Atlantic Salmon Aquaculture Industry Research Articles

Fish welfare in offshore salmon aquaculture

AbstractTo accommodate further growth in the Atlantic salmon aquaculture industry, new production sites may well be established at more exposed locations along the coast or even offshore. Here, fish will encounter strong currents and powerful waves, which are avoided at traditional sheltered locations. Exposed locations offer several advantages and necessitate new technological advancements. However, the most crucial question is whether Atlantic salmon are able to thrive in these more extreme environments. In this review, we describe how strong water currents affect the physiology, behaviour and ultimately the welfare of the fish. If ambient current speeds exceed swimming capacities, fish become fatigued and get stuck on the cage wall leading to unacceptable welfare. The swimming capacity will depend on both the magnitude and duration of the current speeds encountered. Moreover, several environmental and biological factors modulate swimming capabilities, where temperature, body size and health status are particularly important to consider. A series of empirical studies are subsequently used to formulate welfare guidelines with regard to biological limits in exposed aquaculture. In addition, owing to the growing popularity of cleaner fish in salmon aquaculture, we also evaluate their usefulness at exposed sites. Overall, Atlantic salmon is a powerful sustained swimmer, and based on available site surveys of ocean currents, we conclude that the prospects for responsible farming at exposed sites looks promising. However, cleaner fish species such as lumpfish and ballan wrasse are poor swimmers and are therefore not recommended for deployment at exposed sites.

Comparative transcriptome analysis of pilchard orthomyxovirus (POMV) and infectious salmon anaemia virus (ISAV)

The Tasmanian Atlantic salmon (Salmo salar) aquaculture industry had remained relatively free of major viral diseases until the recent emergence of pilchard orthomyxovirus (POMV). The virus originally isolated from wild pilchards in Southern Australia is of great concern to the industry as it can cause high mortality. Despite its classification in the Orthomyxoviridae family, POMV is genetically divergent from infectious salmon anaemia virus (ISAV) and potentially represents a new genus within the family. Previous research has produced a formal case definition for clinical POMV, but the molecular events that underpin viral infection have not been characterized. Here we have undertaken a comparative transcriptome analysis of the response of Atlantic salmon kidney cells (ASK) in vitro to both POMV and ISAV using RNA sequencing, by harvesting cells at 6 and 24 h post infection (hpi). Despite their genomic differences, both orthomyxoviruses induced significant, and in some cases similar, innate antiviral responses. Early up-regulation of pathogen recognition receptor genes, RIG-I and TLR3, was observed in response to both viruses and triggered downstream interferon (IFN) responses. Interferon transcripts (IFN-alpha1 and INF-alpha2) were only induced in POMV infected cells at 24 hpi, but IFN-alpha3 was up-regulated in all time points and with both viruses. In addition, a strong induction of antiviral response genes (Mx and ISG15) was observed during the early infection with both viruses. Analysis of transcription factor binding sites in the up-regulated gene sets indicated that the host response to both viruses was largely driven by interferon regulatory factors (IRF) 1 and 2. Only three genes (slc35f2, odf2, LOC106608698) were differentially expressed in opposite directions, up-regulated with POMV and strongly down-regulated with ISAV at 24 hpi. Differential expression of these transcripts is possibly a consequence of virus divergence, but could also be associated to higher viral loads observed in the infection with POMV. Results from this study improve our understanding of the innate immune responses and host-pathogen interactions between POMV and Atlantic salmon. Early host response genes could potentially be exploited as subclinical biomarkers specific to POMV, and improved the development of tools for disease surveillance.

Prevention not cure: a review of methods to avoid sea lice infestations in salmon aquaculture

AbstractThe Atlantic salmon aquaculture industry still struggles with ectoparasitic sea lice despite decades of research and development invested into louse removal methods. In contrast, methods to prevent infestations before they occur have received relatively little research effort, yet may offer key benefits over treatment‐focused methods. Here, we summarise the range of potential and existing preventative methods, conduct a meta‐analysis of studies trialling the efficacy of existing preventative methods and discuss the rationale for a shift to the prevention‐focused louse management paradigm. Barrier technologies that minimise host–parasite encounter rates provide the greatest protection against lice, with a weighted median 76% reduction in infestation density in cages with plankton mesh ‘snorkels’ or ‘skirts’, and up to a 100% reduction for fully enclosed cages. Other methods such as geographic spatiotemporal management, manipulation of swimming depth, functional feeds, repellents and host cue masking can drive smaller reductions that may be additive when used in combination with barrier technologies. Finally, ongoing development of louse‐resistant salmon lineages may lead to long‐term improvements if genetic gain is maintained, while the development of an effective vaccine remains a key target. Preventative methods emphasise host resistance traits while simultaneously reducing host–parasite encounters. Effective implementation has the potential to dramatically reduce the need for delousing and thus improve fish welfare, productivity and sustainability in louse‐prone salmon farming regions.

Out-of-season spawning affects the nutritional status and gene expression in both Atlantic salmon female broodstock and their offspring

The Atlantic salmon aquaculture industry relies on adjustments of female broodstock spawning season to meet the demand for delivery of embryos outside the natural spawning season. Earlier results from zebrafish have shown that parental micronutrient status program offspring metabolism. Therefore, the main hypothesis of this study was to investigate if out-of-season (off-season) broodstock (spawning in June, in land-based recirculation systems) and their offspring deviate in micronutrient status when compared to broodstock and offspring from normal spawning season. Both seasons of female Atlantic salmon broodstock were fed the same diet and starved for approximately the same time interval prior to spawning. We compared nutrients related to the 1C metabolism (vitamin B12, folate, vitamin B6, methionine), free amino acids (FAAs) and lipid classes in broodstock muscle and liver tissues, and during offspring ontogeny. In general, the off-season broodstock showed higher levels of folate, vitamin B6 and selected FAAs in muscle tissue, and higher levels of folate and lipids (cholesterol and sphingomyelin) in liver tissue compared to normal-season. Furthermore, embryos from off-season had reduced amounts of all the measured lipid classes, like cholesterol and sphingomyelin, and lower levels of one type of folate and changes in FAAs and N-metabolites. We discovered significant differences between the seasons in mRNA levels of genes controlling fatty acid synthesis and 1C metabolism in both broodstock liver and offspring. Moreover, for genes controlling the methylation of DNA; both maintenance and de novo DNA methyltransferases (DNMTs) were expressed at higher levels in off-season compared to normal-season offspring. Our results show, in general that normal spawning season broodstock allocated more nutrients to eggs than off-season. Our results indicate a potential for improved maturation for off-season group to obtain a higher offspring growth potential, and this argues for a reassessment of the nutritional influence from broodstock to offspring and the consequences through nutritional programming.

Efficiency and welfare impact of long-term simultaneous in situ management strategies for salmon louse reduction in commercial sea cages

The Atlantic salmon (Salmo salar) aquaculture industry is faced with an obstacle in sustainability with increasing production, which is the control and prevention of the ectoparasitic salmon louse Lepeophtheirus salmonis. Lice prevention management is steering towards passive applications, and this study aimed to monitor multiple strategies in commercial cages over time, to determine the efficiency of these approaches and their effect on welfare. Four strategies were tested at a commercial scale over a 13-month period, covering a large proportion of a standard production cycle. The additive effect of multiple treatments was established in 12 cages, which were assigned to a prevention strategy of either: cleaner fish only, cleaner fish and functional feed, the previous two factors plus deep attractant lights and submerged feeding, or the previous three factors plus a lice skirt. Environmental profiles and school swimming depth were monitored throughout the study period, and sampling events occurred every 2–6 weeks to assess the infestation and welfare status of salmon. The rate of infestation fluctuated with season; however, the group with all prevention strategies maintained a lower rate of new infestations compared to the groups with cleaner fish or functional feed only. Cages with deep lights and feeding influenced the school swimming depth, with these groups generally swimming deeper; this meant that these cages also swam ~6 m deeper than the halocline when pooled over time. However, even with strong differences in new infestations and vertical distribution, the level of mobile lice was similar among all groups, thus incurring a similar frequency of delousing events. There was no effect of these prevention strategies on overall welfare status of salmon. This study shows the promise of utilising multiple lice prevention approaches and highlights the interaction between environment and infestation pressure.

Veterinary drug use in United States net pen Salmon aquaculture: Implications for drug use policy

Aquaculture now produces nearly half of the seafood consumed globally. Atlantic salmon (Salmo salar) is one of the top aquaculture products and the most valuable farmed marine finfish species in the United States (U.S.). The aim of this study is to better understand veterinary drug use in U.S. net pen Atlantic salmon aquaculture and compare these findings to other salmon producing countries and U.S. livestock. We collected and analyzed records on Atlantic salmon production and veterinary drug use in Maine (2003 to 2017) and Washington (2012 to 2017). Antimicrobial medicated feeds were used in 8% and 93% of production cycles in Maine and Washington, respectively. Oxytetratcycline was the primary drug used in both states. Maine used no antimicrobials in eight of the past 15 yrs., including none in 2017. Emamectin benzoate, an antiparasitic medicated feed, was used in 28% production cycles in Maine (2014 to 2017; avg. 1.1 kg/yr) and no emamectin benzoate was administered in Washington over the time period studied. From 2014 to 2016, the U.S. farmed salmon industry contributed 0.8% ± 0.1% to annual global farmed salmon production and administered 1.2% ± 0.6% of antimicrobials used in global salmon farming. Over the same time period, Norway and Chile accounted for 53% ± 3% and 35% ± 3% of annual global production, and administered 0.06% ± 0.02% and 96% ± 0.09% of antimicrobials used in global salmon farming. Compared to U.S. terrestrial agriculture in 2016, the U.S. Atlantic salmon industry contributed 0.031% to U.S. food animal production and administered 0.057% of antimicrobials available to U.S. food animals. Based on the data we collected, the U.S. Atlantic salmon aquaculture industry is a relatively small user of antimicrobials compared to U.S. beef, pigs, poultry, and Chilean salmon industries. There are relatively few approved drugs in the U.S. to treat aquaculture diseases and more options are needed as well as continued work on vaccines. Antimicrobial resistance is a worldwide public health concern; the overuse or misuse of antimicrobials in any setting can compromise the treatment of bacterial infections. The U.S. net pen Atlantic salmon aquaculture industry appears to be the first U.S. food animal industry to report monthly antimicrobial use at the farm-level to the government. These data are critical to assess public health risks associated with antimicrobial use and resistance, and therefore, are needed from all U.S. food animal industries.

Effects of an aquaculture pesticide (diflubenzuron) on non-target shrimp populations: Extrapolation from laboratory experiments to the risk of population decline

Marine aquaculture production has lately experienced high economic growth, but also concerns related to production and environmental contamination. For the Atlantic salmon aquaculture industry, the ectoparasitic crustacean salmon louse (Lepeophtheirus salmonis) has become a major problem. A common method to control populations of salmon lice within farm cages is treatment by various pharmaceuticals. One of the pesticides used in medicated feed for salmon is diflubenzuron (DFB), which acts as a chitin synthesis inhibitor and thereby interferes with the moulting stages during the development of this crustacean. However, DFB from fish feed may also affect non-target crustaceans such as the northern shrimp (Pandalus borealis), which is an economically and ecologically important species. Nevertheless, the actual risk posed by this chemical to shrimp populations in nature is largely unknown. Laboratory experiments have demonstrated that both larval and adult shrimp exposed to DFB through medicated fish feed have reduced survival compared to control. Moreover, the effects of DFB exposure are more severe under conditions of higher temperature and reduced pH (ocean acidification), which can be expected in a future environment. The aim of this study is to make the individual-level information from laboratory studies more relevant for risk assessment at the population level. We have developed a density-dependent age-structured population model representing a northern shrimp population located in a hypothetical Norwegian fjord containing a fish farm, under both ambient and future environments. Our model is based on thorough documentation of shrimp biology and toxicological effects from the laboratory experiments. Nevertheless, extrapolating the reported individual-level effects of DFB to the population level poses several challenges. Relevant information on shrimp populations in Norwegian fjords is sparse (such as abundances, survival and reproductive rates, and density-dependent processes). The degree of exposure to DFB at different distances from aquaculture farms is also uncertain. We have therefore developed a set of model scenarios representing different DFB application schemes and different degrees of exposure for the shrimp populations. The model predicts effects of DFB exposure on population-level endpoints such as long-term abundance, age structure and the probability of population decline below threshold abundances. These model predictions demonstrate how the risk of DFB to shrimp populations can be enhanced by factors such as the timing (season) of DFB applications, the percentage of the population affected, future environmental conditions and environmental stochasticity.

Sensitivity towards low salinity determined by bioassay in the salmon louse, Lepeophtheirus salmonis (Copepoda: Caligidae)

Sea lice, Lepeophtheirus salmonis (Krøyer, 1837), are ectoparasitic crustacean parasites responsible for economic losses in the Atlantic salmon aquaculture industry in the northern hemisphere. Numerous chemical and non-chemical control methods have been developed, including freshwater bathing. Freshwater bathing is regarded as an environmentally friendly treatment; however, reports of variable treatment efficacies have raised concerns regarding the general applicability of this treatment method. Our study aimed to determine the salinity in parts per thousand (‰) at which median survival (EC50) was obtained at 24 h for the copepodid and pre-adult II stages of L. salmonis from geographically separated populations and to develop a bioassay method for on-site pre-treatment tests. Parasites were separated into four different geographically separated populations which are referred to as population A, B, C, and D. Using a stepwise method, parasites were exposed to a range of salinities (35‰ to 0‰) for 24 h, survival was observed after 24 h and results were classified as unaffected/affected. L. salmonis copepodids illustrated significant differences in tolerance among populations. Population A was the least tolerant, with an EC50 of 17.5‰, whereas population C had an EC50 of 11.3‰. No significant population difference in tolerance was observed among the pre-adult II stages. Pooling the data from pre-adult II L. salmonis from three populations yielded an EC50 of 2.8‰ for both sexes combined, 2.4‰ for males, and 2.6‰ for females. All stages of L. salmonis examined in this study exhibited higher tolerances to low salinity than observed in previous studies. We suggest regular monitoring of freshwater sensitivity in areas where this control option is regularly used to detect possible deviations from baseline sensitivity at an early stage.

Genome wide analysis of infectious salmon anemia resistance in commercial Saint John River Atlantic salmon

Infectious salmon anemia (ISA) is an economically costly viral disease to the global Atlantic salmon aquaculture industry due to the high virulence and high mortality rate. Selective breeding for disease resistance traits using genomic information has been suggested to increase the resistance to various diseases in the breeding population. The aim of this study was to perform a genome wide association (GWA) analysis to identify regions of the genome associated with ISA resistance in the commercial Saint John River population. A total of 2233 parr from 72 families from the 2015 year class were challenged with ISA using a cohabitation trial. The heritability of ISA resistance was 0.16 ± 0.04 on the observed scale for the binary trait of ISA survival at 50% mortality of all cohabitating fish. Binary phenotypes were recorded for all fish with those that survived the challenge being classified as resistant and those that died being classified as susceptible. Selective genotyping using a custom North American 50 K SNP chip was performed by choosing, where possible, four resistant fish and four susceptible fish from each challenged family (n = 572). After quality control a total of 547 fish and 38,954 SNP markers remained for GWA analysis. Two survival traits were analyzed for the genotyped fish: ‘survival to day 37’ when mortality had reached 50% and ‘survival at trial termination’ on day 54. Two SNPs on Ssa03 and on Ssa07 were significant associated with ‘survival to day 37’ but only at the chromosome wide level. More notably nine significant SNPs on nine different chromosomes, were significantly associated with ‘survival at trial termination’ at the genome wide level. The most significant SNP, on Ssa13, explained 8.6% of the phenotypic variation, while SNPs on Ssa12 and Ssa11 explained 7.6% and 6.4% respectively. The remaining six SNPs explained between 4 and 6% of the phenotypic variation, indicating oligogenic architecture of the ‘survival at trial termination’ trait. A total of 33 genes known to be differentially expressed during infection with ISA were located downstream on the same chromosome arm of the nine significant SNPs. Identification of causal mutations is not necessary to implement genomic information into selective breeding programs but improves understanding of the genetic basis of ISA resistance.

Characterization of Glycosaminoglycans in Gaping and Intact Connective Tissues of Farmed Atlantic Salmon (Salmo salar) Fillets by Mass Spectrometry.

In the Atlantic salmon (Salmo salar) aquaculture industry, gaping (the separation of muscle bundles from the connective tissue) is a major quality problem. This study characterized chondroitin sulfate (CS) and heparan sulfate (HS) in the connective tissue of intact and gaping salmon fillets from 30 salmon by mass spectrometry. Statistical difference was detected between gaping and intact tissues only when comparing pairwise samples from the same individual (n = 10). The gaping tissue had a lower content of monosulfated CS disaccharides (p = 0.027), and the relative distribution of CS disaccharides was significantly different (p < 0.05). The HS chains were short (average = 14.09, SD = 4.91), and the intact tissue seemed to have a more uniform HS chain structure compared to the gaping tissue. Time-series samples from the same individuals are recommended for future research to improve the understanding of reasons and implications of these differences.

Dietary β-1,3/1,6-glucans improve the effect of a multivalent vaccine in Atlantic salmon infected with Moritella viscosa or infectious salmon anemia virus

The immunomodulatory properties of β-glucans are well documented for several animal species. However, little is known on their use as enhancers of non-specific and specific responses against Moritella viscosa and infectious salmon anemia (ISA) virus, which are two major pathogens affecting the Atlantic salmon aquaculture industry. The present study assessed the effects of dietary β-1,3/1,6-glucans (MacroGard®) and vaccination on the survival of Atlantic salmon (Salmo salar) post-smolts challenged with M. viscosa or ISA virus. Dietary supplementation decreased mortality in both unvaccinated (55.4%; RPSend 19.9) and vaccinated (9.3%; RPSend 86.6) M. viscosa-challenged fish compared to the non-supplemented groups (69.1% and 28.3% mortality in unvaccinated and vaccinated fish, respectively; RPSend 59 for vaccinate non-supplemented fish). Similarly, mortality of ISA virus-challenged fish decreased from 87.5% in vaccinated fish without supplementation (RPSend 9.2) to 70.9% in the supplemented and vaccinated group (RPSend 26.4). In summary, these findings show that dietary β-glucans increase Atlantic salmon resistance and improve the protective effect of vaccines against the tested pathogens.

Phosphorus nutrition in farmed Atlantic salmon (Salmo salar): Life stage and temperature effects on bone pathologies

Bone health is important for a viable and ethically sound Atlantic salmon aquaculture industry. Two important risk factors for vertebral deformities are dietary phosphorus and water temperature. Here, we explore the interplay between these two factors during a full production of Atlantic salmon. Salmon were fed one of three diets (low 4.4–5.0 g kg−1, medium 7.1–7.6 g kg−1, or high 9.0–9.7 g kg−1 soluble phosphorus) from 3 to 500 g body weight, followed by a common diet of 7.3 g kg−1 soluble phosphorus until harvest size at 4 kg. Additional groups were included to investigate the effects of water temperatures of 10 vs 16 °C (low and high diets only) and the switching of dietary phosphorus levels (from low to medium or high, from medium to low or high, from high to low or medium), starting at seawater transfer (~100 g body weight) and lasting for 4 months (~500 g body weight). During the experimental feeding period, the low phosphorus diet caused reduced bone mineralization and stiffness and a greater prevalence of vertebral deformities, compared to the medium and high phosphorus diets. However, the prevalence of severely deformed fish at harvest was reduced by switching from the low to either the medium or high phosphorus diets for 4 months after seawater transfer, followed by rearing on the standard commercial feed. Concurrently, switching from either the medium or high to a low phosphorus diet for the same period following seawater transfer had no effect on vertebral deformities at harvest. The higher water temperature for 4 months following seawater transfer increased the severity of deformities at harvest, irrespective of dietary phosphorus. Finally, low dietary phosphorus was associated with increased fillet damage, due to ectopic connective tissue around the spine, at harvest. In conclusion, dietary phosphorus levels of 5 g kg−1 for the initial 4 months in seawater are more of a risk factor for vertebral pathologies if preceded by low, but not medium or high, dietary phosphorus in freshwater. However, dietary phosphorus levels may not play a role in temperature induced radiologically detectable vertebral pathologies. Under the reported growing conditions and diet compositions, a combination of 7.5–7.6 g kg−1 soluble phosphorus during freshwater and 5.0 g kg−1 for the first 4 months in seawater, was sufficient for normal bone health and growth in Atlantic salmon.

Physiological performance of ballan wrasse (Labrus bergylta) at different temperatures and its implication for cleaner fish usage in salmon aquaculture

Ballan wrasse (Labrus bergylta) are used extensively as cleaner fish to control salmon lice (Lepeophtheirus salmonis) infestations in the Atlantic salmon (Salmon salar) aquaculture industry. Fish are either cultured or caught in the wild before being transferred to salmon sea cages. Ballan wrasse are a poorly studied species, and fundamental knowledge of physiological performances and environmental limits are therefore needed for better deployment strategies and to predict when animal welfare may be at risk. We acclimated ballan wrasse for a minimum of 2 weeks to 5, 10, 15, 20 and 25 °C, representing the full range of temperatures wrasse may experience in salmon sea cages. Swim tunnel respirometry was performed at each temperature to measure standard and maximum metabolic rates, aerobic scope, and critical swimming speed (Ucrit). No mortalities occurred at any acclimation temperature. However, fish were generally inactive at lower temperatures, as evidenced by low metabolic rates. It was not possible to stimulate fish to swim continuously between 5 and 20 °C, and Ucrit was only obtained at 25 °C as 27 cm s−1 (1.1 body lengths s−1). The aerobic scope increased throughout the thermal interval tested from 129 ± 7 mg O2 kg−1 h−1 at 5 °C to 265 ± 18 mg O2 kg−1 h−1 at 25 °C. Owing to weak swimming capabilities, ballan wrasse deployment at locations with moderate to strong current speeds will likely result in poor welfare. Low metabolic rates and inactivity at 5–10 °C suggests that their efficiency as cleaner fish will be limited in winter and in higher latitude locations. Overall, ballan wrasse differs substantially from Atlantic salmon in physiology, behaviour and morphology, and may not thrive in some farm environments suitable for salmon.

Changes in the liver transcriptome of farmed Atlantic salmon (Salmo salar) fed experimental diets based on terrestrial alternatives to fish meal and fish oil

BackgroundDependence on marine natural resources threatens the sustainability of Atlantic salmon aquaculture. In the present study, Atlantic salmon fed for 14 weeks with an experimental diet based on animal by-products and vegetable oil (ABP) exhibited reduced growth performance compared with others fed a fish meal/fish oil based experimental diet (MAR) and a plant protein/vegetable oil-based experimental diet (VEG). To characterize the molecular changes underlying the differences in growth performance, we conducted a 44 K microarray study of the liver transcriptome of the three dietary groups.ResultsThe microarray experiment identified 122 differentially expressed features (Rank Products, PFP < 10%). Based on their associated Gene Ontology terms, 46 probes were classified as metabolic and growth-relevant genes, 25 as immune-related, and 12 as related to oxidation-reduction processes. The microarray results were validated by qPCR analysis of 29 microarray-identified transcripts. Diets significantly modulated the transcription of genes involved in carbohydrate metabolism (gck and pfkfb4), cell growth and proliferation (sgk2 and htra1), apoptosis (gadd45b), lipid metabolism (fabp3, idi1, sqs), and immunity (igd, mx, ifit5, and mhcI). Hierarchical clustering and linear correlation analyses were performed to find gene expression patterns among the qPCR-analyzed transcripts, and connections between them and muscle and liver lipid composition. Overall, our results indicate that changes in the liver transcriptome and tissue lipid composition were driven by cholesterol synthesis up-regulation by ABP and VEG diets, and the lower carbohydrate intake in the ABP group. Two of the microarray-identified genes (sgk2 and htra1) might be key to explaining glucose metabolism regulation and the dietary-modulation of the immune system in fish. To evaluate the potential of these genes as predictive biomarkers, we subjected the qPCR data to a stepwise discriminant analysis. Three sets of no more than four genes were found to be able to predict, with high accuracy (67–94%), salmon growth and fatty acid composition.ConclusionsThis study provides new findings on the impact of terrestrial animal and plant products on the nutrition and health of farmed Atlantic salmon, and a new method based on gene biomarkers for potentially predicting desired phenotypes, which could help formulate superior feeds for the Atlantic salmon aquaculture industry.

Modeling fitness changes in wild Atlantic salmon populations faced by spawning intrusion of domesticated escapees.

Genetic interaction between domesticated escapees and wild conspecifics represents a persistent challenge to an environmentally sustainable Atlantic salmon aquaculture industry. We used a recently developed eco‐genetic model (IBSEM) to investigate potential changes in a wild salmon population subject to spawning intrusion from domesticated escapees. At low intrusion levels (5%–10% escapees), phenotypic and demographic characteristics of the recipient wild population only displayed weak changes over 50 years and only at high intrusion levels (30%–50% escapees) were clear changes visible in this period. Our modeling also revealed that genetic changes in phenotypic and demographic characteristics were greater in situations where strayers originating from a neighboring wild population were domestication‐admixed and changed in parallel with the focal wild population, as opposed to nonadmixed. While recovery in the phenotypic and demographic characteristics was observed in many instances after domesticated salmon intrusion was halted, in the most extreme intrusion scenario, the population went extinct. Based upon results from these simulations, together with existing knowledge, we suggest that a combination of reduced spawning success of domesticated escapees, natural selection purging maladapted phenotypes/genotypes from the wild population, and phenotypic plasticity, buffer the rate and magnitude of change in phenotypic and demographic characteristics of wild populations subject to spawning intrusion of domesticated escapees. The results of our simulations also suggest that under specific conditions, natural straying among wild populations may buffer genetic changes in phenotypic and demographic characteristics resulting from introgression of domesticated escapees and that variation in straying in time and space may contribute to observed differences in domestication‐driven introgression among native populations.

The cleaning efficacy of lumpfish (Cyclopterus lumpus L.) in Faroese salmon (Salmo salar L.) farming pens in relation to lumpfish size and seasonality

Sea lice infestations causes major economic losses in the Atlantic salmon aquaculture industry, and cleaner fish, e.g. the lumpfish, Cyclopterus lumpus L., are therefore increasingly deployed as a biological control method. However, large variations in the cleaning efficacy have been observed, and in the Faroe Islands, the most prominent variation is seasonal. Over a period of approximately two years 5511 lumpfish stomachs were analysed. The stomach contents, where present, were identified and grouped as, a) sea lice, b) lumpfish feed, c) salmon feed, d) organisms associated with biofouling, e) zooplankton organisms and/or f) other. The presence of zooplankton organisms had a significant, and negative, influence on the cleaning efficacy, while the presence of organisms associated with biofouling had a more moderate, but positive, influence on the prevalence of sea lice in the lumpfish diet. Our findings indicate that biofouling, and the subsequent availability of alternative prey organisms, does not reduce the cleaning efficacy of lumpfish, while zooplankton does, i.e. reducing it by a factor of approximately five. The lumpfish size only seemed to play a minor role in the variation observed in the cleaning efficacy, while it had a significant influence on the proportion of empty stomachs, i.e. the smaller lumpfish (< 50 g) had the highest occurrence of empty stomachs.This is convenient knowledge for implementation in the sea lice strategies of farming sites using lumpfish as cleaner fish in general, but especially in farming areas with large seasonal variations in the zooplankton abundance such as in the Faroes. The high occurrence of empty stomachs in small lumpfish highlights the necessity to adapt husbandry in the first period post lumpfish deployment, especially when the opportunity for naturally occurring food is sparse. Furthermore, our findings of biofouling having a positive influence on the grazing efficacy of lumpfish indicate that net cleaning might have a negative influence on lumpfish grazing efficacy.

Correction: Concurrent jellyfish blooms and tenacibaculosis outbreaks in Northern Norwegian Atlantic salmon (Salmo salar) farms.

[This corrects the article DOI: 10.1371/journal.pone.0187476.].

Concurrent jellyfish blooms and tenacibaculosis outbreaks in Northern Norwegian Atlantic salmon (Salmo salar) farms.

Tenacibaculosis is an increasing problem in the Norwegian Atlantic salmon aquaculture industry causing significant economic losses. In September 2015, two separate outbreaks of suspected tenacibaculosis occurred at two Atlantic salmon farms in Finnmark County in Northern Norway. The events resulted in major losses of smolts newly transferred into seawater. Prior to, and during the outbreaks, large numbers of small jellyfish, identified as Dipleurosoma typicum (Boeck) were observed in the vicinity of the farms and inside the net-pens. This study investigates the possible link between the jellyfish, Tenacibaculum spp. and the tenacibaculosis outbreaks. Bacteriology, histology, scanning and transmission electron microscopy, and real-time RT-PCR screening were performed on both fish and jellyfish samples. Based on the findings, Tenacibaculum finnmarkense was found to be the dominant bacteria associated with the tenacibaculosis outbreaks at both sites and that D. typicum is unlikely to be a vector for this fish pathogenic bacterium. However, results do show that the jellyfish caused direct damage to the fish’s skin and may have exacerbated the bacterial infection by allowing an entry point for bacteria.

Genotyping of Tenacibaculum maritimum isolates from farmed Atlantic salmon in Western Canada.

Mouthrot infections (bacterial stomatitis) have a significant impact on the Atlantic salmon aquaculture industry in Western Canada due to economic losses and fish welfare. Bacteria isolated from lesions in the field have been identified as Tenacibaculum maritimum. Mouthrot is different to classical tenacibaculosis, which is most commonly associated with ulcerative lesions, frayed fins and tail rot. The marine fish pathogen T.maritimum is found worldwide; however, in Western Canada, the knowledge of the genetic profile of T.maritimum is limited. This study looked at increasing this knowledge by genotyping T.maritimum isolates collected from Atlantic salmon from farms in Western Canada. These genotypes were compared to other species of the genus Tenacibaculum, as well as other known sequence types within the species. The Western Canadian isolates belong to two new sequence types within the T.maritimum species. Phylogenetic analysis shows that the isolates form a distinct branch together with T.maritimum NCIMB 2154T separate from other Tenacibaculum type strains, and they are most closely related to strains from Norway and Chile.

Ploidy elicits a whole-genome dosage effect: growth of triploid Atlantic salmon is linked to the genetic origin of the second maternal chromosome set

BackgroundThe Atlantic salmon aquaculture industry is investigating the feasibility of using sterile triploids to mitigate genetic interactions with wild conspecifics, however, studies investigating diploid and triploid performance often show contrasting results. Studies have identified dosage and dosage-compensation effects for gene expression between triploid and diploid salmonids, but no study has investigated how ploidy and parent-origin effects interact on a polygenic trait in divergent lines of Atlantic salmon (i.e. slow growing wild versus fast growing domesticated phenotype). This study utilised two experiments relating to the freshwater growth of diploid and triploid groups of pure wild (0% domesticated genome), pure domesticated (100% domesticated genome), and F1 reciprocal hybrid (33%, 50% or 66% domesticated genome) salmon where triploidy was either artificially induced (experiment 1) or naturally developed/spontaneous (experiment 2).ResultsIn both experiments, reciprocal hybrid growth was influenced by the dosage effect of the second maternal chromosome, with growth increasing as ploidy level increased in individuals with a domesticated dam (from 50% to 66% domesticated genome), and the inverse in individuals with a wild dam (from 50% to 33% domesticated genome).ConclusionsWe demonstrate that the combined effect of ploidy and parent-origin on growth, a polygenic trait, is regulated in an additive pattern. Therefore, in order to maximise growth potential, the aquaculture industry should consider placing more emphasis on the breeding value of the dam than the sire when producing triploid families for commercial production.