How sea lice from salmon farms may cause wild salmonid declines in Europe and North America and be a threat to fishes elsewhere

Fishes farmed in sea pens may become infested by parasites from wild fishes and in turn become point sources for parasites. Sea lice, copepods of the family Caligidae, are the best-studied example of this risk. Sea lice are the most significant parasitic pathogen in salmon farming in Europe and the Americas, are estimated to cost the world industry €300 million a year and may also be pathogenic to wild fishes under natural conditions.Epizootics, characteristically dominated by juvenile (copepodite and chalimus) stages, have repeatedly occurred on juvenile wild salmonids in areas where farms have sea lice infestations, but have not been recorded elsewhere. This paper synthesizes the literature, including modelling studies, to provide an understanding of how one species, the salmon louse, Lepeophtheirus salmonis, can infest wild salmonids from farm sources. Three-dimensional hydrographic models predicted the distribution of the planktonic salmon lice larvae best when they accounted for wind-driven surface currents and larval behaviour. Caligus species can also cause problems on farms and transfer from farms to wild fishes, and this genus is cosmopolitan. Sea lice thus threaten finfish farming worldwide, but with the possible exception of L. salmonis, their host relationships and transmission adaptations are unknown. The increasing evidence that lice from farms can be a significant cause of mortality on nearby wild fish populations provides an additional challenge to controlling lice on the farms and also raises conservation, economic and political issues about how to balance aquaculture and fisheries resource management.     

Temporal and spatial patterns of sea lice levels on sea trout in western Scotland in relation to fish farm production cycles

The relationship between aquaculture and infestations of sea lice on wild sea trout (Salmo trutta) populations is controversial. Although some authors have concluded that there is a link between aquaculture and lice burdens on wild fish, others have questioned this interpretation. Lice levels have been shown to be generally higher on Atlantic salmon farms during the second years of two-year production cycles. Here we investigate whether this pattern relates to lice burdens on wild fish across broad temporal and spatial axes. Within Loch Shieldaig across five successive farm cycles from 2000 to 2009, the percentage of sea trout with lice, and those above a critical level, were significantly higher in the second year of a two-year production cycle. These patterns were mirrored in 2002–2003 across the Scottish west coast. The results suggest a link between Atlantic salmon farms and sea lice burdens on sea trout in the west of Scotland.     

Genomic Resources for Sea Lice: Analysis of ESTs and Mitochondrial Genomes

Sea lice are common parasites of both farmed and wild salmon. Salmon farming constitutes an important economic market in North America, South America, and Northern Europe. Infections with sea lice can result in significant production losses. A compilation of genomic information on different genera of sea lice is an important resource for understanding their biology as well as for the study of population genetics and control strategies. We report on over 150,000 expressed sequence tags (ESTs) from five different species (Pacific Lepeophtheirus salmonis (49,672 new ESTs in addition to 14,994 previously reported ESTs), Atlantic L. salmonis (57,349 ESTs), Caligus clemensi (14,821 ESTs), Caligus rogercresseyi (32,135 ESTs), and Lernaeocera branchialis (16,441 ESTs)). For each species, ESTs were assembled into complete or partial genes and annotated by comparisons to known proteins in public databases. In addition, whole mitochondrial (mt) genome sequences of C. clemensi (13,440 bp) and C. rogercresseyi (13,468 bp) were determined and compared to L. salmonis. Both nuclear and mtDNA genes show very high levels of sequence divergence between these ectoparastic copepods suggesting that the different species of sea lice have been in existence for 37–113 million years and that parasitic association with salmonids is also quite ancient. Our ESTs and mtDNA data provide a novel resource for the study of sea louse biology, population genetics, and control strategies. This genomic information provides the material basis for the development of a 38K sea louse microarray that can be used in conjunction with our existing 44K salmon microarray to study host–parasite interactions at the molecular level. This report represents the largest genomic resource for any copepod species to date.     

Sea louse infection of juvenile sockeye salmon in relation to marine salmon farms on Canada's west coast

Background

Pathogens are growing threats to wildlife. The rapid growth of marine salmon farms over the past two decades has increased host abundance for pathogenic sea lice in coastal waters, and wild juvenile salmon swimming past farms are frequently infected with lice. Here we report the first investigation of the potential role of salmon farms in transmitting sea lice to juvenile sockeye salmon (Oncorhynchus nerka).

Methodology/Principal Findings

We used genetic analyses to determine the origin of sockeye from Canada''s two most important salmon rivers, the Fraser and Skeena; Fraser sockeye migrate through a region with salmon farms, and Skeena sockeye do not. We compared lice levels between Fraser and Skeena juvenile sockeye, and within the salmon farm region we compared lice levels on wild fish either before or after migration past farms. We matched the latter data on wild juveniles with sea lice data concurrently gathered on farms. Fraser River sockeye migrating through a region with salmon farms hosted an order of magnitude more sea lice than Skeena River populations, where there are no farms. Lice abundances on juvenile sockeye in the salmon farm region were substantially higher downstream of farms than upstream of farms for the two common species of lice: Caligus clemensi and Lepeophtheirus salmonis, and changes in their proportions between two years matched changes on the fish farms. Mixed-effects models show that position relative to salmon farms best explained C. clemensi abundance on sockeye, while migration year combined with position relative to salmon farms and temperature was one of two top models to explain L. salmonis abundance.

Conclusions/Significance

This is the first study to demonstrate a potential role of salmon farms in sea lice transmission to juvenile sockeye salmon during their critical early marine migration. Moreover, it demonstrates a major migration corridor past farms for sockeye that originated in the Fraser River, a complex of populations that are the subject of conservation concern.     

Oust the louse: leaping behaviour removes sea lice from wild juvenile sockeye salmon Oncorhynchus nerka

We conducted a manipulative field experiment to determine whether the leaping behaviour of wild juvenile sockeye salmon Oncorhynchus nerka dislodges ectoparasitic sea lice Caligus clemensi and Lepeophtheirus salmonis by comparing sea‐lice abundances between O. nerka juveniles prevented from leaping and juveniles allowed to leap at a natural frequency. Juvenile O. nerka allowed to leap had consistently fewer sea lice after the experiment than fish that were prevented from leaping. Combined with past research, these results imply potential costs due to parasitism and indicate that the leaping behaviour of juvenile O. nerka does, in fact, dislodge sea lice.     

The influence of aquaculture unit proximity on the pattern of Lepeophtheirus salmonis infection of anadromous Salmo trutta populations on the isle of Skye,Scotland

A total of 230 anadromous Salmo trutta (brown trout) were sampled in five sheltered coastal fjords (or sea lochs) on the Isle of Skye, Scotland, U.K., in 2016 at varying distances from active Atlantic salmon Salmo salar farms. Statistical models were developed to investigate potential correlations between salmon lice Lepeophtheirus salmonis burdens on S. trutta hosts and their proximity to S. salar farm cages. Significant correlations were found between lice burdens and fish fork length and proximity to the nearest S. salar farm. The probability of the presence of L. salmonis on fish hosts increased with fish host size and with distance from the nearest S. salar farm, but total lice burdens were highest in fish sampled near S. salar farms and decreased with distance. The proportion of different life‐cycle stages of L. salmonis were also dependent on S. salar farm proximity, with higher juvenile lice numbers recorded at sites near S. salar farm cages. These results highlight the complexity of the relationship between S. trutta and L. salmonis infections on wild fish and emphasize the requirement of further research to quantify these effects to better inform conservation and management strategies, particularly in areas of active S. salar farm facilities.     

Towards direct evidence of the effects of salmon lice (<Emphasis Type="Italic">Lepeophtheirus salmonis</Emphasis> Krøyer) on sea trout (<Emphasis Type="Italic">Salmo trutta</Emphasis> L.) in their natural habitat: proof of concept for a new combination of methods

Studies addressing the impact of salmon lice (Lepeophtheirus salmonis Krøyer) on sea trout in their natural habitat are scarce and mostly limited to prophylaxis-based experiments. The main drawbacks with this approach are that lice infestations on control fish are not known and the anti-parasitic treatment is of unknown efficacy and may have unwanted side-effects. We tested an innovative approach where prophylaxis is replaced with artificial infestation of the fish. Twenty-nine sea trout post-smolts were caught in a farming-free area in southern Norway and half the fish were artificially infested with lice. Survival and behavior of individual fish was investigated using acoustic telemetry. Furthermore, salinity values were extracted from an hydrodynamical model simulation and connected to individual 3D positions. Results from this pilot study show consistent trends in behavioral differences between artificially infested and control fish. All fish that died or prematurely returned to freshwater were artificially infested fish, although results were not statistically significant. Besides, power analysis confirmed the limitations of this small pilot study for delivering statistically significant inferences. We found also indications of artificially infested fish remaining in shallower waters and within shorter distance to low salinity habitats, but only differences in modelled salinity values were statistically significant. Methodological progress and limitations with this original approach are discussed, and we recommend further studies using this combination of methods and the lessons learnt from this pilot study to provide better quantitative evidence on the effect of salmon lice on sea trout in the wild.     

Infestations of wild adult Atlantic salmon (Salmo salar L.) by the ectoparasitic copepod sea louse Lepeophtheirus salmonis Krøyer: prevalence,intensity and the spatial distribution of males and [2pt] females on the host fish

The copepod Lepeophtheirus salmonis Krøyer is a specific ectoparasite of North Atlantic and Pacific salmonids in their marine phases. We compared infestations of L. salmonis on wild Atlantic salmon (Salmo salarL.) captured in estuarine (Firth of Tay, east Scotland; 1995, 1996) and marine coastal waters (Strathy Point, north Scotland; 1998, 1999). Host fish from the Tay were caught by sweep netting, whilst those from Strathy Point were trapped in anchored bagnets. Fish capture method and exposure of the parasites to brackish conditions may both have detrimental effects on the retention of L. salmonis by the host, and hence possibly lead to their being under-estimated on returning adult fish. At Strathy Point, we recorded (i) an infestation prevalence of 100%, (ii) mean log abundances of pre-adult + adult L. salmonis at 19 (1998) and 24 (1999) per fish, (iii) 85/93% of all L. salmonis as being adults and (iv) overall 68/69% females. Fish caught in the upper Firth of Tay showed significantly lower prevalences, intensities and abundances of L. salmonis and probably had lost part or all of their lice burdens prior to capture, whereas those sampled from Strathy Point were apparently minimally affected by capture method or brackish water influences. The loss of parasites for the Tay fish was not markedly biased to males or females, or to pre-adult versus adult developmental stages. There were significantly greater abundances of L. salmonis on two sea-winter fish (30 lice per fish) than on one sea-winter fish (17 lice per fish) sampled at Strathy Point in 1998. There are several possible explanations for such age-related patterns of abundance, but the indications are that (i) initial infestation of smolts occurs in coastal waters, (ii) infestation of hosts in the open ocean is a persistent event, and (iii) oceanic reinfestation outweighs mortality losses of L. salmonis. This parasite typically occupies rather few zones on the host fish covering only a small percentage of the total available body surface area. Female predominance appears to be characteristic of L. salmonisinfestations of wild Atlantic salmon; this is in marked contrast to reports of extreme male dominance on farmed stocks. Adult females predominated on the epidermis adjacent to, and posterior of, the insertion of the anal fin and along the posterior dorsal midline between the dorsal and caudal fins. Males predominated on the sides of the head and along the dorsal midline between the head and the dorsal fin. Mate guarding/precopulatory pairs are formed between pre-adult II females and adult males. The significant correlation between the distribution of pre-adult females and adult males may be indicative of pre-adult females actively seeking out adult males, but more likely is due to the (large) adult females competitively ousting all smaller life stages (female and male) from those preferred zones. Given the relatively low fecundity of adult females, and observations of 100% prevalence of L. salmonis, the infective planktonic copepodid stage evidently is extremely efficient at locating and establishing upon its host fish in the pelagic environment.     

Changes in Atlantic salmon (Salmo salar) epidermal mucus protein composition profiles following infection with sea lice (Lepeophtheirus salmonis)

The mucus protein profile of Atlantic salmon (Salmo salar) and changes due to infection with sea lice (Lepeophtheirus salmonis) were examined. Two-dimensional gel electrophoresis was performed on salmon skin mucus and comparisons between control and infected fish mucus were made. LC MS/MS identified intracellular proteins, calmodulin, actin, and hemopexin and plasma proteins, such as apolipoproteins, lectin, plasminogen and transferrin. Plasma proteins in the mucus may result from either direct expression by epidermal cells, leakage of plasma or via a secondary circulation system. Therefore, RT-PCR was used to measure mRNA of transferrin and lectin in Atlantic salmon skin. Transferrin expression was observed suggesting direct expression by the epidermis. Lectin expression was not detected suggesting another mechanism of entry into mucus, either leakage from plasma or secondary circulation. The lack of observable albumin on 2D gels, suggests that mucus lectin may arise from the secondary circulation route. Interestingly, β-actin was a significant component of Atlantic salmon mucus. Cleaved actin and transferrin fragments were observed and positively correlated with sea lice infection suggestive of proteolytic activity. Increased levels of cleaved transferrin during sea lice infection may activate the nitrous oxide response of salmon macrophages, as part of the fish's immune response to sea lice infection.     

A putative serine protease,SpSsp1, from Saprolegnia parasitica is recognised by sera of rainbow trout, Oncorhynchus mykiss

Saprolegniosis, the disease caused by Saprolegnia sp., results in considerable economic losses in aquaculture. Current control methods are inadequate, as they are either largely ineffective or present environmental and fish health concerns. Vaccination of fish presents an attractive alternative to these control methods. Therefore we set out to identify suitable antigens that could help generate a fish vaccine against Saprolegnia parasitica. Unexpectedly, antibodies against S. parasitica were found in serum from healthy rainbow trout, Oncorhynchus mykiss. The antibodies detected a single band in secreted proteins that were run on a one-dimensional SDS-polyacrylamide gel, which corresponded to two protein spots on a two-dimensional gel. The proteins were analysed by liquid chromatography tandem mass spectrometry. Mascot and bioinformatic analysis resulted in the identification of a single secreted protein, SpSsp1, of 481 amino acid residues, containing a subtilisin domain. Expression analysis demonstrated that SpSsp1 is highly expressed in all tested mycelial stages of S. parasitica. Investigation of other non-infected trout from several fish farms in the United Kingdom showed similar activity in their sera towards SpSsp1. Several fish that had no visible saprolegniosis showed an antibody response towards SpSsp1 suggesting that SpSsp1 might be a useful candidate for future vaccination trial experiments.     

Reduced growth in wild juvenile sockeye salmon Oncorhynchus nerka infected with sea lice

Daily growth rings were examined in the otoliths of wild juvenile sockeye salmon Oncorhynchus nerka to determine whether infection by ectoparasitic sea lice Caligus clemensi and Lepeophtheirus salmonis was associated with reduced host body growth, an important determinant of survival. Over 98% of the sea lice proved to be C. clemensi and the fish that were highly infected grew more slowly than uninfected individuals. Larger fish also grew faster than smaller fish. Finally, there was evidence of an interaction between body size and infection status, indicating the potential for parasite‐mediated growth divergence.     

The Drosophila Polycomb group gene Sex combs extra encodes the ortholog of mammalian Ring1 proteins

In Drosophila, the Polycomb group (PcG) of genes is required for the maintenance of homeotic gene repression during development. Here, we have characterized the Drosophila ortholog of the products of the mammalian Ring1/Ring1A and Rnf2/Ring1B genes. We show that Drosophila Ring corresponds to the Sex combs extra (Sce), a previously described PcG gene. We find that Ring/Sce is expressed and required throughout development and that the extreme Pc embryonic phenotype due to the lack of maternal and zygotic Sce can be rescued by ectopic expression of Ring/Sce. This phenotypic rescue is also obtained by ectopic expression of the murine Ring1/Ring1A, suggesting a functional conservation of the proteins during evolution. In addition, we find that Ring/Sce binds to about 100 sites on polytene chromosomes, 70% of which overlap those of other PcG products such as Polycomb, Posterior sex combs and Polyhomeotic, and 30% of which are unique. We also show that Ring/Sce interacts directly with PcG proteins, as it occurs in mammals.     

Mortality of key fish species released by recreational anglers in an Australian estuary

A field experiment was done to quantify the mortality of fish released during a recreational angling tournament in Botany Bay, Australia. Participating boat-based anglers were divided into two groups, each representing different typical catch-and-release events. The first group (termed the ‘live weigh-in group’) retained the largest two individuals of 4 species (dusky flathead, Platycephalus fuscus, yellowfin bream, Acanthopagrus australis, sand whiting, Sillago ciliata, and trevally, Pseudocaranx dentex) in onboard holding tanks and then presented these to researchers at designated weigh-in times and stations. Gear, operational and handling data were collected before 125 fish were tagged using plastic t-bar tags, returned to the anglers and then released into two sea cages. The second group (termed the ‘immediate-release group’) immediately released 224 fish into two sea cages, after they were tagged and relevant data recorded by onboard observers. This group represented those fish routinely discarded (i) as part of catch-and-immediate-release tournaments and/or (ii) due to minimum legal sizes and/or personal quotas. Appropriate species and numbers of ‘control’ fish were seined and placed into two sea cages. All fish were monitored for mortalities over 10 days. Dusky flathead, yellowfin bream, trevally and snapper, Pagrus auratus accounted for more than 85% of the total catch. Their adjusted mortalities ranged between 0% and 36.6%. Irrespective of the treatment, most yellowfin bream and snapper deaths occurred within 3 h of being hooked and released into the cages, while trevally and dusky flathead showed a delayed mortality over 4 days. Owing to confounding effects due to their confinement, dusky flathead were excluded from further analyses. Anatomical hook location and the time between capture and release were significant predictors of mortality for yellowfin bream and trevally, respectively (p < 0.01), but none of the various gear, operational or handling factors examined were significant for snapper (p > 0.05). The results are discussed in terms of species-specific variabilities in mortalities, their causal effects and better management of catch-and-release events.     

Bestrophin genes are expressed in Xenopus development

The Bestrophin-1/VMD2 gene has been implicated in Best disease, a juvenile-onset vitelliform macular dystrophy. The Bestrophin proteins have anion channel activity, and the four mammalian members share sequence homologies in multiple transmembrane domains and an RFP-tripeptide motif. The expression patterns and functions of the Bestrophin genes in retinal pigment epithelium have been studied intensively, whereas little is known about their roles in vertebrate embryogenesis. This study examined the roles of four Xenopus tropicalis homologs of BEST genes. The xtBest genes showed spatially and temporally distinct expression. xtBest-2 was the only maternally expressed Best gene, and both xtBest-2 and the Xenopus laevis Best-2 gene were expressed at the edge of the blastopore lip including the organizer. Ectopic expression of xBest-2 caused defects in dorsal axis formation and in mesodermal gene expression during gastrulation. These results suggest a new role of the Bestrophin family genes in early vertebrate embryogenesis.     

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.     

The potential of carbaryl as a treatment for sea lice infestations of farmed Atlantic salmon, Salmo salar L.

The treatment of sea lice, Lepeophtheirus salmonis infestations on farmed Atlantic salmon, Salmo salar requires regular bathing of the fish in diachlorvos (DDVP), 2,2-dichlorovinyl dimethyl phosphate, with subsequent release to the marine environment of spent pesticide. The toxicity of a possible alternative compound, carbaryl, l-naphthyl N-methylcarbamate) to kill sea lice and Atlantic salmon was examined. Preliminary studies with carbaryl indicate the potential advantage of a reduced treatment dose was outweighed by the greater persistence of both the parent compound and of its toxic degradation product. Hence this carbamate insecticide is unlikely to be considered by regulatory authorities as an alternative treatment for sea lice infestations.     

Sea lice as a density-dependent constraint to salmonid farming

Fisheries catches worldwide have shown no increase over the last two decades, while aquaculture has been booming. To cover the demand for fish in the growing human population, continued high growth rates in aquaculture are needed. A potential constraint to such growth is infectious diseases, as disease transmission rates are expected to increase with increasing densities of farmed fish. Using an extensive dataset from all farms growing salmonids along the Norwegian coast, we document that densities of farmed salmonids surrounding individual farms have a strong effect on farm levels of parasitic sea lice and efforts to control sea lice infections. Furthermore, increased intervention efforts have been unsuccessful in controlling elevated infection levels in high salmonid density areas in 2009-2010. Our results emphasize host density effects of farmed salmonids on the population dynamics of sea lice and suggest that parasitic sea lice represent a potent negative feedback mechanism that may limit sustainable spatial densities of farmed salmonids.