Modeling Parasite Dynamics on Farmed Salmon for Precautionary Conservation Management of Wild Salmon

Conservation management of wild fish may include fish health management in sympatric populations of domesticated fish in aquaculture. We developed a mathematical model for the population dynamics of parasitic sea lice (Lepeophtheirus salmonis) on domesticated populations of Atlantic salmon (Salmo salar) in the Broughton Archipelago region of British Columbia. The model was fit to a seven-year dataset of monthly sea louse counts on farms in the area to estimate population growth rates in relation to abiotic factors (temperature and salinity), local host density (measured as cohort surface area), and the use of a parasiticide, emamectin benzoate, on farms. We then used the model to evaluate management scenarios in relation to policy guidelines that seek to keep motile louse abundance below an average three per farmed salmon during the March–June juvenile wild Pacific salmon (Oncorhynchus spp.) migration. Abiotic factors mediated the duration of effectiveness of parasiticide treatments, and results suggest treatment of farmed salmon conducted in January or early February minimized average louse abundance per farmed salmon during the juvenile wild salmon migration. Adapting the management of parasites on farmed salmon according to migrations of wild salmon may therefore provide a precautionary approach to conserving wild salmon populations in salmon farming regions.     

Establishment and characterisation of salmon louse (Lepeophtheirus salmonis (Krøyer 1837)) laboratory strains

The salmon louse (Lepeophtheirus salmonis (Krøyer 1837)) is an ectoparasitic copepod which represents a major pathogen of wild and farmed salmonid fishes in the marine environment. In order to facilitate research on this ecologically and economically important parasite, a hatchery and culturing system permitting the closure of the life-cycle of L. salmonis in the laboratory was developed. Here, the hatchery system, breeding practices, and selected louse strains that have been maintained in culture in the period 2002–2009 are presented. The hatchery and culture protocol gave rise to predictable hatching of larvae and infections of host fish, permitting the cultivation of specific strains of L. salmonis for 22 generations. Both in- and out-bred lice and mutant colour strains have been established, and some of these strains were characterised by microsatellite DNA markers confirming their pedigree. No evidence of inbreeding depression, fitness or morphological changes was observed in any of the strains cultured. It is suggested that the culturing system, and the strains produced represent a significant resource for future research on this parasite.     

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.     

EST and Mitochondrial DNA Sequences Support a Distinct Pacific Form of Salmon Louse, <Emphasis Type="Italic">Lepeophtheirus salmonis</Emphasis>

Nuclear deoxyribonucleic acid sequences from approximately 15,000 salmon louse expressed sequence tags (ESTs), the complete mitochondrial genome (16,148bp) of salmon louse, and 16S ribosomal ribonucleic acid (rRNA) and cytochrome oxidase subunit I (COI) genes from 68 salmon lice collected from Japan, Alaska, and western Canada support a Pacific lineage of Lepeophtheirus salmonis that is distinct from that occurring in the Atlantic Ocean. On average, nuclear genes are 3.2% different, the complete mitochondrial genome is 7.1% different, and 16S rRNA and COI genes are 4.2% and 6.1% different, respectively. Reduced genetic diversity within the Pacific form of L. salmonis is consistent with an introduction into the Pacific from the Atlantic Ocean. The level of divergence is consistent with the hypothesis that the Pacific form of L. salmonis coevolved with Pacific salmon (Onchorhynchus spp.) and the Atlantic form coevolved with Atlantic salmonids (Salmo spp.) independently for the last 2.5–11 million years. The level of genetic divergence coincides with the opportunity for migration of fish between the Atlantic and Pacific Ocean basins via the Arctic Ocean with the opening of the Bering Strait, approximately 5 million years ago. The genetic differences may help explain apparent differences in pathogenicity and environmental sensitivity documented for the Atlantic and Pacific forms of L. salmonis. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Life history and virulence are linked in the ectoparasitic salmon louse Lepeophtheirus salmonis

Models of virulence evolution for horizontally transmitted parasites often assume that transmission rate (the probability that an infected host infects a susceptible host) and virulence (the increase in host mortality due to infection) are positively correlated, because higher rates of production of propagules may cause more damages to the host. However, empirical support for this assumption is scant and limited to microparasites. To fill this gap, we explored the relationships between parasite life history and virulence in the salmon louse, Lepeophtheirus salmonis, a horizontally transmitted copepod ectoparasite on Atlantic salmon Salmo salar. In the laboratory, we infected juvenile salmon hosts with equal doses of infective L. salmonis larvae and monitored parasite age at first reproduction, parasite fecundity, area of damage caused on the skin of the host, and host weight and length gain. We found that earlier onset of parasite reproduction was associated with higher parasite fecundity. Moreover, higher parasite fecundity (a proxy for transmission rate, as infection probability increases with higher numbers of parasite larvae released to the water) was associated with lower host weight gain (correlated with lower survival in juvenile salmon), supporting the presence of a virulence–transmission trade‐off. Our results are relevant in the context of increasing intensive farming, where frequent anti‐parasite drug use and increased host density may have selected for faster production of parasite transmission stages, via earlier reproduction and increased early fecundity. Our study highlights that salmon lice, therefore, are a good model for studying how human activity may affect the evolution of parasite virulence.     

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.     

Paranucleospora theridion n. gen., n. sp. (Microsporidia,Enterocytozoonidae) with a Life Cycle in the Salmon Louse (Lepeophtheirus salmonis,Copepoda) and Atlantic Salmon (Salmo salar)

ABSTRACT. Paranucleospora theridion n. gen, n. sp., infecting both Atlantic salmon (Salmo salar) and its copepod parasite Lepeophtheirus salmonis is described. The microsporidian exhibits nuclei in diplokaryotic arrangement during all known life‐cycle stages in salmon, but only in the merogonal stages and early sporogonal stage in salmon lice. All developmental stages of P. theridion are in direct contact with the host cell cytoplasm or nucleoplasm. In salmon, two developmental cycles were observed, producing spores in the cytoplasm of phagocytes or epidermal cells (Cycle‐I) and in the nuclei of epidermal cells (Cycle‐II), respectively. Cycle‐I spores are small and thin walled with a short polar tube, and are believed to be autoinfective. The larger oval intranuclear Cycle‐II spores have a thick endospore and a longer polar tube, and are probably responsible for transmission from salmon to L. salmonis. Parasite development in the salmon louse occurs in several different cell types that may be extremely hypertrophied due to P. theridion proliferation. Diplokaryotic merogony precedes monokaryotic sporogony. The rounded spores produced are comparable to the intranuclear spores in the salmon in most aspects, and likely transmit the infection to salmon. Phylogenetic analysis of P. theridion partial rDNA sequences place the parasite in a position between Nucleospora salmonis and Enterocytozoon bieneusi. Based on characteristics of the morphology, unique development involving a vertebrate fish as well as a crustacean ectoparasite host, and the results of the phylogenetic analyses it is suggested that P. theridion should be given status as a new species in a new genus.     

Effect of environmental salinity on sea lice Lepeophtheirus salmonis settlement success

The sea louse Lepeophtheirus salmonis (Kr?yer, 1837) (Copepoda: Caligidae) is an ectoparasite of salmonid fish. It has earlier been proposed that the free-swimming infectious copepodid stage of L. salmonis gather at river mouths to infect wild Atlantic salmon Salmo salar L. and sea trout S. trutta L. smolts during their seaward migration. This study used aquarium-based methods to investigate the survival, infective ability and behaviour of L. salmonis copepodids exposed to short periods of low salinity levels, such as those encountered at river mouths. Survival of free-swimming copepodids was found to be severely compromised at salinity levels below 29 parts per thousand (ppt). Attachment to an S. salar host did not aid copepodid survival during post-infection exposure to low salinity environment, and a reduction in salinity appears to reduce the ability of copepodids to remain attached to S. salar smolts. Pre-infection exposure of copepodids to reduced salinity levels reduced infection of S. salar. Infection levels at reduced salinity were lower than predicted from the free-swimming survival experiment, suggesting that low salinity compromises the copepodids' ability to sense or respond to the presence of a host. In salinity gradients, copepodids demonstrated avoidance of salinities below 27 ppt, by both altering their swimming behaviour and changing the orientation of passive sinking. Avoidance of low salinity levels may be due to their adverse effects on copepodid physiology, as suggested by the reduction in survival. Sinking rates were also faster in reduced salinity, suggesting that remaining in the water column would be more energetically demanding for the copepodids at reduced salinity. These results show that both survival and host infectivity of L. salmonis are severely compromised by short-term exposure to reduced salinity levels.     

Reduced Lepeophtheirus salmonis larval abundance in a sea loch on the west coast of Scotland between 2002 and 2006

A survey for planktonic sea louse larvae was carried out in Loch Shieldaig, Scotland, between 2002 and 2006, and spanned 2 successive production cycles (Cycles 1 and 2) at a local Atlantic salmon Salmo salar L. farm. The vast majority of the caligid copepodids recovered were Lepeophtheirus salmonis; however, the methodology was unable to determine the species of the caligid nauplii. Greatest densities of nauplii were found at the sampling station adjacent to the salmon farm, and larval densities were low during the fallow period of both cycles. Peaks in nauplius densities occurred around the same time in the 2 cycles, but the peaks were significantly lower during Cycle 2 than Cycle 1. Lepeophtheirus salmonis copepodid densities varied temporally, but not spatially. During most of Cycle 2, copepodid densities were significantly lower than those recovered during Cycle 1. Numbers of gravid L. salmonis at the local salmon farm correlated significantly with densities of louse nauplii and L. salmonis copepodids in the water at time lags of 0 and 1 wk, and 1 and 2 wk, respectively. This survey demonstrated a reduction in densities of L. salmonis larvae in the plankton (an indication of L. salmonis infectious pressure) between the 2 cycles and indicated that the farm was an important source of L. salmonis larvae. The application of anti-louse treatments using emamectin benzoate reduced the numbers of gravid L. salmonis at the farm, and this was the main factor influencing the apparent reduction in L. salmonis infectious pressure in the loch between cycles.     

The osmotic response of salmon louse,Lepeophtheirus salmonis (Copepoda: Caligidae), during the transition from sea water to fresh water

Summary The osmotic changes in haemolymph and body tissues of the ectoparasitic salmon louse,Lepeophtheirus salmonis, have been studied upon transfer from sea water (SW) to dilute sea water (37% SW), and then to fresh water (FW). The parasite shows osmoconformity in SW but hyperosmotic regulation in 37% SW regardless of whether it is attached to the salmon host or free swimming in the water. The same conclusion is reached by haemolymph Cl^– measurements. In FW, the osmotic tolerance and response of attached and free swimming parasites differ: Attached animals maintain steady haemolymph osmolality and Cl^– concentration and survive for at least 1 week, while free swimming parasites quickly become diluted and start to die within 8 h.Acclimation to 37% SW is accompanied by changes in body tissue water content and in the content of ninhydrin positive substances and specific amino acids which suggest the presence of cell volume regulation. Glycine is the dominating free amino acid in the cephalothorax tissues but alanine, proline and taurine also occur in high amounts. Lysine is found to increase significantly during FW acclimation of attached parasites. A breakdown of cell volume regulation is suggested to limit the survival of attached salmon louse in fresh water.Abbreviations FW fresh water - NPS ninhydrin positive substances - SW sea water     

Evolution of virulence under intensive farming: salmon lice increase skin lesions and reduce host growth in salmon farms

Parasites rely on resources from a host and are selected to achieve an optimal combination of transmission and virulence. Human‐induced changes in parasite ecology, such as intensive farming of hosts, might not only favour increased parasite abundances, but also alter the selection acting on parasites and lead to life‐history evolution. The trade‐off between transmission and virulence could be affected by intensive farming practices such as high host density and the use of antiparasitic drugs, which might lead to increased virulence in some host–parasite systems. To test this, we therefore infected Atlantic salmon (Salmo salar) smolts with salmon lice (Lepeophtheirus salmonis) sampled either from wild or farmed hosts in a laboratory experiment. We compared growth and skin damage (i.e. proxies for virulence) of hosts infected with either wild or farmed lice and found that, compared to lice sampled from wild hosts in unfarmed areas, those originating from farmed fish were more harmful; they inflicted more skin damage to their hosts and reduced relative host weight gain to a greater extent. We advocate that more evolutionary studies should be carried out using farmed animals as study species, given the current increase in intensive food production practices that might be compared to a global experiment in parasite evolution.     

Sea lice and salmon population dynamics: effects of exposure time for migratory fish

The ecological impact of parasite transmission from fish farms is probably mediated by the migration of wild fishes, which determines the period of exposure to parasites. For Pacific salmon and the parasitic sea louse, Lepeophtheirus salmonis, analysis of the exposure period may resolve conflicting observations of epizootic mortality in field studies and parasite rejection in experiments. This is because exposure periods can differ by 2–3 orders of magnitude, ranging from months in the field to hours in experiments. We developed a mathematical model of salmon–louse population dynamics, parametrized by a study that monitored naturally infected juvenile salmon held in ocean enclosures. Analysis of replicated trials indicates that lice suffer high mortality, particularly during pre-adult stages. The model suggests louse populations rapidly decline following brief exposure of juvenile salmon, similar to laboratory study designs and data. However, when the exposure period lasts for several weeks, as occurs when juvenile salmon migrate past salmon farms, the model predicts that lice accumulate to abundances that can elevate salmon mortality and depress salmon populations. The duration of parasite exposure is probably critical to salmon–louse population dynamics, and should therefore be accommodated in coastal planning and management where fish farms are situated on wild fish migration routes.     

The impact of Piscirickettsia salmonis infection on genome-wide DNA methylation profile in Atlantic Salmon

Salmon rickettsial septicaemia (SRS), caused by the bacteria Piscirickettsia salmonis (P. salmonis), is responsible for significant mortality in farmed Atlantic salmon in Chile. Currently there are no effective treatments or preventive measures for this disease, although genetic selection or genome engineering to increase salmon resistance to SRS are promising strategies. The accuracy and efficiency of these strategies are usually influenced by the available biological background knowledge of the disease. The aim of this study was to investigate DNA methylation changes in response to P. salmonis infection in the head kidney and liver tissue of Atlantic salmon, and the interaction between gene expression and DNA methylation in the same tissues. The head kidney and liver methylomes of 66 juvenile salmon were profiled using reduced representation bisulphite sequencing (RRBS), and compared between P. salmonis infected animals (3 and 9 days post infection) and uninfected controls, and between SRS resistant and susceptible fish. Methylation was correlated with matching RNA-Seq data from the same animals, revealing that methylation in the first exon leads to an important repression of gene expression. Head kidney methylation showed a clear response to the infection, associated with immunological processes such as actin cytoskeleton regulation, phagocytosis, endocytosis and pathogen associated pattern receptor signaling. Our results contribute to the growing understanding of the role of methylation in regulation of gene expression and response to infectious diseases and could inform the incorporation of epigenetic markers into genomic selection for disease resistant and the design of diagnostic epigenetic markers to better manage fish health in salmon aquaculture.     

Exocrine glands of Lepeophtheirus salmonis (Copepoda: Caligidae): Distribution,developmental appearance,and site of secretion

Exocrine glands of blood‐feeding parasitic copepods are believed to be important in host immune response modulation and inhibition of host blood coagulation, but also in the production of substances for integument lubrication and antifouling. In this study, we aimed to characterize the distribution of different types of salmon louse (Lepeophtheirus salmonis) exocrine glands and their site of secretion. The developmental appearance of each gland type was mapped and genes specifically expressed by glands were identified. Three types of tegumental (teg 1–3) glands and one labial gland type were found. The first glands to appear during development were teg 1 and teg 2 glands. They have ducts extending both dorsally and ventrally suggested to be important in lubricating the integument. Teg 1 glands were found to express two astacin metallopeptidases and a gene with fibronectin II domains, while teg 2 glands express a heme peroxidase. The labial glands were first identified in planktonic copepodids, with reservoirs that allows for storage of glandular products. The last gland type to appear during development was named teg 3 and was not seen before the preadult I stage when the lice become more virulent. Teg 3 glands have ducts ending ventrally at the host‐parasite contact area, and may secrete substances important for the salmon lice virulence. Salmon lice teg 3 and labial glands are thus likely to be especially important in the host‐parasite interaction. Proteins secreted from the salmon louse glands to its salmonid host skin or blood represents a potential interface where the host immune system can meet and elicit effective responses to sea lice antigens. The present study thus represents a fundamental basis for further functional studies and identification of possible vaccine candidates. J. Morphol. 277:1616–1630, 2016. © 2016 Wiley Periodicals, Inc.     

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.     

The importance of escaped farmed rainbow trout (Oncorhynchus mykiss) as a vector for the salmon louse (Lepeophtheirus salmonis) depends on the hydrological conditions in the fjord

The objectives of the study were to see if escaped rainbow trout (Oncorhynchus mykiss) spread rapidly or not from fish farms, and to test whether the hydrological conditions in a fjord influence their vertical distribution and importance as vector for the salmon lice (Lepeophtheirus salmonis). Fifty farmed rainbow trout were tagged with acoustic transmitters including depth sensors and released from two of 11 fish farms in the fjord system. In addition, unintentionally escaped rainbow trout were recaptured for analysis of salmon lice and stomach content. Dispersal out of the fjord system was limited. Most fish stayed in the vicinity of and moved between the fish farms but fed primarily on a variety of indigestible items. They moved in the warm relatively fresh surface layer from late spring until early autumn where the risk of being infested with salmon lice was low. They swam gradually deeper and became much more infested with salmon lice as the surface layers cooled and salinity and temperature gradients became less distinct over the course of the winter. The observed post-escapement behavior may challenge the control of the spread of diseases and parasites between neighboring farms and to wild fish, but also increases opportunities for recapture.     

Trophic discrimination factor of nitrogen isotopes within amino acids in the dobsonfly Protohermes grandis (Megaloptera: Corydalidae) larvae in a controlled feeding experiment

The trophic discrimination factor (TDF) of nitrogen isotopes (¹⁵N/¹⁴N) within amino acids, between a stream‐dwelling dobsonfly larva (Protohermes grandis: Megaloptera; Corydalidae) and its diet (chironomid larvae), was determined in controlled feeding experiments. Last‐instar larvae of P. grandis were collected from the Yozawa‐gawa River, central Japan, and reared in the laboratory. After fed to satiation for 1 month, one group of larvae was each fed one living chironomid larva per day for 4 weeks, while a second group was starved for 8 weeks. The larvae were harvested at intervals and the nitrogen isotopic composition of glutamic acid (δ¹⁵N_Glu) and phenylalanine (δ¹⁵N_Phe) were determined to calculate TDF. The mean TDF of satiated and starved larvae were 7.1‰ ± 0.5‰ (n = 3) and 7.3‰ ± 0.5‰ (n = 5), respectively. Thus, the TDF for P. grandis larvae in this study was similar to that reported for other arthropods (approximately 7‰) and was independent of satiation or starvation. A previous study of wild P. grandis larvae, based on the δ¹⁵N_Glu and δ¹⁵N_Phe values, estimated its trophic position (TP) as approximately 2.0 ± 0.1 (n = 5), a low value close to that of algivores, although they are generally characterized as carnivores (usually accepted as TP ≥ 3). The TDF for P. grandis larvae suggests that their low TPs in nature were caused by incorporation of vascular plant‐derived amino acids (with a different δ¹⁵N profile from that of algae) and not by an unusually low TDF or by the effects of the satiation/starvation on amino acid metabolism.     

Genomewide single nucleotide polymorphism discovery in Atlantic salmon (Salmo salar): validation in wild and farmed American and European populations

A considerable number of single nucleotide polymorphisms (SNPs) are required to elucidate genotype–phenotype associations and determine the molecular basis of important traits. In this work, we carried out de novo SNP discovery accounting for both genome duplication and genetic variation from American and European salmon populations. A total of 9 736 473 nonredundant SNPs were identified across a set of 20 fish by whole‐genome sequencing. After applying six bioinformatic filtering steps, 200 K SNPs were selected to develop an Affymetrix Axiom^® myDesign Custom Array. This array was used to genotype 480 fish representing wild and farmed salmon from Europe, North America and Chile. A total of 159 099 (79.6%) SNPs were validated as high quality based on clustering properties. A total of 151 509 validated SNPs showed a unique position in the genome. When comparing these SNPs against 238 572 markers currently available in two other Atlantic salmon arrays, only 4.6% of the SNP overlapped with the panel developed in this study. This novel high‐density SNP panel will be very useful for the dissection of economically and ecologically relevant traits, enhancing breeding programmes through genomic selection as well as supporting genetic studies in both wild and farmed populations of Atlantic salmon using high‐resolution genomewide information.     

Exposure of goldsinny, rock cook and corkwing wrasse to low temperature and low salinity: survival, blood physiology and seasonal variation

Wrasse used as cleaner fish with farmed Atlantic salmon Salmo salar can be subjected to large and rapid temperature and salinity fluctuations in late autumn and early winter, when summer-warmed surface water is affected by early snowmelt episodes. Because of their containment in sea cages, wrasse which are essentially acclimated to summer temperatures may be rapidly exposed to winter conditions. Short-term tolerance of low temperature and low salinity by three species of wrasse, goldsinny Ctenolabrus rupestris rock cook Centrolabrus exoletus corkwing Crenilabrus melops caught during the summer, and winter-caught corkwing, was investigated. A 3–day period at 30 or 32‰ salinity and temperature 8, 6 or 4° C (for summer-caught fish; 4° C only for winter-caught) was followed by a decline in salinity to 24, 16 or 8‰ over c. 36 h, followed by a further 24 h at these salinities held constant, at each of the three temperatures. Controls in 30 or 32‰ were maintained at 8, 6 or 4° C. Mortality of summer-caught corkwing and rock cook was high at 4° C, whereas the influence of salinity on mortality was small. Mortality of goldsinny was low or zero in all treatments. Surviving corkwing and rock cook after 3 days at 4° C and 32‰ salinity had elevated plasma osmolality: in summer-caught corkwing, plasma [Cl°] and [Na⁺] were high, whereas in rock cook only [Na⁺] was high. Haematocrit was low in summer-caught corkwing, high in rock cook. In survivors of all three species at the end of the experiment, values of all these parameters were comparable with those of fish at the beginning of the experiment, except that survivors at low salinity (8, 16‰) had low plasma osmolality, at all temperatures, and in surviving rock cook in these treatments haematocrit was high and plasma [Cl^?] was low. Winter-caught corkwing had higher osmolality, [Na⁺] and [Cl^?] than summer-caught corkwing; there was no difference in haematocrit. Survival of wintercaught corkwing exposed to four salinities at 4° C was much higher than that of summercaught corkwing under the same conditions. Little change in blood physiology was recorded for winter-caught corkwing, with only fish subjected to 8‰ and 4° C showing signs of osmoregulatory stress. The interspecific and seasonal differences in survival and blood physiology at low temperature and low salinity are discussed in relation to wrasse survival over winter, both in the field and in salmon farms.