The efficacy of emamectin benzoate against infestations of Lepeophtheirus salmonis on farmed Atlantic salmon (Salmo salar L) in Scotland, 2002-2006

Spatiotemporal patterns of mortality events in farmed Atlantic salmon in British Columbia, Canada, using publicly available data

Monitoring mortality is an essential strategy for fish health management. Commercial marine finfish sites in British Columbia, Canada, are required to report mortality events (MEs) to Fisheries and Oceans Canada (DFO), which makes these data publicly available. This study aimed to analyze the spatial and temporal patterns of ME composition and total MEs. Between June 2011 and June 2022, 561 MEs were reported. The annual incidence ranged from 1.36 (95% CI: 0.55-2.81) MEs per 100 active site-months in 2013 to 17.98 (95% CI: 13.26-23.84) MEs per 100 active site-months in 2022, with a broadly increasing trend over the period under consideration. The primary causes of MEs were low levels of dissolved oxygen, fish health treatments, and harmful algal blooms (HABs). Both HABs and low dissolved oxygen followed similar patterns, increasing from 2014, peaking in 2019, and declining thereafter. Treatment-related MEs were first reported in 2017 and saw a sharp increase in subsequent years, becoming the leading cause of MEs by 2020. Nearly all treatment-related MEs were linked to sea lice treatments, highlighting the urgent need for adaptive strategies to mitigate these impacts. Sites on the west coast of Vancouver Island demonstrated a higher risk of reporting MEs compared to Mainland sites, likely due to their higher levels of exposure to fluctuating oceanographic conditions. Long-term climate change and persistent periods of warming events, such as marine heat waves, are warming the oceans, altering water parameters, and likely increasing the occurrence and severity of HABs and low dissolved oxygen-related MEs. Further studies are needed to quantify the effects of ocean warming on salmon aquaculture and the resulting increase in fish mortalities.

QTL mapping provides new insights into emamectin benzoate resistance in salmon lice, Lepeophtheirus salmonis

BACKGROUND

The salmon louse (Lepeophtheirus salmonis) is a parasite of wild and farmed salmonid fish, causing huge economic damage to the commercial farming of Atlantic salmon (Salmo salar) in the northern hemisphere. The avermectin emamectin benzoate (EMB) is widely used for salmon delousing. While resistance to EMB is widespread in Atlantic populations of L. salmonis, the molecular mechanisms of resistance remain to be elucidated. The aim of the present work was to obtain insights into potential EMB resistance mechanisms by identifying genetic and transcriptomic markers associated with EMB resistance.

RESULTS

Crosses were performed between EMB-susceptible and -resistant L. salmonis, sourced from two parental strains isolated in Scotland, producing fully pedigreed families. The EMB susceptibility of individual parasites was characterised using time-to-response bioassays. Parasites of two families were subjected to double digest restriction site-associated DNA sequencing (ddRAD-seq) for simultaneous discovery of single nucleotide polymorphisms (SNPs) and genotyping. Data analysis revealed that EMB resistance is associated with one quantitative trait locus (QTL) region on L. salmonis chromosome 5. Marker-trait association was confirmed by genotyping assays for 7 SNPs in two additional families. Furthermore, the transcriptome of male parasites of the EMB-susceptible and -resistant L. salmonis parental strains was assessed. Among eighteen sequences showing higher transcript expression in EMB-resistant as compared to drug-susceptible lice, the most strongly up-regulated gene is located in the above QTL region and shows high homology to β spectrin, a cytoskeleton protein that has roles in neuron architecture and function. Further genes differentially regulated in EMB-resistant lice include a glutathione S-transferase (GST), and genes coding for proteins with predicted roles in mitochondrial function, intracellular signalling or transcription.

CONCLUSIONS

Major determinants of EMB resistance in L. salmonis are located on Chromosome 5. Resistance can be predicted using a limited number of genetic markers. Genes transcriptionally up-regulated in EMB resistant parasites include a β spectrin, a cytoskeletal protein with still incompletely understood roles in neuron structure and function, as well as glutathione S-transferase, an enzyme with potential roles in the biochemical defence against toxicants.

Insights from Hi-C data regarding the Pacific salmon louse (Lepeophtheirus salmonis) sex chromosomes

Salmon lice, Lepeophtheirus salmonis (family Caligidae), are ectoparasites that have negatively impacted the salmon aquaculture industry and vulnerable wild salmon populations. Researchers have studied salmon lice to better understand their biology to develop effective control strategies. In this study, we updated the chromosome-level reference genome assembly of the Pacific subspecies of L. salmonis using Hi-C data. The previous version placed contigs/scaffolds using an Atlantic salmon louse genetic map. By utilizing Hi-C data from Pacific salmon lice, we were able to properly assign locations to contigs/scaffolds previously unplaced or misplaced. This resulted in a more accurate genome assembly and a more comprehensive characterization of the sex chromosome unique to females (W). We found evidence that the same ZW-ZZ mechanism is common in both Atlantic and Pacific subspecies of salmon lice using PCR assays. The W chromosome was approximately 800 kb in size, which is ∼30 times smaller than the Z chromosome (24 Mb). The W chromosome contained 61 annotated genes, including 32 protein-coding genes, 27 long noncoding RNA (lncRNA) genes, and 2 pseudogenes. Among these 61 genes, 39 genes shared homology to genes found on other chromosomes, while 20 were unique to the W chromosome. Two genes of interest on the W chromosome, prohibitin-2 and kinase suppressor of ras-2, were previously identified as potential sex-linked markers in the salmon louse. However, we prioritized the 20 unique genes on the W chromosome as sex-determining candidates. This information furthers our understanding of the biology of this ectoparasite and may help in the development of more effective management strategies.

Fine-scale environmentally associated spatial structure of lumpfish (Cyclopterus lumpus) across the Northwest Atlantic

Lumpfish, Cyclopterus lumpus, have historically been harvested throughout Atlantic Canada and are increasingly in demand as a solution to controlling sea lice in Atlantic salmon farms-a process which involves both the domestication and the transfer of lumpfish between geographic regions. At present, little is known regarding population structure and diversity of wild lumpfish in Atlantic Canada, limiting attempts to assess the potential impacts of escaped lumpfish individuals from salmon pens on currently at-risk wild populations. Here, we characterize the spatial population structure and genomic-environmental associations of wild populations of lumpfish throughout the Northwest Atlantic using both 70K SNP array data and whole-genome re-sequencing data (WGS). At broad spatial scales, our results reveal a large environmentally associated genetic break between the southern populations (Gulf of Maine and Bay of Fundy) and northern populations (Newfoundland and the Gulf of St. Lawrence), linked to variation in ocean temperature and ice cover. At finer spatial scales, evidence of population structure was also evident in a distinct coastal group in Newfoundland and significant isolation by distance across the northern region. Both evidence of consistent environmental associations and elevated genome-wide variation in F ST values among these three regional groups supports their biological relevance. This study represents the first extensive description of population structure of lumpfish in Atlantic Canada, revealing evidence of broad and fine geographic scale environmentally associated genomic diversity. Our results will facilitate the commercial use of lumpfish as a cleaner fish in Atlantic salmon aquaculture, the identification of lumpfish escapees, and the delineation of conservation units of this at-risk species throughout Atlantic Canada.

Direct evidence of increased natural mortality of a wild fish caused by parasite spillback from domestic conspecifics

Parasite spillback from domestic animals can distort the balance between host and parasites in surrounding wildlife, with potential detrimental effects on wild populations. In aquatic environments, parasite spillback from aquaculture to wild salmon is one of the most contentious sustainability debates. In a 19 year time series of release group studies of Atlantic salmon, we demonstrated that (i) the effect of subjecting out-migrating salmon smolts to parasite treatment on marine survival has been reduced over a time, (ii) the relation between salmon lice levels in the out-migration route of the salmon and effect of treatment against the parasite is weak, but also (iii) the return rates in both treated and untreated groups of salmon are negatively correlated with salmon lice levels, and (iv) returns of wild salmon to the region are similarly negatively correlated with salmon lice levels during the out-migration year. Our study suggests that salmon lice can have a large effect on wild salmon populations that is not revealed with randomized control trials using antiparasitic drugs. This should be better accounted for when considering the impacts of farms on wild salmon populations.

A century of parasitology in fisheries and aquaculture

Fish parasitological research associated with fisheries and aquaculture has expanded remarkably over the past century. The application of parasites as biological tags has been one of the fields in which fish parasitology has generated new insight into fish migration and stock assessments worldwide. It is a well-established discipline whose methodological issues are regularly reviewed and updated. Therefore, no concepts or case-studies will be repeated here; instead, we summarize some of the main recent findings and achievements of this methodology. These include the extension of its use in hosts other than bony fishes; the improvements in the selection of parasite tags; the recognition of the host traits affecting the use of parasite tags; and the increasingly recognized need for integrative, multidisciplinary studies combining parasites with classical methods and modern techniques, such as otolith microchemistry and genetics. Archaeological evidence points to the existence of parasitic problems associated with aquaculture activities more than a thousand years ago. However, the main surge of research within aquaculture parasitology occurred with the impressive development of aquaculture over the past century. Protozoan and metazoan parasites, causing disease in domesticated fish in confined environments, have attracted the interest of parasitologists and, due to their economic importance, funding was made available for basic and applied research. This has resulted in a profusion of basic knowledge about parasite biology, physiology, parasite-host interactions, life cycles and biochemistry. Due to the need for effective control methods, various solutions targeting host-parasite interactions (immune responses and host finding), genetics and pharmacological aspects have been in focus.

Control of parasitic diseases in aquaculture

Finfish aquaculture in freshwater and marine environments is continuously expanding globally, and the potential for a substantial further increase is well documented. The industry is supplying fish products for human consumption to the same extent as capture fisheries, and new fish species for domestication are still being selected by the industry. The challenge faced by all aquacultured species, classical and novel, is the range of pathogens associated with each new fish type. A fish host in its natural environment carries a series of more or less specific parasites (specialists and generalists). Some of these show a marked ability to propagate in aquaculture settings. They may then elicit disease when infection intensities in the confined aquaculture environment reach high levels. In addition, the risk of transmission of parasites from aquaculture enterprises to wild fish stocks adds to the parasitic challenge. Control programmes of various kinds are needed and these may include chemotherapeutants and medicines as the farmer's first and convenient choice, but mechanical, biological, immunological and genetic control methods are available solutions. New methods are still to be developed by scrutinizing the life cycle of each particular parasite species and pin-pointing the vulnerable stage to be targeted. As parasites exhibit a huge potential for adaptation to environmental changes, one must realize that only one approach rarely is sufficient. The present work therefore elaborates on and advocates for implementation of integrated control strategies for diseases caused by protozoan and metazoan parasites.

Assessment of the effect of sub-lethal acute toxicity of Emamectin benzoate in Labeo rohita using multiple biomarker approach

Emamectin benzoate (EMB) is a potent neurotoxin agent, widely used for ectoparasites control in aquaculture, but their detailed toxicological implications in Labeo rohita are unknown. Thus, this study was conceptualized to determine the LC₅₀ and to investigate the effects of two sub-lethal concentrations 1/50th of 96 h LC₅₀ (1.82 μgL^-1) and 1/10thof 96 h LC₅₀ (9.1 μgL^-1) on hemato-immunological and biochemical responses in L. rohita (mean weight 25.54 ± 2.3 g and length 10.35 ± 2.4 cm) for a period of 24 h, 48 h, and 72 h. LC₅₀ of EMB were 163 μgL^-1, 112 μgL^-1, 99 μgL^-1 and 91 μgL^-1 at 24 h, 48 h, 72 h, and 96 h respectively. The safe limit at 96 h LC₅₀ of EMB was 2.30 μgL^-1. In EMB treated fish, red blood cells, white blood cells, hemoglobin, and hematocrit counts were reduced (p < 0.05) significantly. Superoxide dismutase (SOD) activity in the liver and kidney declined (p < 0.05) at 72 h while in gill and muscle the activity increased significantly. Glutathione-s-transferase (GST) activity in the liver, gill, and kidney increased (p < 0.05) while muscle decreased significantly. Catalase (CAT) activity in liver, gill, and muscle decreased while in kidney increases. Glutamic-oxaloacetic acid transaminase (GOT) activity and Glutamate pyruvate transaminase (GPT) activity were increased in liver, kidney, and muscle tissue. The change in serum triglycerides, serum protein level was noticed. The level of cortisol, heat shock protein 70 (HSP70), and HSP90 increased (p < 0.05) while the immunological responses like immunoglobulin M (IgM) and complement 3(C3) activity decreased (p < 0.05) in EMB exposed fish. Thus, EMB exposure at two sub-lethal concentrations in L. rohita induces several hemato-immuno, and biochemical alterations in blood, serum, and different organs. The overall result of the present study indicated that EMB is toxic to fish even for a short-term exposure and low doses, and therefore utmost caution should be taken to prevent their drainage into water bodies.

Anemia in salmon aquaculture: Scotland as a case study

Anemia in salmonid aquaculture is a recognized blood disorder resulting from the reduction of hemoglobin concentration and/or erythrocyte count. Because of sub-optimal oxygen supply to the tissues, as a negative impact of anemia fish will experience reduced growth and poor health. This health challenge may be linked with several factors including anthropogenic changes in the marine environment, infectious etiology (viral, bacterial, and parasitic), nutritional deficiencies, or hemorrhaging. From the mid-late summer of 2017 to 2019, Scottish salmon farming companies began to report the occurrence of anemic events in open-net marine sites. At that time, the industry had little understanding of the pathogenesis and possible mechanisms of anemia and limited the ability to formulate effective mitigation strategies. Clinical examination of fish raised suspicion of anemia and this was confirmed by generating a packed cell volume value by centrifugation of a microhematocrit tube of whole anticoagulated blood. Company health team members, including vets and biologists, reported discoloration of gills and local hemorrhages. This paper reviews various commercially significant cases and lesser-known cases of anemia in cultured salmonid species induced by various biological factors. The current methods available to assess hematology are addressed and some future methods that could be adopted in modern day fish farming are identified. An account of the most recent anemic event in Scottish farmed Atlantic salmon (Salmo salar) is presented and discussed as a case study from information provided by two major Scottish salmon producers. The percent of total marine sites (n = 80) included in this case study, that reported with suspected or clinical anemia covering the period mid-late summer 2017 to 2019, was between 1 and 13%. The findings from this case study suggest that anemia experienced in most cases was regenerative and most likely linked to blood loss from the gills.

Pharmacological characterization of novel heteromeric GluCl subtypes from C. elegans and parasitic nematodes

BACKGROUND AND PURPOSE

Macrocyclic lactones (MLs) are the most widely used broad-spectrum anthelmintic drugs for the treatment of parasitic nematodes impacting both human and animal health. MLs are known to act as agonist of the nematode glutamate-gated chloride channels (GluCls). However, for many important nematode species, the GluCls subunit composition and pharmacological properties remain largely unknown. In order to get new insights about the GluCl diversity and MLs mode of action, we identified and pharmacologically characterized receptors made of highly conserved GluCl subunits from the model nematode Caenorhabditis elegans, the human filarial nematode Brugia malayi and the horse parasite Parascaris univalens.

EXPERIMENTAL APPROACH

AVR-14, GLC-2, GLC3 and GLC-4 are the most conserved GluCl subunits throughout the Nematoda phylum. For each nematode species, we investigated the ability of these subunits to form either homomeric or heteromeric GluCls when expressed in Xenopus laevis oocytes and performed the detailed pharmacological characterization of the functional channels.

KEY RESULTS

Here, a total of 14 GluCls have been functionally reconstituted and heteromers formation was inferred from pharmacological criteria. Importantly, we report that the GLC-2 subunit plays a pivotal role in the composition of heteromeric GluCls in nematodes. In addition, we describe a novel GluCl subtype, made of the GLC-2/GLC-3 subunit combination, for which a high concentration of the anthelmintics ivermectin and moxidectin reversibly potentiate glutamate-induced response.

CONCLUSION AND IMPLICATIONS

This study brings new insights into the diversity of GluCl subtypes in nematodes and promote novel drug targets for the development of next generation anthelmintic compounds.

Genomic Characterization of the Barnacle Balanus improvisus Reveals Extreme Nucleotide Diversity in Coding Regions

Barnacles are key marine crustaceans in several habitats, and they constitute a common practical problem by causing biofouling on man-made marine constructions and ships. Despite causing considerable ecological and economic impacts, there is a surprising void of basic genomic knowledge, and a barnacle reference genome is lacking. We here set out to characterize the genome of the bay barnacle Balanus improvisus (= Amphibalanus improvisus) based on short-read whole-genome sequencing and experimental genome size estimation. We show both experimentally (DNA staining and flow cytometry) and computationally (k-mer analysis) that B. improvisus has a haploid genome size of ~ 740 Mbp. A pilot genome assembly rendered a total assembly size of ~ 600 Mbp and was highly fragmented with an N50 of only 2.2 kbp. Further assembly-based and assembly-free analyses revealed that the very limited assembly contiguity is due to the B. improvisus genome having an extremely high nucleotide diversity (π) in coding regions (average π ≈ 5% and average π in fourfold degenerate sites ≈ 20%), and an overall high repeat content (at least 40%). We also report on high variation in the α-octopamine receptor OctA (average π = 3.6%), which might increase the risk that barnacle populations evolve resistance toward antifouling agents. The genomic features described here can help in planning for a future high-quality reference genome, which is urgently needed to properly explore and understand proteins of interest in barnacle biology and marine biotechnology and for developing better antifouling strategies.

Losing the 'arms race': multiresistant salmon lice are dispersed throughout the North Atlantic Ocean

Nothing lasts forever, including the effect of chemicals aimed to control pests in food production. As old pesticides have been compromised by emerging resistance, new ones have been introduced and turned the odds back in our favour. With time, however, some pests have developed multi-pesticide resistance, challenging our ability to control them. In salmonid aquaculture, the ectoparasitic salmon louse has developed resistance to most of the available delousing compounds. The discovery of genetic markers associated with resistance to organophosphates and pyrethroids made it possible for us to investigate simultaneous resistance to both compounds in approximately 2000 samples of salmon lice from throughout the North Atlantic in the years 2000-2016. We observed widespread and increasing multiresistance on the European side of the Atlantic, particularly in areas with intensive aquaculture. Multiresistant lice were also found on wild Atlantic salmon and sea trout, and also on farmed salmonid hosts in areas where delousing chemicals have not been used. In areas with intensive aquaculture, there are almost no lice left that are sensitive to both compounds. These results demonstrate the speed to which this parasite can develop widespread multiresistance, illustrating why the aquaculture industry has repeatedly lost the arms race with this highly problematic parasite.

Accurate Analysis of Tricarboxylic Acid Cycle Metabolites and Anion Components in Hemocytes by IC-CD/ESI-MS for Quantifying Insecticide Impairment on Cellular Immunity in Mythimna separata

Insecticides are more broadly known to affect insect cellular immunity, but the components in hemocytes and their response to insecticide stress are still unknown. In this paper, a method based on trifluoroacetic acid extraction, followed by IC-CD/ESI-MS analysis, was developed to simultaneously determine tricarboxylic acid (TCA) cycle metabolites and anion components in hemocytes from Mythimna separata larvae. Validation gave excellent selectivity, recovery (88.7-107.6%), linear correlation (r² > 0.9961), precision (<3.89%), LOD (0.002-0.006 mg/L), LOQ (0.006-0.020 mg/L), and a short chromatographic run. The method was verified by administration of 4-((3-chloro-4-fluorophenyl)amino)-7-methoxyquinazolin-6-yl 3-(1,3-dioxoiso-indolin-2-yl) propanoate (QDP) or emamectin benzoate (EMB) to hemocytes in vitro and larvae in vivo. TCA metabolites including citrate, α-ketoglutarate, fumarate, malate, and oxaloacetate, and anions including acetate, oxalate, chloride, carbonate, and sulfate were identified and clearly separated. QDP and EMB showed a biphasic dose effect on TCA metabolites, and the contrary hormesis paralleled the different actions of QDP and EMB. The inhibition or improvement of cellular immunity depended on the QDP concentration. In conclusion, a highly sensitive, reliable, and robust method was developed, enabling the monitoring of hemocyte immunity by the quantification of TCA metabolites and anion components in minute hemocyte samples.

Emamectin Benzoate Treatment of Hybrid Grouper Infected With Sea Lice in Hong Kong

Sea lice (Copepoda: Caligidae) are ectoparasites which negatively impact marine aquaculture species around the world. There are a limited number of treatments licensed for use against sea lice in tropical and semi-tropical farmed fish species. Emamectin benzoate (EB) was an effective pharmaceutical drug against sea lice infestations in several salmon industries before resistance to the product developed. This drug has not been extensively tested in marine fish within Asia. The objective of this study was to determine whether this drug could be used to treat oral infections with sea lice in hybrid grouper (Mycteroperca tigris × Epinephelus lanceolatus) cultured in saltwater net-pen sites in Hong Kong. We observed an overall reduction in sea lice infections over time, starting on the last day of the treatment up to the end of our study (i.e., 14 days after the last EB treatment). We also observed a large variation in concentrations of EB in fish on the last day of the treatment, which provides an explanation for the variation in response to the treatment. It also suggests that distribution of the medication to fish in saltwater net-pens is difficult, especially when medication is hand-mixed in the feed and possibly unevenly distributed in the daily rations. Overall, this study provides preliminary evidence that EB could be used to treat sea lice found in Hong Kong and potentially in other regions of SE Asia.

Bias in self-reported parasite data from the salmon farming industry

Many industries are required to monitor themselves in meeting regulatory policies intended to protect the environment. Self-reporting of environmental performance can place the cost of monitoring on companies rather than taxpayers, but there are obvious risks of bias, often addressed through external audits or inspections. Surprisingly, there have been relatively few empirical analyses of bias in industry self-reported data. Here, we test for bias in reporting of environmental compliance data using a unique data set from Canadian salmon farms, where companies monitor the number of parasitic sea lice on fish in open sea pens, in order to minimize impacts on wild fish in surrounding waters. We fit a hierarchical population-dynamics model to these sea-louse count data using a Bayesian approach. We found that the industry's monthly counts of two sea-louse species, Caligus clemensi and Lepeophtheirus salmonis, increased by a factor of 1.95 (95% credible interval: 1.57, 2.42) and 1.18 (1.06, 1.31), respectively, in months when counts were audited by the federal fisheries department. Consequently, industry sea-louse counts are less likely to trigger costly but mandated delousing treatments intended to avoid sea-louse epidemics in wild juvenile salmon. These results highlight the potential for combining external audits of industry self-reported data with analyses of their reporting to maintain compliance with regulations, achieve intended conservation goals, and build public confidence in the process.

Trends in de-lousing of Norwegian farmed salmon from 2000-2019-Consumption of medicines, salmon louse resistance and non-medicinal control methods

The salmon louse Lepeophtheirus salmonis has been a substantial obstacle in Norwegian farming of Atlantic salmon for decades. With a limited selection of available medicines and frequent delousing treatments, resistance has emerged among salmon lice. Surveillance of salmon louse sensitivity has been in place since 2013, and consumption of medicines has been recorded since the early 80’s. The peak year for salmon lice treatments was 2015, when 5.7 times as many tonnes of salmonids were treated compared to harvested. In recent years, non-medicinal methods of delousing farmed fish have been introduced to the industry. By utilizing data on the annual consumption of medicines, annual frequency of medicinal and non-medicinal treatments, the aim of the current study was to describe the causative factors behind salmon lice sensitivity in the years 2000–2019, measured through toxicity tests–bioassays. The sensitivity data from 2000–2012 demonstrate the early emergence of resistance in salmon lice along the Norwegian coast. Reduced sensitivity towards azamethiphos, deltamethrin and emamectin benzoate was evident from 2009, 2009 and 2007, respectively. The annual variation in medicine consumption and frequency of medicinal treatments correlated well with the evolution in salmon louse sensitivity. The patterns are similar, with a relatively small response delay from the decline in the consumption of medicines in Norway (2016 and onward) to the decline in measured resistance among salmon louse (2017 and onward). 2017 was the first year in which non-medicinal treatments outnumbered medicinal delousing treatments as well as the peak year in numbers of cleanerfish deployed. This study highlights the significance of avoiding heavy reliance on a few substance groups to combat ectoparasites, this can be a potent catalyst for resistance evolution. Further, it demonstrates the importance of transparency in the global industry, which enables the industry to learn from poor choices in the past.

Assessing the Impact of Physical and Anthropogenic Environmental Factors in Determining the Habitat Suitability of Seagrass Ecosystems

Blue Carbon ecosystems such as mangroves, saltmarshes and seagrasses have been shown to sequester large amounts of carbon, and subsequently are receiving renewed interest from policy experts in light of climate change. Globally, seagrasses remain the most understudied of these ecosystems, with their total geographic extent largely unknown due to challenges in mapping dynamic coastal environments. As such, species distribution models (SDMs) have been used to identify areas of high suitability, in order to inform our understanding of where unmapped meadows may be located or to identify suitable sites for restoration and/or enhancement efforts. However, many SDMs parameterized to project seagrass distributions focus on physical and not anthropogenic variables (i.e., dredging, aquaculture), which can have negative impacts on seagrass meadows. Here we used verified datasets to identify the potential distribution of Zostera marina and Zostera noltei at a national level for the Republic of Ireland, using 19 environmental variables including both physical and anthropogenic. Using the Maximum Entropy method for developing the SDM, we estimated approximately 95 km2 of suitable habitat for Z. marina and 70 km2 for Z. noltei nationally with high accuracy metrics, including Area Under the Curve (AUC) values of 0.939 and 0.931, respectively for the two species. We found that bathymetry, maximum sea-surface temperature (SST) and minimum salinity were the most important environmental variables that explained the distribution of Z. marina and that high standard deviation of SST, mean SST and maximum salinity were the most important variables in explaining the distribution of Z. noltei. At a national level, we noted that it was primarily physical variables that determined the geographic distribution of seagrass, not anthropogenic variables. We unexpectedly modelled areas of high suitability in locations of anthropogenic disturbance (i.e., dredging, high pollution risk), although this may be due to the binary nature of SDMs capturing presence-absence and not the size and condition of the meadows, suggesting a need for future research to explore the finer scale impacts of anthropogenic activity. Subsequently, this research should foster discussion for researchers and practitioners working on sustainability projects related to Blue Carbon.

Impact of a candidate vaccine on the dynamics of salmon lice (Lepeophtheirus salmonis) infestation and immune response in Atlantic salmon (Salmo salar L.)

Infection with parasitic copepod salmon louse Lepeophtheirus salmonis, represents one of the most important limitations to sustainable Atlantic salmon (Salmo salar L.) farming today in the North Atlantic region. The parasite exerts negative impact on health, growth and welfare of farmed fish as well as impact on wild salmonid populations. It is therefore central to ensure continuous low level of salmon lice with the least possible handling of the salmon and drug use. To address this, vaccination is a cost-effective and environmentally friendly control approach. In this study, efficacy of a vaccine candidate, containing a peptide derived from ribosomal protein P0, was validated post infestation with L. salmonis, at the lab-scale. The sampling results showed good potential of the vaccine candidate when administered intraperitoneally in the host, in reducing the ectoparasite load, through reduction of adult female lice counts and fecundity and with greater presumptive effect in F1 lice generation. The sampling results correlated well with the differential modulation of pro-inflammatory, Th1, Th2 and T regulatory mediators at the transcript level at different lice stages. Overall, the results supports approximately 56% efficacy when administered by intraperitoneal injection. However, additional validation is necessary under large-scale laboratory trial for further application under field conditions.

Identification, functional characterization and expression profiling of cytochrome p450 1A (CYP1A) gene in Labeo rohita against emamectin benzoate

The cytochrome p450 1A (CYP1A) plays vital role in detoxification of xenobiotic compounds in living organisms. In the present study, full-length CYP1A gene was sequenced from liver of Labeo rohita and mRNA expression analysis were carried out at 0, 2, 4, 8, 12, 24, 48, 72, 96 and 120 h (h) time points after emamectin benzoate treatment. The full-length cDNA sequence of CYP1A was 1741 bp which consist of open reading frame (ORF) of 1618 bp, 5'-untranslated region (UTR) 48 bp and 75 bp 3'-UTR respectively. ORF encodes 526 amino acids with a molecular mass a 59.05 kDa and an isoelectric point of 8.74. The subcellular localization confirmed presence of the CYP1A protein was higher in plasma membrane (45.8%), followed by the mitochondrial region (13.9%) and nuclear region (9.2%). The CYP1A protein interaction was found to intermingle more with other CYP family proteins. Analysis of tissue distribution revealed that CYP1A gene was predominantly expressed in the liver compared to other tissues kidney, gills, muscle and intestine. Furthermore, present study reveals that CYP1A mRNA level in emamectin benzoate treated group @ 20 mgkg-1 body was significantly (p < 0.05) higher compared with the control. The CYP1A mRNA expression levels were found upregulating with time and highest expression levels at 24 h. Histological examination found that emamectin benzoate treated liver revealed vacuolisation, hepatocyte infiltrations, cytoplasmic degeneration of hepatocytes compared to control. Overall, present results lay a strong basis for CYP1A is important biomarker for drug detoxification in aquatic animals.

Can Rock-Rubble Groynes Support Similar Intertidal Ecological Communities to Natural Rocky Shores?

Despite the global implementation of rock-rubble groyne structures, there is limited research investigating their ecology, much less than for other artificial coastal structures. Here we compare the intertidal ecology of urban (or semi-urban) rock-rubble groynes and more rural natural rocky shores for three areas of the UK coastline. We collected richness and abundance data for 771 quadrats across three counties, finding a total of 81 species, with 48 species on the groynes and 71 species on the natural rocky shores. We performed three-way analysis of variance (ANOVA) on both richness and abundance data, running parallel analysis for rock and rock-pool habitats. We also performed detrended correspondence analysis on all species to identify patterns in community structure. On rock surfaces, we found similar richness and abundance across structures for algae, higher diversity and abundance for lichen and mobile animals on natural shores, and higher numbers of sessile animals on groynes. Rock-pool habitats were depauperate on groynes for all species groups except for sessile animals, relative to natural shores. Only a slight differentiation between groyne and natural shore communities was observed, while groynes supported higher abundances of some ‘at risk’ species than natural shores. Furthermore, groynes did not differ substantially from natural shores in terms of their presence and abundance of species not native to the area. We conclude that groynes host similar ecological communities to those found on natural shores, but differences do exist, particularly with respect to rock-pool habitats.

Safety of emamectin benzoate administered in feed to Nile tilapia Oreochromis niloticus (L.)

Emamectin benzoate (EB) premix top-coated onto feed is extensively used to treat ectoparasitic crustacean infestations in aquaculture. This study evaluated the safety of EB-dosing in Nile tilapia Oreochromis niloticus at the recommended dose and dosage of 50 μg/kg biomass/day for 7 consecutive days (1X) and compared with control and 10 times the recommended dose (10X). Depletion of EB-residues in the edible muscle of 1X-dosed Nile tilapia was also studied. Mortality, behavioural changes, feed consumption, biomass, EB-residue depletion, and histopathological alterations in the kidney, liver and intestine were determined at slated intervals. Significant dose-dependent reduction in feed intake and biomass and insignificant mortalities were noted in 1X and 10X EB-dosed fish. In 1X EB-dosed fish muscle, the residues peaked on day 7 EB-dosing (9.72 ng/g) and decreased subsequently. Nevertheless, the residue levels were within the acceptable limit of the European Commission and the Canadian Food Inspection Agency even during the EB-dosing period. Histologically, tubule degeneration in the kidney, mild glycogen vacuolation in the liver, and loss of absorptive vacuoles, inflammation and disintegration of the epithelial layer in the intestine of Nile tilapia fed the 1X EB-diet were observed. The fish reverted back to their normal functions with time upon termination of oral-EB-dosing. This work contributed scientific data on the safety of EB particularly on the feed intake, growth reduction, mortality, histopathological alterations, and EB-residue levels in the edible tissues of Nile tilapia fed at the recommended dose and dosage, which suggested that EB-therapy might be reasonably risky in a tropical climate.

Toxic effects of the Emamectin Benzoate exposure on cultured human bronchial epithelial (16HBE) cells

Pesticides pollution has caused serious environmental problems in recent years, and mounting evidence has shown that more and more insecticides have serious risk in human health. Emamectin Benzoate formally regarded as a highly safety insecticide based on its exclusive targets, but the cytotoxicity to human lung was ignored for a long time. In the present study, bioassay experiments were used to assess the toxicity of the Emamectin Benzoatein on human non-target cells including cell viability assay, DNA damage assay, flow cytometer assay and western blotting assay. The results indicated that Emamectin Benzoatecan cause the inhibition of the proliferation, cytochrome c release, activation of caspase-3/9 and increase Bax/Bcl-2 ratio, which means it induced the cytotoxicity on 16HBE cells associated with the mitochondrial apoptosis. Besides, the DNA damge caused by the Emamectin Benzoate suggest it has a potential genotoxic effect on human lung cells.

Sea lice exposure to non-lethal levels of emamectin benzoate after treatments: a potential risk factor for drug resistance

The avermectin derivative emamectin benzoate (EMB) has been widely used by salmon industries around the world to control sea lice infestations. Resistance to this anti-parasitic drug is also commonly reported in these industries. The objective of this study was to quantify the number of sea lice potentially exposed to sub-lethal concentrations of EMB while fish clear the drug after treatments. We assessed juvenile sea lice abundance after 38 EMB treatments on six Atlantic salmon farms, in a small archipelago in British Colombia, Canada, between 2007 and 2018. We fitted a standard EMB pharmacokinetic curve to determine the time when fish treated with this product would have EMB tissue concentrations below the recommended target therapeutic level. During the study, we estimated that for each sea lice treatment there was, on average, an abundance of 0.12 juvenile sea lice per fish during the time period when the concentrations of EMB would have been lower than 60ppb, the recommended therapeutic treatment level for sea lice. The findings from this study on metaphylactic anti-parasitic treatments identify a potential driver for drug resistance in sea lice that should be further explored.

Genome-wide survey of cytochrome P450 genes in the salmon louse Lepeophtheirus salmonis (Krøyer, 1837)

Background

The salmon louse (Lepeophtheirus salmonis) infests farmed and wild salmonid fishes, causing considerable economic damage to the salmon farming industry. Infestations of farmed salmon are controlled using a combination of non-medicinal approaches and veterinary drug treatments. While L. salmonis has developed resistance to most available salmon delousing agents, relatively little is known about the molecular mechanisms involved. Members of the cytochrome P450 (CYP) superfamily are typically monooxygenases, some of which are involved in the biosynthesis and metabolism of endogenous compounds, while others have central roles in the detoxification of xenobiotics. In terrestrial arthropods, insecticide resistance can be based on the enhanced expression of CYPs. The reported research aimed to characterise the CYP superfamily in L. salmonis and assess its potential roles in drug resistance.

Methods

Lepeophtheirus salmonis CYPs were identified by homology searches of the genome and transcriptome of the parasite. CYP transcript abundance in drug susceptible and multi-resistant L. salmonis was assessed by quantitative reverse transcription PCR, taking into account both constitutive expression and expression in parasites exposed to sublethal levels of salmon delousing agents, ecdysteroids and environmental chemicals.

Results

The above strategy led to the identification of 25 CYP genes/pseudogenes in L. salmonis, making its CYP superfamily the most compact characterised for any arthropod to date. Lepeophtheirus salmonis possesses homologues of a number of arthropod CYP genes with roles in ecdysteroid metabolism, such as the fruit fly genes disembodied, shadow, shade, spook and Cyp18a1. CYP transcript expression did not differ between one drug susceptible and one multi-resistant strain of L. salmonis. Exposure of L. salmonis to emamectin benzoate or deltamethrin caused the transcriptional upregulation of certain CYPs. In contrast, neither ecdysteroid nor benzo[a]pyrene exposure affected CYP transcription significantly.

Conclusions

The parasite L. salmonis is demonstrated to possess the most compact CYP superfamily characterised for any arthropod to date. The complement of CYP genes in L. salmonis includes conserved CYP genes involved in ecdysteroid biosynthesis and metabolism, as well as drug-inducible CYP genes. The present study does not provide evidence for a role of CYP genes in the decreased susceptibility of the multiresistant parasite strain studied.

Bioaccumulation of selected veterinary medicinal products (VMPs) in the blue mussel (Mytilus edulis)

Veterinary medicinal products (VMPs) are widely used within the fish farming industry to control sea lice infestations. There is concern that wild and farmed mussels in the vicinity to these fish farms may be exposed and subsequently bioaccumulate these chemicals, which could pose a threat to human health. To understand the fate of these chemicals in the environment, controlled laboratory exposures were performed to establish the uptake and depuration of selected VMPs in the blue mussel (Mytilus edulis). The VMPs included teflubenzuron, emamectin benzoate and deltamethrin. The effects of salinity on the bioaccumulation of teflubenzuron were also investigated to see whether mussels in brackish waters exhibit different bioaccumulation dynamics. Salinity had no significant effect on the uptake or depuration curves for teflubenzuron down to 15‰. The uptake rate constants (k₁) for teflubenzuron, emamectin benzoate and deltamethrin in mussels were 192, 4.82 and 2003, with kinetic bioconcentration factors (BCFs) of 1304, 49 and 2516. Depuration rate constants (k₂) were also found to differ between the three VMPs at 0.147, 0.048 and 0.796 for teflubenzuron, emamectin benzoate and deltamethrin, with calculated elimination half-lives (t_1/2)of 4.7, 14 and 0.87 days. The longer elimination half-lives for teflubenzuron and emamectin benzoate, suggest that these chemicals accumulate in blue mussels and therefore have the potential to bioaccumulate in wild and farmed mussel populations in the environment.

Modelling sea lice control by lumpfish on Atlantic salmon farms: interactions with mate limitation, temperature and treatment rules

Atlantic salmon farming is one of the largest aquaculture sectors in the world. A major impact on farm economics, fish welfare and, potentially, nearby wild salmonid populations, is the sea louse ectoparasite Lepeophtheirus salmonis. Sea louse infestations are most often controlled through application of chemicals, but in most farming regions, sea lice have evolved resistance to the small set of available chemicals. Therefore, alternative treatment methodologies are becoming more widely used. One increasingly common alternative treatment involves the co-culture of farmed salmon with cleaner fish, which prey on sea lice. However, despite their wide use, little is understood about the situations in which cleaner fish are most effective. For example, previous work suggests that a low parasite density results in sea lice finding it difficult to acquire mates, reducing fecundity and population growth. Other work suggests that environmental conditions such as temperature and external sea louse pressure have substantial impact on this mate limitation threshold and may even remove the effect entirely. We used an Agent-Based Model (ABM) to simulate cleaner fish on a salmon farm to explore interactions between sea louse mating behaviour, cleaner fish feeding rate, temperature and external sea louse pressure. We found that sea louse mating has a substantial effect on sea louse infestations under a variety of environmental conditions. Our results suggest that cleaner fish can control sea louse infestations most effectively by maintaining the population below critical density thresholds.

Genetic and phenotypic differentiation of lumpfish (Cyclopterus lumpus) across the North Atlantic: implications for conservation and aquaculture

Demand for lumpfish (Cyclopterus lumpus) has increased exponentially over the last decade, both for their roe, which is used as a caviar substitute, and increasingly also as cleaner fish to control sea lice in salmon farming. The species is classified as Near Threatened by the IUCN and there are growing concerns that over-exploitation of wild stocks and translocation of hatchery-reared lumpfish may compromise the genetic diversity of native populations. We carried out a comparative analysis of genetic and phenotypic variation across the species' range to estimate the level of genetic and phenotypic differentiation, and determined patterns of gene flow at spatial scales relevant to management. We found five genetically distinct groups located in the West Atlantic (USA and Canada), Mid Atlantic (Iceland), East Atlantic (Faroe Islands, Ireland, Scotland, Norway and Denmark), English Channel (England) and Baltic Sea (Sweden). Significant phenotypic differences were also found, with Baltic lumpfish growing more slowly, attaining a higher condition factor and maturing at a smaller size than North Atlantic lumpfish. Estimates of effective population size were consistently low across the North East Atlantic (Iceland, Faroe Islands and Norway), the area where most wild lumpfish are fished for their roe, and also for the aquaculture industry. Our study suggests that some lumpfish populations are very small and have low genetic diversity, which makes them particularly vulnerable to over-exploitation and genetic introgression. To protect them we advocate curtailing fishing effort, closing the breeding cycle of the species in captivity to reduce dependence on wild stocks, restricting the translocation of genetically distinct populations, and limiting the risk of farm escapes.

Sensitivity assessment of sea lice to chemotherapeutants: Current bioassays and best practices

Traditional bioassays are still necessary to test sensitivity of sea lice species to chemotherapeutants, but the methodology applied by the different scientists has varied over time in respect to that proposed in "Sea lice resistance to chemotherapeutants: A handbook in resistance management" (2006). These divergences motivated the organization of a workshop during the Sea Lice 2016 conference "Standardization of traditional bioassay process by sharing best practices." There was an agreement by the attendants to update the handbook. The objective of this article is to provide a baseline analysis of the methodology for traditional bioassays and to identify procedures that need to be addressed to standardize the protocol. The methodology was divided into the following steps: bioassay design; material and equipment; sea lice collection, transportation and laboratory reception; preparation of dilution; parasite exposure; response evaluation; data analysis; and reporting. Information from the presentations of the workshop, and also from other studies, allowed for the identification of procedures inside a given step that need to be standardized as they were reported to be performed differently by the different working groups. Bioassay design and response evaluation were the targeted steps where more procedures need to be analysed and agreed upon.

RNA sequencing reveals distinct gene expression patterns during the development of parasitic larval stages of the salmon louse (Lepeophtheirus salmonis)

The salmon louse (Lepeophtheirus salmonis), an ectoparasitic copepod on salmonids, has become a major threat for the aquaculture industry. In search for new drugs and vaccines, transcriptome analysis is increasingly used to find differently regulated genes and pathways in response to treatment. However, the underlying gene expression changes going along with developmental processes could confound such analyses. The life cycle of L. salmonis consists of eight stages divided by moults. The developmental rate of salmon lice on the host is not uniform. Individual- and sex-related differences are found leading to individuals of unlike developmental status at same sampling time point after infection. In this study, we analyse L. salmonis from a time series by RNA sequencing applying a method of separating individuals of different instar age independent of sampling time point. Lice of four stages divided into up to four age groups within the stage were analysed in triplicate (total of 66 samples). Gene expression analysis shows that the method for sorting individuals was successful. Many genes show cyclic expression patterns over the moulting cycles. Overall gene expression differs more between lice of different age within the same stage than between lice of different stage but same instar age.

The drivers of sea lice management policies and how best to integrate them into a risk management strategy: An ecosystem approach to sea lice management

The control of sea lice infestations on cultivated Atlantic salmon is a major issue in many regions of the world. The numerous drivers which shape the priorities and objectives of the control strategies vary for different regions/jurisdictions. These range from the animal welfare and economic priorities of the producers, to the mitigation of any potential impacts on wild stocks. Veterinary ethics, environmental impacts of therapeutants, and impacts for organic certification of the produce are, amongst others, additional sets of factors which should be considered. Current best practice in both EU and international environmental law advocates a holistic ecosystem approach to assessment of impacts and risks. The issues of biosecurity and ethics, including the impacts on the stocks of species used as cleaner fish, are areas for inclusion in such a holistic ecosystem assessment. The Drivers, Pressures, State, Impacts, Responses (DPSIR) process is examined as a decision-making framework and potential applications to sea lice management are outlined. It is argued that this is required to underpin any integrated sea lice management (ISLM) strategy to balance pressures and outcomes and ensure a holistic approach to managing the issue of sea lice infestations on farmed stock on a medium to long-term basis.

Response of Caligus rogercresseyi (Boxshall & Bravo, 2000) to treatment with Hydrogen Peroxide: Recovery of parasites, fish infestation and egg viability under experimental conditions

Hydrogen peroxide (HP) is used to remove C. rogercresseyi from fish but little is known about its effect on this species. This study determined EC₅₀ and concentration immobilizing 100% of specimens, capacity of parasites exposed to HP to recover and infest fish, and effect on survival into the copepodid stage. EC₅₀ and concentration immobilizing 100% of specimens were estimated by exposing parasites for 20 min to 11 concentrations and evaluating effect at 1 and 24 h post-exposure. Capacity to recover and infest fish, and survival into copepodid were evaluated by exposing parasites and eggs to HP for 20 min. Recovery and fish infestation were evaluated at 25 and 24 h post-exposure, respectively. Eggs were grown until control reached the copepodid stage and survival calculated. EC₅₀ was 709.8 ppm.100% immobilization was obtained at 825 ppm. Male and female recover 0.5 and 1 h post-exposure, respectively. Percentage of parasites exposed and not exposed to HP that were recovered on fish was not significantly different. Survival to copepodid was lower in those exposed to HP. HP effect is greater on copepodids, but 100% of the mobile stages are immobilized under 825 ppm causing detachment from fish and potentially driven away, reducing infestation risk.

A multivariable assessment of the spatio-temporal distribution of pyrethroids performance on the sea lice Caligus rogercresseyi in Chile

Synthetic pyrethroids have been widely used in Chile to control the sea lice Caligus rogercresseyi, a major ectoparasite of farmed salmon. Although resistance of C. rogercresseyi to pyrethroids has been reported in Chile, there is no information regarding the geographic extent of this problem. In this study we explored the spatial and temporal variation of C. rogercresseyi's response to pyrethroids in Chile from 2012 to 2013. We modeled lice abundance one week after treatment with a linear mixed-effects regression, and then we performed spatial and spatio-temporal cluster analyses on farm-level effects and on treatment-level residuals, respectively. Results indicate there were two areas where the post-treatment lice counts were significantly higher than in the rest of the study area. These spatial clusters remained even once we adjusted for environmental and management predictors, suggesting unmeasured factors (e.g. resistance) were causing the clustering. Further investigation should be carried out to confirm this hypothesis.

A 200K SNP chip reveals a novel Pacific salmon louse genotype linked to differential efficacy of emamectin benzoate

Antiparasitic drugs such as emamectin benzoate (EMB) are relied upon to reduce the parasite load, particularly of the sea louse Lepeophtheirus salmonis, on farmed salmon. The decline in EMB treatment efficacy for this purpose is an important issue for salmon producers around the world, and particularly for those in the Atlantic Ocean where widespread EMB tolerance in sea lice is recognized as a significant problem. Salmon farms in the Northeast Pacific Ocean have not historically experienced the same issues with treatment efficacy, possibly due to the relatively large population of endemic salmonid hosts that serve to both redistribute surviving lice and dilute populations potentially under selection by introducing naïve lice to farms. Frequent migration of lice among farmed and wild hosts should limit the effect of farm-specific selection pressures on changes to the overall allele frequencies of sea lice in the Pacific Ocean. A previous study using microsatellites examined L. salmonis oncorhynchi from 10 Pacific locations from wild and farmed hosts and found no population structure. Recently however, a farm population of sea lice was detected where EMB bioassay exposure tolerance was abnormally elevated. In response, we have developed a Pacific louse draft genome that complements the previously-released Atlantic louse sequence. These genomes were combined with whole-genome re-sequencing data to design a highly sensitive 201,279 marker SNP array applicable for both subspecies (90,827 validated Pacific loci; 153,569 validated Atlantic loci). Notably, kmer spectrum analysis of the re-sequenced samples indicated that Pacific lice exhibit a large within-individual heterozygosity rate (average of 1 in every 72 bases) that is markedly higher than that of Atlantic individuals (1 in every 173 bases). The SNP chip was used to produce a high-density map for Atlantic sea louse linkage group 5 that was previously shown to be associated with EMB tolerance in Atlantic lice. Additionally, 478 Pacific louse samples from farmed and wild hosts obtained between 2005 and 2014 were also genotyped on the array. Clustering analysis allowed us to detect the apparent emergence of an otherwise rare genotype at a high frequency among the lice collected from two farms in 2013 that had reported elevated EMB tolerance. This genotype was not observed in louse samples collected from the same farm in 2010, nor in any lice sampled from other locations prior to 2013. However, this genotype was detected at low frequencies in louse samples from farms in two locations reporting elevated EMB tolerance in 2014. These results suggest that a rare genotype present in Pacific lice may be locally expanded in farms after EMB treatment. Supporting this hypothesis, 437 SNPs associated with this genotype were found to be in a region of linkage group 5 that overlaps the region associated with EMB resistance in Atlantic lice. Finally, five of the top diagnostic SNPs within this region were used to screen lice that had been subjected to an EMB survival assay, revealing a significant association between these SNPs and EMB treatment outcome. To our knowledge this work is the first report to identify a genetic link to altered EMB efficacy in L. salmonis in the Pacific Ocean.

A pedigree-based experiment reveals variation in salinity and thermal tolerance in the salmon louse, Lepeophtheirus salmonis

The salmon louse is a highly abundant ectoparasitic copepod of salmonids in the North Pacific and Atlantic. Widespread and rapid development of resistance to chemical agents used to delouse salmonids on marine farms is now threatening the continued development of the aquaculture industry and have served as a potent catalyst for the development of alternative pest management strategies. These include freshwater and warm-water treatments to which the louse is sensitive. However, given the well-documented evolutionary capacity of this species, the risk of developing tolerance towards these environmental treatments cannot be dismissed. Two common-garden experiments were performed using full-sibling families of lice identified by DNA parentage testing to investigate whether one of the fundamental premises for evolution, in this context genetic variation in the capacity of coping with fresh or warm water, exists within this species. Significant differences in survival were observed among families in both experiments, although for the salinity experiment, it was not possible to unequivocally disentangle background mortality from treatment-induced mortality. Thus, our data demonstrate genetic variation in tolerance of warm water and are suggestive of genetic variation in salinity tolerance. We conclude that extensive use of these environmental-based treatments to delouse salmonids on commercial farms may drive lice towards increased tolerance.

Managing aquatic parasites for reduced drug resistance: lessons from the land

Atlantic salmon farming is one of the largest aquaculture industries in the world. A major problem in salmon farms is the sea louse ectoparasite Lepeophtheirus salmonis, which can cause stress, secondary infection and sometimes mortality in the salmon host. Sea lice have substantial impacts on farm economics and potentially nearby wild salmonid populations. The most common method of controlling sea louse infestations is application of chemicals. However, most farming regions worldwide have observed resistance to the small set of treatment chemicals that are available. Despite this, there has been little investigation of treatment strategies for managing resistance in aquaculture. In this article, we compare four archetypical treatment strategies inspired by agriculture, where the topic has a rich history of study, and add a fifth strategy common in aquaculture. We use an agent-based model (ABM) to simulate these strategies and their varying applications of chemicals over time and space. We analyse the ABM output to compare how the strategies perform in controlling louse abundance, number of treatments required and levels of resistance in the sea louse population. Our results indicated that among the approaches considered applying chemicals in combination was the most effective over the long term.

The mechanism (Phe362Tyr mutation) behind resistance in Lepeophtheirus salmonis pre-dates organophosphate use in salmon farming

The salmon louse is an ectoparasitic copepod of salmonids in the marine environment, and represents a global challenge to salmon aquaculture. A major issue is the reliance of the industry on a limited number of chemicals to delouse salmonids on farms, and the high levels of resistance that lice have developed to all of these agents. However, for most of these chemicals, resistance and dispersal mechanisms are unknown. We recently demonstrated that the Phe362Tyr mutation is the primary cause of organophosphate resistance in lice collected on Norwegian farms. In the present study, we genotyped >2000 lice collected throughout the entire North Atlantic in the period 1998–2016, using Phe362Tyr and nine tightly linked SNPs. Our results showed that the Phe362Tyr mutation is strongly linked to lice survival following chemical treatment on farms located throughout the North Atlantic, demonstrating for the first time, that this mutation represents the primary mechanism for organophosphate resistance in salmon lice across the North Atlantic. Additionally, we observed multiple and diverse high frequency haplotypes linked with the allele conveying resistance to organophosphate. We, therefore, conclude that Phe362Tyr is not a de novo mutation, but probably existed in salmon lice before the introduction of organophosphates in commercial aquaculture.

Evaluating bath treatment effectiveness in the control of sea lice burdens on Atlantic salmon in New Brunswick, Canada

The use of medicinal bath treatment for sea lice is becoming more common, due to increasing resistance to in-feed treatments with emamectin benzoate. Common treatment modalities in New Brunswick, Canada, include Salmosan administered by tarpaulin or wellboat, and Paramove administered by wellboat. In this study, we assessed the effectiveness of these treatment modalities in the field between 2010 and 2015 using a web-based sea lice data management system (Fish-iTrends^© ). Effectiveness was evaluated for adult female (AF) and for pre-adult and adult male (PAAM) life stages separately. We also investigated the impact of variability in pretreatment lead and post-treatment lag time on effectiveness measures. There were 1185 treatment events at 57 farms that uniquely matched our pre- and post-treatment count criteria. The effectiveness of treatment modality was significantly influenced by season, pretreatment level of sea lice and by lead and lag times. In summer, Salmosan administered by tarpaulin had the greatest effectiveness on both AF and PAAM, when pretreatment levels were above 10 sea lice; whereas in autumn, the performance of treatment modalities varied significantly, depending on the pretreatment levels for the life stages. Ignoring the lead or lag time effect generally resulted in an underestimation of treatment effectiveness.

Effect of timing of count events on estimates of sea lice abundance and interpretation of effectiveness following bath treatments

Effectiveness of sea lice bath treatment is often assessed by comparing pre- and post-treatment counts. However, in practice, the post-treatment counting window varies from the day of treatment to several days after treatment. In this study, we assess the effect of post-treatment lag time on sea lice abundance estimates after chemical bath treatment using data from the sea lice data management program (Fish-iTrends) between 2010 and 2014. Data on two life stages, (i) adult female (AF) and (ii) pre-adult and adult male (PAAM), were aggregated at the cage level and log-transformed. Average sea lice counts by post-treatment lag time were computed for AF and PAAM and compared relative to treatment day, using linear mixed models. There were 720 observations (treatment events) that uniquely matched pre- and post-treatment counts from 53 farms. Lag time had a significant effect on the estimated sea lice abundance, which was influenced by season and pre-treatment sea lice levels. During summer, sea lice were at a minimum when counted 1 day post-treatment irrespective of pre-treatment sea lice levels, whereas in the spring and autumn, low levels were observed for PAAM over a longer interval of time, provided the pre-treatment sea lice levels were >5-10.

Lessons from sea louse and salmon epidemiology

Effective disease management can benefit from mathematical models that identify drivers of epidemiological change and guide decision-making. This is well illustrated in the host-parasite system of sea lice and salmon, which has been modelled extensively due to the economic costs associated with sea louse infections on salmon farms and the conservation concerns associated with sea louse infections on wild salmon. Consequently, a rich modelling literature devoted to sea louse and salmon epidemiology has been developed. We provide a synthesis of the mathematical and statistical models that have been used to study the epidemiology of sea lice and salmon. These studies span both conceptual and tactical models to quantify the effects of infections on host populations and communities, describe and predict patterns of transmission and dispersal, and guide evidence-based management of wild and farmed salmon. As aquaculture production continues to increase, advances made in modelling sea louse and salmon epidemiology should inform the sustainable management of marine resources.

Characterization of IgM in Norwegian cleaner fish (lumpfish and wrasses)

The use of cleaner fish in Norwegian aquaculture has to a large extent been based on wild catches, but breeding of lumpfish and ballan wrasse is currently increasing. Due to disease problems and required vaccine development, tools to study immune responses and a better understanding of the immune system in these species is demanded. The present study comprises lumpfish (Cyclopterus lumpus) and five species of wrasses: Ballan wrasse (Labrus bergylta), rock cook (Centrolabrus exoletus), cuckoo wrasse (Labrus mixtus), corkwing wrasse (Symphodus melops), and goldsinny wrasse (Ctenolabrus rupestris). We present a comparison of the IgM sequences, phylogenetic relationship to other teleosts and characteristic features of IgM in the species studied. The lumpfish IgM heavy chain sequence was assembled from high throughput cDNA sequencing whereas the wrasse sequences were determined by molecular cloning. The secreted form of the IgM heavy chain from all species consisted of four constant Ig domains. IgM was purified from lumpfish and ballan wrasse sera by gel filtration followed by anion exchange chromatography, and polyclonal sera were produced against these proteins. Antisera against ballan wrasse IgM showed cross-reactivity to all analyzed species of wrasses, some cross-reactivity to lumpfish, very low reaction to salmon, and no reaction to cod. Anti- IgM sera against lumpfish cross-reacted to the light chain of all species studied. Wrasses and lumpfish IgM showed high binding affinities for protein A. IgM concentration in adult ballan wrasse (700-800 g) was measured by single radial immunodiffusion assay and found to be 13.4 mg/ml which is about 36% of the total protein concentration. The IgM concentration in lumpfish (600-3600 g) was estimated to 1-2.6 mg/ml, which corresponds to approximately 3% of the total protein concentration.

Evaluating the effect of synchronized sea lice treatments in Chile

The sea louse is considered an important ectoparasite that affects farmed salmonids around the world. Sea lice control relies heavily on pharmacological treatments in several salmon-producing countries, including Chile. Among options for drug administration, immersion treatments represent the majority of antiparasitic control strategies used in Chile. As a topical procedure, immersion treatments do not induce a long lasting effect; therefore, re-infestation from neighbouring farms may undermine their efficacy. Synchronization of treatments has been proposed as a strategy to improve immersion treatment performance, but it has not been evaluated so far. Using a repeated-measures linear mixed-effect model, we evaluated the impact of treatment synchronization of neighbouring farms (within 10km seaway distance) on the adult lice mean abundance from weeks 2 to 8 post-treatment on rainbow trout and Atlantic salmon farms in Chile, while controlling for external and internal sources of lice before the treatments, and also for environmental and fish-related variables. Results indicate that treatment synchronization was significantly associated with lower adult lice levels from weeks 5 to 7 after treatment. This relationship appeared to be linear, suggesting that higher levels of synchronization may result in lower adult sea lice levels during these weeks. These findings suggest that synchronization can improve the performance of immersion delousing treatments by keeping sea lice levels low for a longer period of time. Our results may be applicable to other regions of the world where immersion treatments are widely used.

Variation in pre-treatment count lead time and its effect on baseline estimates of cage-level sea lice abundance

Treatment efficacy studies typically use pre-treatment sea lice abundance as the baseline. However, the pre-treatment counting window often varies from the day of treatment to several days before treatment. We assessed the effect of lead time on baseline estimates, using historical data (2010-14) from a sea lice data management programme (Fish-iTrends). Data were aggregated at the cage level for three life stages: (i) chalimus, (ii) pre-adult and adult male and (iii) adult female. Sea lice counts were log-transformed, and mean counts by lead time relative to treatment day were computed and compared separately for each life stage, using linear mixed models. There were 1,658 observations (treatment events) from 56 sites in 5 Bay Management Areas. Our study showed that lead time had a significant effect on the estimated sea lice abundance, which was moderated by season. During the late summer and autumn periods, counting on the day of treatment gave significantly higher values than other days and would be a more appropriate baseline estimate, while during spring and early summer abundance estimates were comparable among counts within 5 days of treatment. A season-based lead time window may be most appropriate when estimating baseline sea lice levels.

Time-to-response toxicity analysis as a method for drug susceptibility assessment in salmon lice

The salmon louse Lepeophtheirus salmonis (Krøyer, 1837) is an ectoparasite causing infections of wild and farmed Atlantic salmon (Salmo salar L.) in the Northern hemisphere. While L. salmonis control at commercial mariculture sites increasingly employs non-medicinal approaches, such as cage designs reducing infection rates and biological control through cleaner fish, anti-parasitic drugs are still a requirement for effective fish health care. With only a limited range of salmon delousing agents available, all of which have been in use for more than a decade, drug resistance formation has been reported for different products. Successful resistance management requires reliable susceptibility assessment, which is usually achieved through L. salmonis bioassays. These tests involve the exposure of parasites to different drug concentrations and require significant numbers of suitable L. salmonis stages. The present study reports an alternative bioassay that is based on time-to-response toxicity analyses and can be carried out with limited parasite numbers. The assay determines the median effective time (ET₅₀), i.e., the time required until impaired swimming and/or attachment behaviour becomes apparent in 50% of parasites, by conducting repeated examinations of test animals starting at the time point where exposure to a set drug concentration commences. This experimental approach further allows the estimation of the apparent drug susceptibility of individual L. salmonis by determining their time to response, which may prove useful in experiments designed to elucidate associations between genetic factors and the drug susceptibility phenotype of parasites. Three laboratory strains of L. salmonis differing in susceptibility to emamectin benzoate were characterised using standard 24 h bioassays and time-to-response toxicity assays. While both the median effective concentration (EC₅₀) and the ET₅₀ showed variability between experimental repeats, both types of bioassay consistently discriminated susceptible and drug-resistant L. salmonis laboratory strains.

STATEMENT OF RELEVANCE

Infections by sea lice cause significant costs to the global salmon farming industry, which have been estimated to exceed €300 million per year worldwide. Control of sea lice still relies to a significant extent on chemical delousing; however, chemical control is threatened by resistance formation. Resistance can be combated by rotation between different drugs and strategic implementation of non-medicinal strategies. However, resistance management requires reliable and feasible methods of susceptibility assessment. The present study is a technical note introducing a novel approach to susceptibility assessments in sea lice. The method can be applied in susceptibility assessments on farms, where it offers the advantage of a reduced requirement of parasites for testing. In addition, the novel method allows deriving the times of parasite require to show a response after drug treatment has started, thus providing a variable characterizing the drug susceptibility phenotype of individual parasites. Accordingly, the bioassay approach presented here will be useful for studies aiming at unravelling the genetic determinants of drug resistance.

Sex-biased gene expression and sequence conservation in Atlantic and Pacific salmon lice (Lepeophtheirus salmonis)

BACKGROUND

Salmon lice, Lepeophtheirus salmonis (Copepoda: Caligidae), are highly important ectoparasites of farmed and wild salmonids, and cause multi-million dollar losses to the salmon aquaculture industry annually. Salmon lice display extensive sexual dimorphism in ontogeny, morphology, physiology, behavior, and more. Therefore, the identification of transcripts with differential expression between males and females (sex-biased transcripts) may help elucidate the relationship between sexual selection and sexually dimorphic characteristics.

RESULTS

Sex-biased transcripts were identified from transcriptome analyses of three L. salmonis populations, including both Atlantic and Pacific subspecies. A total of 35-43 % of all quality-filtered transcripts were sex-biased in L. salmonis, with male-biased transcripts exhibiting higher fold change than female-biased transcripts. For Gene Ontology and functional analyses, a consensus-based approach was used to identify concordantly differentially expressed sex-biased transcripts across the three populations. A total of 127 male-specific transcripts (i.e. those without detectable expression in any female) were identified, and were enriched with reproductive functions (e.g. seminal fluid and male accessory gland proteins). Other sex-biased transcripts involved in morphogenesis, feeding, energy generation, and sensory and immune system development and function were also identified. Interestingly, as observed in model systems, male-biased L. salmonis transcripts were more frequently without annotation compared to female-biased or unbiased transcripts, suggesting higher rates of sequence divergence in male-biased transcripts.

CONCLUSIONS

Transcriptome differences between male and female L. salmonis described here provide key insights into the molecular mechanisms controlling sexual dimorphism in L. salmonis. This analysis offers targets for parasite control and provides a foundation for further analyses exploring critical topics such as the interaction between sex and drug resistance, sex-specific factors in host-parasite relationships, and reproductive roles within L. salmonis.

Susceptibility of goldsinny wrasse, Ctenolabrus rupestris L. (Labridae), to viral haemorrhagic septicaemia virus (VHSV) genotype III: Experimental challenge and pathology

Cleaner fish, such as wrasse, are being increasingly used to combat the sea lice infestation of Atlantic salmon (Salmo salar L.) in many European countries. To determine susceptibility of the goldsinny wrasse (Ctenolabrus rupestris L.) and pathogenesis of the viral haemorrhagic septicaemia virus (VHSV) genotype III isolate 12-654, previously associated with VHSV infection in the Shetland Islands in 2012, fish were experimentally challenged by intraperitoneal injection (IP), bath immersion and cohabitation routes. Cumulative proportion of moribund wrasse reached 17% following the virus immersion challenge while by the IP-route moribunds exceeded 50% within 14days post-challenge. Typical signs of VHS as reported in rainbow trout (Oncorhynchus mykiss), were not observed in moribund goldsinny wrasse. The most pronounced histopathological changes, consistent regardless of the route of infection, were observed within the heart and included atrium myofibril degeneration, focal infiltration and multifocal necrosis, with prominent swelling of the endocardium and occasional detachment. Pathological changes in the atrium were associated with presence of the viral antigen as confirmed by a positive immunohistochemical staining. Virus clearance and heart tissue recovery were noted although further experiments are required to confirm these observations. The results of a cohabitation experiment confirmed that goldsinny wrasse shed viable virus and therefore represent a risk of virus transmission to other VHSV susceptible species. Similarities between the pathology in goldsinny wrasse induced through the controlled experimental challenges and that of wrasse spp. from an infection occurrence in Shetland are discussed.

Surveillance of the Sensitivity towards Antiparasitic Bath-Treatments in the Salmon Louse (Lepeophtheirus salmonis)

The evolution of drug resistant parasitic sea lice is of major concern to the salmon farming industry worldwide and challenges sustainable growth of this enterprise. To assess current status and development of L. salmonis sensitivity towards different pesticides used for parasite control in Norwegian salmon farming, a national surveillance programme was implemented in 2013. The programme aims to summarize data on the use of different pesticides applied to control L. salmonis and to test L. salmonis sensitivity to different pesticides in farms along the Norwegian coast. Here we analyse two years of test-data from biological assays designed to detect sensitivity-levels towards the pesticides azamethiphos and deltamethrin, both among the most common pesticides used in bath-treatments of farmed salmon in Norway in later years. The focus of the analysis is on how different variables predict the binomial outcome of the bioassay tests, being whether L. salmonis are immobilized/die or survive pesticide exposure. We found that local kernel densities of bath treatments, along with a spatial geographic index of test-farm locations, were significant predictors of the binomial outcome of the tests. Furthermore, the probability of L. salmonis being immobilized/dead after test-exposure was reduced by odds-ratios of 0.60 (95% CI: 0.42–0.86) for 2014 compared to 2013 and 0.39 (95% CI: 0.36–0.42) for low concentration compared to high concentration exposure. There were also significant but more marginal effects of parasite gender and developmental stage, and a relatively large random effect of test-farm. We conclude that the present data support an association between local intensities of bath treatments along the coast and the outcome of bioassay tests where salmon lice are exposed to azamethiphos or deltamethrin. Furthermore, there is a predictable structure of L. salmonis phenotypes along the coast in the data, characterized by high susceptibility to pesticides in the far north and far south, but low susceptibility in mid Norway. The study emphasizes the need to address local susceptibility to pesticides and the need for restrictive use of pesticides to preserve treatment efficacy.