Lepeophtheirus Salmonis Research Articles

Effect of diel light cycles on vertical migration patterns of Lepeophtheirus salmonis (Krøyer, 1837) copepodids measured in an in situ mesocosm

The ectoparasitic salmon louse (Lepeophtheirus salmonis Krøyer, 1837) persists as a major hindrance for continued growth of the Norwegian salmon industry. Shielding technologies or management strategies are being used to reduce the likelihood of contact in the net pens between the salmon and the sea lice’s infectious copepodid stage. Knowledge about the vertical distribution and diel migration patterns of the planktonic sea lice stages are inevitably a premise for such water-depth-dependent avoidance technologies to be efficient; however, not many studies have investigated diel vertical migration patterns of sea lice in situ or in relevant scales. In this work, we aimed to monitor the diel migration of L. salmonis copepodids in the vertical plane without the influence of hydrographical forces, using sufficient water column depth of 11 m, natural thermoclines, and natural exogenous light conditions. We followed a cohort of copepodids over the course of 4 days and measured their vertical distribution in the water column twice per hour in a custom made mesocosm, using automatic particle detection and machine learning to analyse position and migration patterns. The L. salmonis copepodid population displayed reverse diel migration, and we found a statistically significant interaction between depth and time. During the day, copepodids were found in the upper 1 m of the water column, before the majority of the population descended to 1–2 m depth during night-time. A small fraction of the population remained in the upper part of the water column also during night-time. A significantly higher fraction of the copepodid population resided in the upper metre during daylight hours compared to other depths. These results could be utilised as input in sea lice dispersal models which are important management tools in today’s sea lice prevention strategies and regulatory frameworks.

Effects of regional coordination of salmon louse control in reducing negative impacts of salmonid aquaculture on wild salmonids

Parasitic salmon lice (Lepeophtheirus salmonis) are a constraint to the sustainable growth of salmonids in open net pens, and this issue has caused production to level off in recent years in the most aquaculture-intensive areas of Norway. The maximum allowed biomass at a regional level is regulated by using the so-called “traffic light” system, where salmon louse-induced mortality of migrating wild salmon post-smolts is evaluated against set targets. As a case study, we have investigated how a specific aquaculture-intensive area can reduce its louse levels sufficiently to achieve a low impact on wild salmon. Analyses of the output from a virtual post-smolt model that uses data on the reported number of salmon lice in fish farms as key input data and estimates the salmon louse-induced mortality of wild out-migrating Atlantic salmon post-smolts, suggested that female louse abundance on the local farms must be halved in spring to reach the goal implied by the traffic light system. The outcome of a modelling scenario simulating a proposed new plan for coordinated production and fallowing proved beneficial, with an overall reduction in louse infestations and treatment efforts. The interannual variability in louse abundance in spring, however, increased for this scenario, implying unacceptably high louse abundance when many farms were in their second production year. We then combined the scenario with coordinated production with other louse control measures. Only measures that reduced the density of farmed salmonids in open cages in the study area resulted in reductions in salmon louse infestations to acceptable levels. This could be achieved either by stocking with larger fish to reduce exposure time or by reducing fish numbers, e.g. by producing in closed units.

Genetic architecture and accuracy of predicted genomic breeding values for sea lice resistance in the St John River aquaculture strain of North American Atlantic salmon

Sea lice infections cause significant economic losses in Atlantic salmon (Salmo salar) farming. The objectives of this study were to (1) estimate the heritability of resistance to sea lice in the USDA's breeding program for North American (N.A.) Atlantic salmon; (2) elucidate the genetic architecture of sea lice resistance; and (3) assess the accuracy and bias of predicted breeding values for sea lice resistance using cross-validation analysis (CVA) and progeny testing of selection candidates (PTSC). Fish from year-class (YC) 2014 n=967 and YC 2018 n=941 were challenged with sea lice (Lepeophtheirus salmonis) and genotyped with the new USDA 50K SNP chip developed for the N.A. Atlantic salmon. Heritability estimates for sea lice count in the two year-classes were between 0.07 and 0.10. GWAS with weighted ssGBLUP (wssGBLUP) highlighted the polygenic architecture of the trait. With CVA, the genomic prediction methods x¯=0.54 had higher accuracy of predictions than the pedigree-based method x¯=0.43; and ssGBLUP had the highest accuracy of BV predictions (0.50–0.62) among the genomic prediction methods. With PTSC approach, the pedigree-based method PBLUP had higher estimated accuracy than the genomic prediction methods (0.48 vs. 0.30). Bias of BV predictions with CVA x¯=0.87range0.57−1.06 was lower than with PTSC x¯=1.22range0.44−2.35. Our retrospective assessment of the accuracy of indirect sib-based selection to improve sea lice resistance in the St John River aquaculture stock that is commonly used for farming of Atlantic salmon in North America demonstrated that substantial genetic gains can be obtained. Due to the polygenic architecture of the trait, the estimated accuracy of the ssGBLUP model was better than the wssGBLUP model. In our assessment, a larger training sample size will be needed to achieve optimal results with the whole genome-enabled selective breeding method due to the polygenic architecture of sea lice resistance coupled with low heritability of the trait.

Association of ectoparasite Lepeophtheirus salmonis counts on farmed Atlantic salmon and wild sea trout in Scotland.

Parasitic sea lice (Copepoda: Caligidae) colonising marine salmonid (Salmoniformes: Salmonidae) aquaculture production facilities have been implicated as a possible pressure on wild salmon and sea trout populations. This investigation uses monitoring data from the mainland west coast and Western Isles of Scotland to estimate the association of the abundance of adult female Lepeophtheirus salmonis (Krøyer) colonising farmed Atlantic salmon Salmo salar L. with the occurrence of juvenile and mobile L. salmonis on wild sea trout, anadromous S. trutta L. The associations were evaluated using generalised linear mixed models incorporating farmed adult female salmon louse abundances which are temporally lagged relative to dependent wild trout values. The pattern of lags, which is consistent with time for L. salmonis development between egg and infective stage, was evaluated using model deviances. A significant positive association is identified between adult female L. salmonis abundance on farms and juvenile L. salmonis on wild trout. This association is consistent with a causal relationship in which increases in the number of L. salmonis copepodids originating from lice colonising farmed Atlantic salmon cause an increase of L. salmonis abundance on wild sea trout.

Comparison of DNA metabarcoding and morphological diet analysis of lumpfish (Cyclopterus lumpus) as methods of estimating sea lice cleaning efficacy inside commercial Atlantic salmon (Salmo salar) sea cages

Lumpfish (Cyclopterus lumpus) are often used as cleaner fish inside salmonid sea cages for biological control of ectoparasitic sea lice. We compared traditional morphological diet assessments with DNA metabarcoding as methods of estimating lumpfish cleaning efficacy as a function of its size, and of its ingestion of alternative prey. Traditional morphological analysis was performed on 122 ethanol-preserved stomachs. The stomachs were removed in the field - following eDNA protocols – from lumpfish collected from industrial Atlantic salmon sea cages in southern Newfoundland. Morphological stomach analysis gave semi-quantitative estimates of prey counts and/or biomass at the order or family level; it showed that shorter lumpfish ate more gammarid amphipods whereas longer lumpfish ate more formulated feed pellets designed for salmon. No sea lice were confidentially morphologically identified in the lumpfish stomachs. In contrast, COI metabarcoding was able to confidently identify the presence of the salmon louse (Lepeophtheirus salmonis) in 61 of 82 successfully barcoded lumpfish stomachs. Metabarcoding showed that diurnally-migrating krill and gammarid amphipods were present in more stomachs in 2018 whereas formulated feed pellets – identified from COI sequences classified as domestic poultry and non-indigenous fish species - were present in more stomachs in 2020. We argue that, when weighed by the total number of reads, the counts per stomach of the number of Global Estimated Sequence Variants (ESV) classified as a particular prey species could be used to estimate the single-species indices of abundance needed for lumpfish breeding values. The number of ESVs classified as the salmon louse could be used to estimate an index of lumpfish cleaning efficacy. As proof-of-concept, we show that 83% of the variance in the number of Norwegian krill heads observed per stomach was explained by two weighed covariates: 1) Norwegian krill ESV counts and 2) lumpfish stomach contents weight.

The impact of dissolved oxygen concentration on the swimming performance and welfare of ballan wrasse (Labrus bergylta Ascanius, 1767).

Sea lice (Lepeophtheirus salmonis [Krøyer, 1838]) are a key issue for salmon aquaculture, contributing to increased mortality for both wild and farmed salmon if no action is taken. Using cleaner fish can be an effective, drug-free treatment method, and ballan wrasse (Labrus bergylta) is a hardy wrasse species that displays cleaning behavior. With concerns about the overharvest of wild ballan wrasse, many companies farm this species, but the optimal ranges of a wide variety of rearing parameters are still unknown. This study investigated the effect of 6-week exposure to four dissolved oxygen (DO) levels (125%, 100%, 85%, and 75% DO saturation as the percentage of air) on ballan wrasse. Survival; growth (specific growth rate, SGR); condition factor (CF); and primary (cortisol), secondary (glucose, lactate, magnesium), and tertiary stress indicators (swimming performance) were investigated. There were no differences in SGR, CF, survival, or cortisol level among the groups at the end of the 6 weeks. There was variation in the magnitude of the cortisol response to an acute stressor at the end of the 6-week period, with the 75% DO treatment exhibiting a 3.3-fold increase in cortisol compared to a 5.2-fold increase in the control group (100%), which could suggest chronic stress. Relative critical swimming speed (RUcrit) was measured to investigate swimming performance once all groups were returned to 100% DO saturation. The 75% RUcrit was lower than the 100% treatment (1.7 ± 0.18 body length [BL]/s compared to 2.5 ± 0.16 BL/s). Overall, these results suggest that DO levels of 75% trigger physiological changes and therefore may negatively affect welfare.

Consequences of reduced effectiveness of salmon lice treatments for lice control

The effective control of ectoparasitic salmon lice, Lepeophtheirus salmonis, in fish farms is challenged by the salmon lice having developed resistance towards several antiparasitic drugs and by the effectiveness of non-medicinal treatments being limited by considerations of fish welfare. When new antiparasitics are introduced to the market, these should be used sparingly to slow resistance development. Using a population model for salmon lice parameterised for salmonid fish farms in Norway, we quantified how reduced treatment effectiveness influences treatment frequency and lice abundance. Furthermore, we investigated when in the production cycle a highly effective lice treatment leads to the largest reduction in the total number of treatments, mean lice abundance and lice larvae production. Results showed that reductions in treatment effectiveness to lower than 50% led to the steepest increases in treatment frequency and mean lice abundance, as well as to increased risk that lice abundance increased beyond control. The timing of the most effective treatment had only moderate effects on the total treatment need and the mean number of adult female lice through the production cycle, but large effect on the production of lice larvae in spring. These findings imply that farmers can optimise the timing of the most effective treatment to reduce the release of lice larvae in the period of year when wild salmonids are in coastal waters, without compromising total treatment need or mean lice levels.

Louse‐induced mortality thresholds in Atlantic salmon of wild‐origin

AbstractWild salmonid fish are faced with the challenge of infections from the ectoparasitic salmon louse, which thus dictates the management of salmon aquaculture in areas where farms and wild populations co‐exist. The mortality risk of wild Atlantic salmon is used as an indicator of threat levels when monitoring wild populations, and therefore the aim of this study was to robustly investigate the effect of infection intensity on probability of mortality in salmon of wild origin. To assess critical infection levels, recently‐smoltified Atlantic salmon were infected with intensities ranging from 0 to 2.5 lice g−1 and exposed to a 48‐h simulated migration. The first mortalities occurred when preadults appeared and ceased as lice became adults. Infection intensity negatively affected mortality probability and survival time, whereas the simulated migration had no influence on the probability of mortality. Individual‐level effects of infection on host mortality is difficult to quantify in the field; however, the correlation between louse load and mortality probability obtained in this study can facilitate indirect estimates of mortality risk of wild salmon populations and contribute to conservation efforts associated with this parasite.

Salmon louse labial gland enzymes: implications for host settlement and immune modulation.

Salmon louse (Lepeophtheirus salmonis) is a skin- and blood-feeding ectoparasite, infesting salmonids. While feeding, labial gland proteins from the salmon louse may be deposited on the Atlantic salmon (Salmo salar) skin. Previously characterized labial gland proteins are involved in anti-coagulation and may contribute to inhibiting Atlantic salmon from mounting a sufficient immune response against the ectoparasite. As labial gland proteins seem to be important in the host-parasite interaction, we have, therefore, identified and characterized ten enzymes localized to the labial gland. They are a large group of astacins named L. salmonis labial gland astacin 1-8 (LsLGA 1-8), one serine protease named L. salmonis labial gland serine protease 1 (LsLGSP1), and one apyrase named L. salmonis labial gland apyrase 1 (LsLGAp1). Protein domain predictions showed that LsLGA proteins all have N-terminal ShK domains, which may bind to potassium channels targeting the astacins to its substrate. LsLGA1 and -4 are, in addition, expressed in another gland type, whose secrete also meets the host-parasite interface. This suggests that LsLGA proteins may have an anti-microbial function and may prevent secondary infections in the wounds. LsLGAp1 is predicted to hydrolyze ATP or AMP and is, thereby, suggested to have an immune dampening function. In a knockdown study targeting LsLGSP1, a significant increase in IL-8 and MMP13 at the skin infestation site was seen under LsLGSP1 knockdown salmon louse compared to the control, suggesting that LsLGSP1 may have an anti-inflammatory effect. Moreover, most of the identified labial gland proteins are expressed in mature copepodids prior to host settlement, are not regulated by starvation, and are expressed at similar or higher levels in lice infesting the salmon louse-resistant pink salmon (Oncorhynchus gorbuscha). This study, thereby, emphasizes the importance of labial gland proteins for host settlement and their immune dampening function. This work can further contribute to anti-salmon louse treatment such as vaccine development, functional feed, or gene-edited salmon louse-resistant Atlantic salmon.

Histopathological assessment of Atlantic salmon exposed to calcium oxide particles: a controlled clinical study

Particulate calcium oxide (CaO) has been successfully used for the control of sea urchin and starfish populations. Recent studies have proven its efficacy in killing planktonic copepods, making it a promising and cheap option for salmon louse control in the Atlantic salmon (Salmo salar) farming industry. Additionally, preliminary laboratory tests demonstrated that exposure to 0.2 g/L–0.6 g/L of fine CaO particles induced significant mortality of salmon louse (Lepeophtheirus salmonis) in the planktonic stages. The present study investigated the effects of water treatment with fine CaO particles [0.2 g/L or 127.4 g/m2; 0.1 mm–0.3 mm], conducted twice a week for three consecutive weeks at 5°C and 12°C on Atlantic salmon post-smolt in a flow-through system. The study compared mortality rates and histopathology of skin, eyes, gills, and intestines between treated and untreated control salmon. The results indicated that CaO exposure did not induce fish mortality or histopathological damages in the skin, eyes, or intestines. Although there were no significant effects of CaO exposure on gill inflammation or hyperplasia, the exposure did increase the occurrence of gill vascular injuries and necrosis in small portions of the tissue (< 10% of the respiratory gill tissue in most of the samples) by 60% and 35%, respectively. The effect was not modulated by temperature or time. The results indicate that the CaO treatment protocol used in this study is not lethal to Atlantic salmon and is safe for most analyzed tissues. Nonetheless, the induction of vascular damage and necrosis in the gill, albeit in small tissue portions, emphasizes the need for further research. It is crucial to develop a protocol that avoids such lesions, ensuring the secure application of CaO as an antiparasitic treatment in salmon aquaculture.

Novel large‐scale mapping highlights poor state of sea trout populations

Abstract The state of sea trout in 1251 Norwegian watercourses was assessed based on a scoring system for human pressures, abundance data, and local knowledge. Over 16,000 km of rivers and lakes were available to sea trout in these watercourses, spanning from the temperate to Arctic regions. Sea trout were classified to be in a good or very good state in fewer than 25% of the watercourses and in a poor or very poor state in almost 40%. Twenty‐nine watercourses had lost their sea trout populations. Salmon lice from aquaculture salmon farms had by far the largest adverse effect on sea trout among the human impact factors, both in the number of watercourses (83%) and river area affected (60%), and the total effect on sea trout abundance. Agriculture and hydropower production also had strong adverse impacts (35% and 19% of watercourses), but substantially lower than that caused by salmon lice. Culverts related to road crossings and other habitat alterations also had impacts on sea trout in many watercourses (27%). Exploitation of sea trout has been reduced in Norway in recent years, both in the marine and freshwater fisheries. Yet, the exploitation pressure was moderate or high in almost 14% of the watercourses where the state of sea trout was poor or very poor, suggesting a high potential for overexploitation in these. The state of sea trout was best in the northern sparsely populated areas. However, distribution of watercourses with sea trout in a poor or very poor state was more linked to aquaculture, agriculture, and hydropower production than human population density. The developed approach for large‐scale mapping of state and pressures, which is vital for prioritizing management measures, may inspire other nations in their conservation effort for this important species.

Use of whole-genome sequence data for fine mapping and genomic prediction of sea louse resistance in Atlantic salmon.

Sea louse (Lepeophtheirus salmonis) infestation of Atlantic salmon (Salmo salar) is a significant challenge in aquaculture. Over the years, this parasite has developed immunity to medicinal control compounds, and non-medicinal control methods have been proven to be stressful, hence the need to study the genomic architecture of salmon resistance to sea lice. Thus, this research used whole-genome sequence (WGS) data to study the genetic basis of the trait since most research using fewer SNPs did not identify significant quantitative trait loci. Mowi Genetics AS provided the genotype (50k SNPs) and phenotype data for this research after conducting a sea lice challenge test on 3,185 salmon smolts belonging to 191 full-sib families. The 50 k SNP genotype was imputed to WGS using the information from 197 closely related individuals with sequence data. The WGS and 50k SNPs of the challenged population were then used to estimate genetic parameters, perform a genome-wide association study (GWAS), predict genomic breeding values, and estimate its accuracy for host resistance to sea lice. The heritability of host resistance to sea lice was estimated to be 0.21 and 0.22, while the accuracy of genomic prediction was estimated to be 0.65 and 0.64 for array and WGS data, respectively. In addition, the association test using both array and WGS data did not identify any marker associated with sea lice resistance at the genome-wide level. We conclude that sea lice resistance is a polygenic trait that is moderately heritable. The genomic predictions using medium-density SNP genotyping array were equally good or better than those based on WGS data.

Molecular quantification of parasitic sea louse larvae depends on species and life stage.

Sea lice cause substantial economic and environmental harm to Norway's aquaculture industry and wild salmonid populations. Rapid, accurate quantification of lice larval densities in coastal waters remains the greatest bottleneck for providing empirical data on infestation risk within wild salmon habitats and aquaculture production regions. We evaluated the capability of droplet digital PCR (ddPCR) as an absolute quantification method for the planktonic stages of two parasitic louse species, Lepeophtheirus salmonis (Krøyer) and Caligus elongatus (von Nordman). Results demonstrated linear relationships between the DNA quantity measured and the number of spiked larvae for both species and life stages. However, L. salmonis contained a significantly greater number of DNA copies than C. elongatus individuals and for C. elongatus, nauplii displayed a significantly higher number of DNA copies than copepodids. Our results suggest that ddPCR can effectively enumerate louse larvae, but interpreting ddPCR results differ between the two louse species. Obtaining larval abundance estimates from marine plankton samples will depend on the nauplii to copepodid ratio for C. elongatus, but not for L. salmonis.

Exploring the impact of thermal delousing on gill health and microbiome dynamics in farmed Atlantic Salmon

The economic cost of salmon louse (Lepeophtheirus salmonis and Caligus rogercresseyi) infestations in the Atlantic salmon (Salmo salar) industry has been estimated to be around $900 million annually. This high cost has driven a concerted effort to develop, examine, understand, and implement various methods for louse control. Husbandry interventions utilising warm water exposure have been highly successful in complementing traditional chemotherapeutants, especially as the efficiency of the latter has reduced in recent years. In this study, we sought to examine the impact of thermal delousing on gill health in two commercial sites with different historical husbandry and treatment interventions prior to and post-thermal treatment. Methods to characterise gill health and the response to thermal treatment included a detailed examination of the gill for microparasites using both histology and qPCR and targeted immune gene expression analysis, most notably antigen-presenting cells (mhc ii), proinflammatory cytokines (il-1β and tnf-α) and inhibitory cytokines (tgf-β and il-10). Furthermore, we examined the bacterial communities present on the gill surface using 16S rRNA amplicon sequencing. Data obtained from these trials indicated a minimal impact on gill microparasite prevalence in response to the thermal treatment. The expression of immune markers exhibited a significant decrease across both sites after treatment. Intriguingly, marked differences in the gill bacterial communities in response to treatment between the two sites were clearly observed. This divergence could be attributed to the notable differences in husbandry history and health status of the fish at the two sites prior to the thermal treatment. Our data suggest that microbiome diversity is an informative indicator of fish gill health and could be used to define appropriate interventions when treating sea lice.

Reduced Infestation Levels of Lepeophtheirus salmonis in Atlantic Salmon (Salmo salar) following Immersion Exposure to Probiotic Aliivibrio spp.

Salmon lice (Lepeophtheirus salmonis) constitute a major challenge during the production of farmed Atlantic salmon in Norway. Preventive measures are considered to have a higher impact on sustainable control than lice treatment. Therefore, the studies presented here aimed to document the preventive effects of probiotic Aliivibrio spp. on lice infestation in experimental challenges. A reduction in salmon lice attachment success (58–65%) was observed in two separate aquarium trials, where Atlantic salmon were exposed to different compositions of Aliivibrio species 91 and 155 days prior to lice challenge. In a third trial, no difference in attachment was observed in groups exposed to probiotics 58 days prior to lice challenge compared to controls. However, a relative reduction in lice counts was seen on movable stages later in the trial. High levels of probiotic bacteria had no impact on lice viability in an in vitro bioassay on the preadult life stage; thus, the mechanism behind the preventative effect remains unknown. In conclusion, probiotic Aliivibrio bacteria can likely be used as a preventive tool to reduce salmon louse infestations in the salmon industry. The mechanism is still unknown, and this novel approach to lice control warrants further investigation to understand its optimal use and potential.

Evolutionary predictions for a parasite metapopulation: Modelling salmon louse resistance to pest controls in aquaculture.

Pests often evolve resistance to pest controls used in agriculture and aquaculture. The rate of pest adaptation is influenced by the type of control, the selective pressure it imposes, and the gene flow between farms. By understanding how these factors influence evolution at the metapopulation level, pest management strategies that prevent resistance from evolving can be developed. We developed a model for the metapopulation and evolutionary dynamics of the salmon louse (Lepeophtheirus salmonis), which is a major parasite affecting salmon aquaculture. Different management scenarios were simulated across a network of salmon farms covering half of Norway, and their effects on louse epidemiology and evolution were investigated. We compared louse controls that differed in how they were deployed through time (discrete vs. continuous), how they impacted the louse life cycle, and in their overall efficacy. We adjusted the strength of selection imposed by treatments, the dominance effect of the resistant allele, and the geographic location at which resistance originated. Continuously acting strategies (e.g., louse-resistant salmon) were generally more effective than discrete strategies at controlling lice, especially when they increased louse mortality during early developmental stages. However, effective strategies also risked imposing frequent and/or strong selection on lice, thus driving rapid adaptation. Resistant alleles were more likely to be lost through genetic drift when they were recessive, had a low-fitness advantage, or originated in low-farm-density areas. The north-flowing current along the Norwegian coastline dispersed resistant genes from south to north, and limited gene flow in the opposite direction. We demonstrate how evolutionary models can produce quantitative predictions over large spatial and temporal scales and for a range of pest control scenarios. Quantitative outputs can be translated into practical management decisions applied at a regional level to minimise the risk of resistance developing.

Transcriptome analysis revealed immune responses in the kidney of Atlantic salmon (Salmo salar) co-infected with sea lice (Lepeophtheirus salmonis) and infectious salmon anemia virus

Sea lice (Lepeophtheirus salmonis) and infectious salmon anemia virus (ISAv) are two of the most important pathogens in Atlantic salmon (Salmo salar) farming and typically cause substantial economic losses to the industry. However, the immune interactions between hosts and these pathogens are still unclear, especially in the scenario of co-infection. In this study, we artificially infected Atlantic salmon with sea lice and ISAv, and investigated the gene expression patterns of Atlantic salmon head kidneys in response to both lice only and co-infection with lice and ISAv by transcriptomic analysis. The challenge experiment indicated that co-infection resulted in a cumulative mortality rate of 47.8 %, while no mortality was observed in the lice alone infection. We identified 240 differentially expressed genes (DEGs) under the lice alone infection, of which 185 were down-regulated and 55 were up-regulated, while a total of 994 DEGs were identified in the co-infection, of which 206 were down-regulated and 788 were significantly up-regulated. The pathway enrichment analysis revealed that single-infection significantly suppressed the innate immune system (e.g., the complement system), whereas co-infection induced a strong immune response, leading to the activation of immune-related signaling pathways such as Toll-like receptors and NOD-like receptors pathways, as well as significant upregulation of genes related to the activation of interferon and MH class I protein complex. Our results provide the first global transcriptomic study of gene expression in the Atlantic salmon head kidney in response to co-infection with sea lice and ISAv, and provided the baseline knowledge for understanding the immune responses during co-infection.

Do DOM quality and origin affect the uptake and accumulation of lipid-soluble contaminants in coastal filter feeders? An experimental simulation of teflubenzuron exposure to blue mussels

The increased export of terrestrial dissolved organic matter (terrDOM) to coastal marine ecosystems may affect local filter feeders and the local food web via the altered uptake of organic material and associated contaminants. To compare terrDOM to marine DOM (marDOM) as contaminant vectors to coastal biota, we exposed blue mussels (Mytilus sp.) to the different DOM types in combination with teflubenzuron, a widely applied lipophilic aquaculture medicine targeting salmon lice (Lepeophtheirus salmonis). A 16-day exposure of the blue mussels to DOM and teflubenzuron was followed by a depuration phase of 20 days without teflubenzuron. We calculated teflubenzuron adsorption rates and bioaccumulation factors (BAF) using a Bayesian model, expecting teflubenzuron uptake to be greater with terrDOM than marDOM due to the higher prevalence of large amphipathic humic acids in terrDOM. Humic acids have strong absorption properties and are able to envelope lipophilic molecules. Thus, humic acids can function as an efficient contaminant vector when taken up by filter feeders. Although there were varying degrees of overlap, the mussels tended to accumulate higher amounts of teflubenzuron in the DOM treatments than in the seawater control (bioaccumulation factor [BAF] in seawater: median 106 L/kg; 2.5 %–97.5 % percentile: 69–160 L/kg). Contrary to expectations, mussels exposed to marDOM showed a trend toward more bioaccumulation of teflubenzuron than those exposed to terrDOM (BAF marine 144 L/kg; 102–221 L/kg versus BAF terrestrial: 121 L/kg; 82–186 L/kg). The highest teflubenzuron accumulation was observed with the 50:50 mixture of marDOM and terrDOM (BAF mix: 165 L/kg; 117–244 L/kg). The slight difference in DOM-type accumulation rates observed in this experiment—especially the accumulation rate of terrDOM compared to that of the seawater-only treatment type—was not considered environmentally relevant. Further studies are necessary to see if the observed trends transfer to complex environmental systems.

The decline in sea lice numbers during freshwater treatments in salmon aquaculture

Knowledge of using freshwater as a method to remove sea lice from farmed Atlantic salmon (Salmo salar) is limited. A field study was conducted on a specially designed well boat for delousing serving salmon farms to investigate the fate of sea lice on the salmon during freshwater treatment. The parasitic sea lice (Lepeophtheirus salmonis and Caligus elongatus) were registered at five different freshwater treatments at three aquaculture sites. Crowded salmon were pumped onboard to the starboard well and port well, with each well having a capacity of 1600 m3. The range of mean weight for the treated salmon was between 2.87 and 4.65 kg. The first sampling was made during crowding of the salmon from the net pens, followed by three samplings in each freshwater well during the treatment, and one sampling during the unloading of salmon back to the cages. Using freshwater as a delousing method generally had a high efficiency for L. salmonis and C. elongatus. There was a rapid reduction for all stages of L. salmonis and of C. elongatus in the initial phase of the freshwater treatment. The mean reduction for the earliest developmental stages of L. salmonis (sessile and preadult) was 70% and 63%. In contrast, females with egg strings and C. elongatus were almost completely removed within the first sampling (88% and 94%). For the adult stages (male and female without egg strings), there was a slower reduction (49% and 44%), respectively, at the first sampling. It is unclear how strongly the mechanical effect caused by crowding in the initial phase of the treatment affected attached sea lice. The rapid initial decrease in lice numbers might be a combination of freshwater and mechanical effects. There was further a high final reduction during unloading for sessile and preadult (89% and 93%), and for the adult males and females without egg strings and females with egg strings (82%, 88%, and 89%). Some variation was found between stages of L. salmonis as well as between the five treatments. The earlier life stages start with variable numbers before treatment (1.3–6.1 lice fish−1), ending with relatively similar numbers post-treatment. For the older stages, there were variations in lice numbers before freshwater treatment (0.7–2.1 lice fish−1) and after treatments (0.0–0.8 lice fish−1).

Identification and characterization of two salmon louse heme peroxidases and their potential as vaccine antigens

Salmon louse, Lepeophtheirus salmonis, represents major challenge for salmon farming. Current treatments impose welfare issues and are costly, whereas prophylactic measures are unavailable. Two salmon louse heme peroxidases (LsPxtl-1 and LsPxtl-2) were tested for their importance for parasite development and as potential vaccine candidates. LsPxtl-1 possesses two heme peroxidase domains and is expressed in ovaries and gut, whereas LsPxtl-2 encodes one domain and contains N-terminal signal peptide and an Eph receptor ligand-binding domain. LsPxtl-1, but not LsPxtl-2, knockdown in nauplius II stage caused poor swimming and death, indicating its importance for parasite development. Immunizations using single DNA plasmid injection encoding the peroxidases or heterologous prime (DNA) and boost (recombinant LsPxtl-2 protein) gave non-significant reduction in lice numbers. Single injection gave low specific antibody levels compared with the prime-boost. The findings suggest LsPxtl-1 is important for parasite development but formulations and vaccination modalities used did not significantly reduce lice infestation.