. Statistically significant reductions in the number of L. salmonis per fish were recorded. Maximum efficacy was observed toward chalimus and preadult stages of L. salmonis, and was achieved approximately 26 d post-medication. No adverse drug reactions or palatability problems were associated with the treatments.     

Transmission dynamics of parasitic sea lice from farm to wild salmon

Marine salmon farming has been correlated with parasitic sea lice infestations and concurrent declines of wild salmonids. Here, we report a quantitative analysis of how a single salmon farm altered the natural transmission dynamics of sea lice to juvenile Pacific salmon. We studied infections of sea lice (Lepeophtheirus salmonis and Caligus clemensi) on juvenile pink salmon (Oncorhynchus gorbuscha) and chum salmon (Oncorhynchus keta) as they passed an isolated salmon farm during their seaward migration down two long and narrow corridors. Our calculations suggest the infection pressure imposed by the farm was four orders of magnitude greater than ambient levels, resulting in a maximum infection pressure near the farm that was 73 times greater than ambient levels and exceeded ambient levels for 30 km along the two wild salmon migration corridors. The farm-produced cohort of lice parasitizing the wild juvenile hosts reached reproductive maturity and produced a second generation of lice that re-infected the juvenile salmon. This raises the infection pressure from the farm by an additional order of magnitude, with a composite infection pressure that exceeds ambient levels for 75 km of the two migration routes. Amplified sea lice infestations due to salmon farms are a potential limiting factor to wild salmonid conservation.     

Sex‐selective Predation by Threespine Sticklebacks on Sea Lice: A Novel Cleaning Behaviour

Cleaning interactions have been described in a wide range of fish species and other taxa. We observed a novel cleaning behaviour during a study of the transmission dynamics of sea lice (Lepeophtheirus salmonis) between juvenile pink salmon (Oncorhynchus gorbuscha) and threespine sticklebacks (Gasterosteus aculeatus) in the Broughton Archipelago, British Columbia, Canada. Experiments showed that sticklebacks significantly reduced the number of sea lice on individual juvenile salmon. Adult female lice were preferentially consumed by sticklebacks, and gravid female lice also experienced egg string cropping. Overall, 76% of gravid female lice experienced either consumption, egg string cropping, or both by sticklebacks. This preference by sticklebacks for female parasites may stem from female lice being larger than males and the added nutritional value of egg strings on gravid females. Cleaning by sticklebacks can potentially have an impact on sea louse populations on wild juvenile salmon.     

Commercial trials using emamectin benzoate to control sea lice Lepeophtheirus salmonis infestations in Atlantic salmon Salmo salar

Two trials were conducted at commercial salmon farms to evaluate the efficacy of emamectin benzoate (Slice, 0.2% aquaculture pre-mix, Schering-Plough Animal Health) as a treatment for sea lice Lepeophtheirus salmonis (Kr?yer) and Caligus elongatus Nordmann infestations in Atlantic salmon Salmo salar L. Trials were carried out in 15 m2 commercial sea pens, at temperatures of 5.5 to 7.5 degrees C and 10.8 to 13.8 degrees C. Each pen was stocked with 14,000 to 17,500 fish with mean weights of 0.44 to 0.74 and 1.33 to 1.83 kg. Fish were naturally infested with sea lice at the start of each trial. At Day -1, samples of 10 or 15 fish were taken from each pen to determine pre-treatment numbers of lice. Emamectin benzoate was administered in feed, to 4 replicate pens, at a dose of 50 micrograms kg-1 biomass d-1 for 7 consecutive days (Days 0 to 6). Sea lice were counted again, between Days 7 and 77, and comparisons made with untreated control fish. Despite adverse weather conditions, wide variations in fish weights and exposure to new infestations, treatment was effective against chalimus and motile stages of L. salmonis. In the autumn trial, efficacy at Day 27 was 89%, and lice numbers remained lower on treated fish than on control fish 64 d from the start of treatment. In the winter trial, reductions in lice numbers at low temperatures were slower but good efficacy was achieved by Day 35. Although control fish had to be treated with hydrogen peroxide at Day 21, fish treated only with emamectin benzoate on Days 0 to 6 still had 89% fewer lice than control fish at Day 35. There were very few C. elongatus present, but at the end of both trials numbers were lower on treated fish. No adverse effects were associated with treatment of fish with emamectin benzoate.     

The abundance and distribution of Lepeophtheirus salmonis (Copepoda: Caligidae) on pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon in coastal British Columbia

In total, 23,750 specimens of the salmon louse, Lepeophtheirus salmonis, were collected from 3,907 juvenile pink and 3,941 chum salmon caught within the Broughton Archipelago during a 2-yr survey. The prevalence on pink salmon was significantly higher than on chum salmon in 2004 (62.3% and 58.6%, respectively) and in 2005 (26.4% and 23.1%, respectively). The mean abundance on chum salmon was significantly higher than on pink salmon in 2004 (7.0 +/- 0.3 and 2.8 +/- 0.2, respectively), whereas in 2005 the mean abundance did not differ between species (0.6 +/- 0.1 and 0.5 +/- 0.0, respectively). The mean intensity on chum salmon was significantly higher than on pink salmon in 2004 (12.0 +/- 0.4 and 4.5 +/- 0.2, respectively) and in 2005 (2.5 +/- 0.2 and 1.7 +/- 0.1, respectively). The prevalence, intensity, and abundance of L. salmonis were significantly higher on salmon belonging to both host species in 2004 compared with 2005. In both years, a majority of pink and chum salmon had 2 or fewer lice. In general, a decline in abundance of L. salmonis over the 3 collection periods in each year coincided with an increased percentage of motile developmental stages. The abundance was lowest on fish collected from zones in which the seawater surface salinity was also lowest. Seawater surface temperature was higher and salinity was lower in 2004 compared with 2005. The spatial and temporal trends in the abundance of L. salmonis in relation to host size, infestation rates, and seawater salinity and temperature, evident in both years, must be considered in future studies assessing the role of farmed salmon in the epizootiology of this parasite on juvenile salmon in this area.     

Sea lice--major pathogens of farmed atlantic salmon

The most important metazoan parasites of farmed Atlantic salmon are the sea lice Lepeophtheirus salmonis and Caligus elongatus. Adults of these caligid copepods are responsible for serious damage to the skin of salmon, which may die unless treated with organophosphorus pesticides. Alan Pike presents what is known of the biology of these important pathogens and highlights the shortcomings of this knowledge in terms of useful insights which could lead to more effective control measures.     

Enzymes released from Lepeophtheirus salmonis in response to mucus from different salmonids

Adult and mobile preadult sea lice Lepophtheirus salmonis were incubated with mucus samples from rainbow trout (Oncorhynchus mykiss), coho salmon (O. kisutch), Atlantic salmon (Salmo salar), and winter flounder (Pseudopleuronectes americanus) to determine the response of L. salmonis to fish skin mucus as assessed by the release of proteases and alkaline phosphatase. There was variation in the release of respective enzymes by sea lice in response to different fish. As well, sealice collected from British Columbia responded differently than New Brunswick sea lice to coho salmon mucus. Fish mucus and seawater samples were also analyzed using protease gel zymography to observe changes in the presence of low molecular weight (LMW) proteases after L. salmonis incubation. Significantly higher proportions of sea lice secreted multiple bands of L. salmonis-derived LMW proteases after incubation with rainbow trout or Atlantic salmon mucus in comparison with seawater, coho salmon, or winter flounder mucus. Susceptibility to L. salmonis infections may be related to the stimulation of LMW proteases from L. salmonis by fish mucus. The resistance of coho salmon to L. salmonis infection may be due to agents in their mucus that block the secretion of these LMW proteases or factors may exist in the mucus of susceptible species that stimulate their release.     

Frontal filament morphogenesis in the salmon louse Lepeophtheirus salmonis.

The objective of the present study was to understand how and when the frontal filament (FF) in the salmon louse Lepeophtheirus salmonis is produced by examining the sequence of morphological changes leading to FF production in the copepodid and early chalimus stages. Atlantic salmon (Salmo salar) were heavily infested with newly molted copepodids. Sea lice were sampled prior to infestation and at 1, 2, 3, 4, 5, 6, 7, 8, and 9 days postinfestation. FF morphogenesis from newly molted copepodid to chalimus II, i.e., through 2 molts, was studied using high-resolution light microscopy of serial transverse and sagittal resin sections. Three groups of cells, identified as A, B, and C, are thought to be involved in the production of the secretions (S1 and S2) that form the filament material. The amount and shape of S1 and S2 and their association with B- and C-group cells, respectively, changed with the molt cycle. The following scenario for FF development is proposed: the first secretion to form after the molt for both copepodid and chalimus stages is S1, and it is formed by B-group cells and becomes the basal plate of the external FE C-group cells produce S2 during mid-intermolt to premolt stage. The S2 becomes the stem of the external FE In premolt larvae, S1 and S2 were contained within a cuticle-lined invagination that had a form similar to that of the extruded filament. The axial duct present in both copepodid and chalimus originates from the A-group cells and probably carries a secretion used to attach the filament to the host. This study provides strong evidence that L. salmonis produces a new filament with each molt, creating the possibility of using a sea lice control method based on interference with filament production more feasible.     

Ecology of sea lice parasitic on farmed and wild fish

Sea lice, especially Lepeophtheirus salmonis and Caligus spp., have the greatest economic impact of any parasite in salmonid fish farming and are also a threat to wild salmonids. Here, I review how the biology and ecology of various louse and host species influence their pathogenicity and epidemiology. Recent discoveries of new species and genotypes emphasize the need for more basic research on louse taxonomy and host preferences. Louse development rates are strongly dependent on temperature, and increasing mean sea temperatures are likely to increase infestation pressure on farms and wild fish, as well as affecting the geographical distribution of hosts and parasites. Despite progress in finding L. salmonis larvae in the plankton and in modelling louse production in several countries, more data on larval behaviour and distribution are required to develop dispersal and transmission models for both L. salmonis and Caligus spp. This knowledge could be used to take measures to reduce the risks of lice affecting farmed and wild fish.     

Identifying epidemiological factors affecting sea lice Lepeophtheirus salmonis abundance on Scottish salmon farms using general linear models

The variation in Lepeophtheirus salmonis sea lice numbers across 40 Scottish salmon farm sites during 1996 to 2000 was analysed using mean mobile abundance for 3 important 6 mo periods within the production cycle. Using statistical regression techniques, over 20 management and environmental variables suspected to have an effect on controlling lice populations were investigated as potential risk factors. The findings and models developed provide a picture of mobile L. salmonis infestation patterns on Scottish farm sites collectively. The results identified level of treatment, type of treatment, cage volume, current speed, loch flushing time and sea lice levels in the preceding 6 mo period to be key explanatory factors. Factors such as stocking density, site biomass, water temperature and the presence of neighbours, previously cited to be important correlates of sea lice risk from analysis of individual sites over time, were not found to be important. Variation in mobile abundance in the first half of the second year of production could be adequately explained (adjusted R2 between 55 and 72%) by the recorded data, suggesting that there is scope for management to control L. salmonis abundance, though much of the variation remains unexplained.     

Historical control clinical trial to assess the effectiveness of teflubenzuron for treating sea lice on Atlantic salmon

A historical control clinical trial was performed to assess the effectiveness of teflubenzuron in controlling sea lice Lepeoptheirus salmonis burdens on farmed Atlantic salmon Salmo salar over time. The study site comprised 9 sea cages, all of which were treated. The teflubenzuron was administered in the feed, at a dosage of 10 mg kg(-1) biomass d(-1), over a treatment period of 7 d. At 1 wk post-treatment, sea lice chalimus and mobile stages were reduced by 92 and 74% (both p < 0.001), respectively. At 2 wk post-treatment, chalimus stages were reduced by 41% and mobile stages 61% (both p < 0.001) compared to pre-treatment levels. At 3 wk post-treatment, chalimus stages were still 36% (p < 0.001) lower than pre-treatment levels, but mobile stages had increased to above pre-treatment levels. Our results show that the effects of teflubenzuron are limited to a 3 wk duration, but that with appropriate management, farms could benefit from these reduced lice burdens for longer periods.     

Chemoreception in the salmon louse Lepeophtheirus salmonis: an electrophysiology approach

The search for effective and long-term solutions to the problems caused by salmon lice Lepeophtheirus salmonis (Kr?yer, 1837) has increasingly included biological/ecological mechanisms to combat infestation. One aspect of this work focuses on the host-associated stimuli that parasites use to locate and discriminate a compatible host. In this study we used electrophysiological recordings made directly from the antennule of adult lice to investigate the chemosensitivity of L. salmonis to putative chemical attractants from fish flesh, prepared by soaking whole fish tissue in seawater. There was a clear physiological response to whole fish extract (WFX) with threshold sensitivity at a dilution of 10 . When WFX was size fractionated, L. salmonis showed the greatest responses to the water-soluble fractions containing compounds between 1 and 10 kDa. The results suggest that the low molecular weight, water-soluble compounds found in salmon flesh may be important in salmon lice host choice.     

Effects of sea lice (Lepeophtheirus salmonis Kröyer, 1837) infestation on macrophage functions in Atlantic salmon (Salmo salar L.)

Experiments were conducted to determine the effects of sea lice, Lepeophtheirus salmonis, on non-specific defence mechanisms in Atlantic salmon, Salmo salar, by experimentally infesting hatchery-reared 1 and 2 year old post-smolts, S1 and S2, with laboratory grown infective copepodids at moderate to high infection intensities ranging from 15-285 lice per fish. The effects of sea lice-induced stress were investigated by measuring the blood levels of cortisol and glucose as indicators of primary and secondary stress responses, and by changes in macrophage respiratory burst activity and phagocytosis as indicators of tertiary stress responses as well as non-specific defence mechanisms. Fish were sampled prior to sea lice infestation at day 0 and at days 3, 7, 14 and 21 post-infestation. Sea lice were at copepodid stage at day 3, at chalimus stages at days 7 and 14, and at pre-adult stage at day 21. Blood levels of cortisol and glucose were found to be significantly increased at day 21 in fish-infested with the highest levels. Macrophage respiratory burst and phagocytic activities were found to be significantly decreased only at day 21. These results indicate that sea lice do not suppress host defence mechanisms during the earlier stages of infestation. They do have effects on the development of chronic stress and on the host non-specific defence mechanisms soon after the lice reach the pre-adult stage.     

Development and application of real-time PCR for specific detection of Lepeophtheirus salmonis and Caligus elongatus larvae in Scottish plankton samples

Lepeophtheirus salmonis and Caligus elongatus are important parasites of wild and cultured salmonids in the Northern Hemisphere. These species, generically referred to as sea lice, are estimated to cost the Scottish aquaculture industry in excess of pound 25 million per annum. There is great interest in countries such as Ireland, Scotland, Norway and Canada to sample sea lice larvae in their natural environment in order to understand lice larvae distribution and improve parasite control. Microscopy is currently relied on for use in the routine identification of sea lice larvae in plankton samples. This method is, however, limited by its time-consuming nature and requirement for highly skilled personnel. The development of alternative methods for the detection of sea lice larvae which might be used to complement and support microscopic examinations of environmental samples is thus desirable. In this study, a genetic method utilising a real-time PCR Taqman-MGB probe-based assay targeting the mitochondrial cytochrome oxidase I (mtCOI) gene was developed, which allowed species-specific detection of L. salmonis and C. elongatus larvae from unsorted natural and spiked plankton samples. Real-time PCR is a rapid, sensitive, highly specific and potentially quantitative technique. This study demonstrated its suitability for the routine identification of L. salmonis and C. elongatus in mixed plankton samples. The real-time PCR assay developed has considerable potential for use in complementing, supporting and reducing reliance on time-consuming conventional microscopic examination for the specific identification of sea lice larvae in plankton samples.     

Does increased abundance of sea lice influence survival of wild Atlantic salmon post‐smolt?

A synthesis of results from two projects was assessed to analyse possible influence of sea lice Lepeophtheirus salmonis on marine Atlantic salmon Salmo salar survival. During the years 1992–2004, trawling for wild migrating post-smolts was performed in Trondheimsfjord, a fjord in which no Atlantic salmon aquaculture activity is permitted. Prevalence and intensity of sea lice infections on migrating wild post-smolts differed between years. A correlation analysis between 1 sea-winter (SW) Atlantic salmon catch statistics from the River Orkla (a Trondheimsfjord river) and sea lice infections on the migrating smolts in the Trondheimsfjord was not significant. Up to 2% reduction in adult returns due to sea-lice infection was expected. In addition, experimental releases from 1996 to 1998 with individually tagged groups of hatchery-reared Atlantic salmon smolts given protection against sea-lice infection was performed. Higher recaptures of adult Atlantic salmon from 1998 treated smolts compared to the control group may correspond to high abundance of sea lice found on the wild smolt, and may indicate influence on post-smolt mortality. These studies indicate that post-smolt mortality in Trondheimsfjord is marginally influenced by sea lice infection; however, the methods for assessing wild smolt mortality might be insufficient. Higher infections of sea lice farther out in the fjord may indicate more loss in Atlantic salmon returns in some years.     

Salmon lice, Lepeophtheirus salmonis, infestation as a causal agent of premature return to rivers and estuaries by sea trout, Salmo trutta, juveniles

A field experiment conducted in the River Lønningdalselven in spring 1992 supports the hypothesis that salmon lice, Lepeophtheirus salmonis, infestations may cause premature return of sea trout, Salmo trutta, juveniles, either to estuaries or to rivers. When lice infested (exposed) and uninfested (control) sea trout juveniles (post smolts) were released simultaneously into the sea, exposed fish returned to the estuarine area earlier compared with controls. Within the following two days, exposed sea trout migrated further into freshwater. At that time they were infested with a median of 62.5 lice, dominated by chalimus larvae and late juveniles. Exposed sea trout suffered from an osmoregulatory failure in sea water and this is considered one reason for infested fish returning to brackish water. While only a few control fish returned to the estuary on the day of release, some more returned to freshwater the following four days. During this time they had become heavily infested with copepodids, and carried a median of 150.0 lice. It is suggested that physiological stress and high infection pressure in the sea results in sea trout juveniles returning to estuaries and freshwater.     

Temporal stability of sea louse Lepeophtheirus salmonis Krøyer populations on Atlantic salmon Salmo salar L. of wild, farm and hybrid parentage

Atlantic salmon salmo salar smolts of wild, hybrid and farmed parentage were individually tagged then reared in a sea cage for 8 months. The fish were sampled three times during this period. On all occasions, farmed Atlantic salmon displayed the highest abundance and density of sea lice Lepeophtheirus salmonis , whilst no significant differences were observed between hybrid and wild Atlantic salmon. Percentage variation between the lowest and highest infected groups was as high as 175 and 144% for L. salmonis abundance and density respectively (sample 2). The temporal stability of interindividual sea lice infection levels was investigated pair‐wise between samples using correlation (sample 1 v . 2, 1 v . 3 and 2 v . 3). When calculated using sea louse abundance, correlations ranged from r ² = 0·11, P  < 0·01 to r ² = 0·39, P  < 0·001, but, when the effects of fish size were controlled for by converting abundance to density, all correlations were <  r ² = 0·1. Therefore, these data indicate that a fish's relative infection level in one sample was a weak predictor of its relative infection level in another sample. This suggests that identification of individual Atlantic salmon that display reduced susceptibility to sea lice, may be problematic.