Commercial trials using emamectin benzoate to control sea lice Lepeophtheirus salmonis infestations in Atlantic salmon Salmo salar

Two trials were conducted at commercial salmon farms to evaluate the efficacy of emamectin benzoate (Slice, 0.2% aquaculture pre-mix, Schering-Plough Animal Health) as a treatment for sea lice Lepeophtheirus salmonis (Kr?yer) and Caligus elongatus Nordmann infestations in Atlantic salmon Salmo salar L. Trials were carried out in 15 m2 commercial sea pens, at temperatures of 5.5 to 7.5 degrees C and 10.8 to 13.8 degrees C. Each pen was stocked with 14,000 to 17,500 fish with mean weights of 0.44 to 0.74 and 1.33 to 1.83 kg. Fish were naturally infested with sea lice at the start of each trial. At Day -1, samples of 10 or 15 fish were taken from each pen to determine pre-treatment numbers of lice. Emamectin benzoate was administered in feed, to 4 replicate pens, at a dose of 50 micrograms kg-1 biomass d-1 for 7 consecutive days (Days 0 to 6). Sea lice were counted again, between Days 7 and 77, and comparisons made with untreated control fish. Despite adverse weather conditions, wide variations in fish weights and exposure to new infestations, treatment was effective against chalimus and motile stages of L. salmonis. In the autumn trial, efficacy at Day 27 was 89%, and lice numbers remained lower on treated fish than on control fish 64 d from the start of treatment. In the winter trial, reductions in lice numbers at low temperatures were slower but good efficacy was achieved by Day 35. Although control fish had to be treated with hydrogen peroxide at Day 21, fish treated only with emamectin benzoate on Days 0 to 6 still had 89% fewer lice than control fish at Day 35. There were very few C. elongatus present, but at the end of both trials numbers were lower on treated fish. No adverse effects were associated with treatment of fish with emamectin benzoate.     

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.     

Spawning behaviour of a farmed escaped female Atlantic salmon (Salmo salar)

Excavation of stranded redds revealed differences in spawning behaviour between farmed and wild Atlantic salmon. The redd of a farmed fish contained more egg pockets (nine v . average of two) and fewer eggs per pocket (averages: 459 v . 707). No other pocket measures differed.     

Migratory behaviour of wild and farmed Atlantic salmon (Salmo salar) during spawning

Migratory behaviour at spawning of wild and newly-escaped farmed Atlantic salmon was analysed by radio telemetry in the River Alta, North Norway. Spawning areas were located by aerial surveys. Farmed females moved significantly more than wild females ( P <0.01). There was no such difference between the two groups of males. About 83% of the wild fish stayed within identified spawning areas for 1 day or longer. The corresponding figure for farmed salmon was only 43% ( P <0.05). Wild salmon stayed 8.1 days inside spawning areas and farmed salmon 5.2 days. The present results suggest that escaped farmed salmon had reduced spawning success compared with wild fish.     

Changes in physiological parameters and feeding behaviour of Atlantic salmon Salmo salar infected with sea lice Lepeophtheirus salmonis

Atlantic salmon Salmo salar L. artificially infected with salmon lice Lepeophtheirus salmonis (Kr?yer 1837) recovered from detrimental physiological changes and skin damage induced by preadult lice as the parasites matured. Growth rates of Atlantic salmon remained unaffected by lice infection, but food consumption decreased with increasing feeding and movement of the lice prior to and post-mating, correlating with the appearance of head erosions and detrimental changes in physiological integrity. Food consumption of the fish increased as the lice moulted to the adult stage and gravid female lice settled in a posterior location on the fish, subsequently reducing the impact of infection and allowing recovery of the skin damage. However, the impact of preadults was limited, as the decrease in food consumption of fish at 21 d post-infection had no effect on either the specific growth rate or condition factor of the fish. Furthermore, the intensity of lice infections at each of the sample days was not correlated with food consumption, specific growth rate or any of the haematological or physiological parameters measured, either before or after infection, indicating that lice intensity was independent of social dominance/subordinance. This work has provided the first evidence that infected fish can recover from the detrimental changes caused by lice infection, even when they are still infected with lice. If fish can survive the preadult stage of lice, then the mortal impact of lice infections is greatly reduced.     

Transport-associated proteins in Atlantic salmon (Salmo salar)

 The major histocompatibility complex (Mhc) regions of mice, rats, and humans all contain a pair of related genes, TAP1 and TAP2, which encode members of a large superfamily of proteins of similar structure and function. A functional TAP1/TAP2 heterodimer is probably required for efficient presentation of antigens to CD8⁺ T cells. This heterodimer resides in the membrane of the endoplasmic reticulum, and transports peptides from the cytoplasm into the endoplasmic reticulum lumen for binding to Mhc class I molecules. The TAP transporter demonstrates specificity for both peptide sequence and length, and in rats, allelic variation in the sequence of the transporter molecules results in differential ability to transport particular peptides. Here we report two expressed Sasa-TAP2 loci, both of which are polymorphic, as well as an expressed Sasa-TAP1 locus from Atlantic salmon. The Sasa-TAP2A locus has a genomic organization similar to the human TAP2 equivalent. Received: 23 December 1996 / Revised: 26 February 1997     

An epidemiological study of cataracts in seawater farmed Atlantic salmon Salmo salar

Cataracts in farmed Atlantic salmon have been known for many years, but the aetiology and importance of the disease have not been clarified. A cross-sectional field study of 51 cages of Atlantic salmon at 49 randomly selected sea sites was performed during the summer of 1998. The target population was spring and autumn entry groups of the 1997 generation salmon. Approximately 15 fish from each cage, altogether 777 fish, were autopsied by the same person. Each eye of the fish was scored for cataracts on a scale from 0 to 4 using an otoscope lamp with magnification. The weight and length of each fish were measured. The prevalence of cataracts was 83 % and 79% in spring entry groups and autumn entry groups, respectively. The overall mean cataract index (mean score of both eyes) was 1.23, being significantly higher in the spring entry groups (1.36) than the autumn entry groups (0.85). The final results in the spring entry groups showed that the fish groups with higher weight at sea transfer also had a higher cataract index at inspection. The risk of development of cataracts varied significantly among the offspring from the 5 strains represented in the study. Fish from sites located in 2 counties in the southern part of Norway had a significantly higher cataract index than fish farmed in the northernmost county in the study. For the autumn entry groups none of the explanatory variables was significant. In the spring entry groups a significant negative relationship was observed between the cataract score and the weight of the fish at the time of inspection (Pearson's r = -0.17), while the corresponding correlation for the autumn released groups was r = -0.10. Among the spring entry groups the average weight of the fish with the highest cataract score was estimated to about a third of the weight of the fish with no visible cataracts.     

A novel betaproteobacterial agent of gill epitheliocystis in seawater farmed Atlantic salmon (Salmo salar)

Epitheliocystis, a disease characterised by cytoplasmic bacterial inclusions (cysts) in the gill and less commonly skin epithelial cells, has been reported in many marine and freshwater fish species and may be associated with mortality. Previously, molecular and ultrastructural analyses have exclusively associated members of the Chlamydiae with such inclusions. Here we investigated a population of farmed Atlantic salmon from the west coast of Norway displaying gill epitheliocystis. Although 'Candidatus Piscichlamydia salmonis', previously reported to be present in such cysts, was detected by PCR in most of the gill samples analysed, this bacterium was found to be a rare member of the gill microbiota, and not associated with the observed cysts as demonstrated by fluorescence in situ hybridization assays. The application of a broad range 16 S rRNA targeted PCR assay instead identified a novel betaproteobacterium as an abundant member of the gill microbiota. Fluorescence in situ hybridization demonstrated that this bacterium, tentatively classified as 'Candidatus Branchiomonas cysticola', was the cyst-forming agent in these samples. While histology and ultrastructure of 'Ca. B. cysticola' cysts revealed forms similar to the reticulate and intermediate bodies described in earlier reports from salmon in seawater, no elementary bodies typical of the chlamydial developmental cycle were observed. In conclusion, this study identified a novel agent of epitheliocystis in sea-farmed Atlantic salmon and demonstrated that these cysts can be caused by bacteria phylogenetically distinct from the Chlamydiae.     

Identification and localization of a putative ATP-binding cassette transporter in sea lice (Lepeophtheirus salmonis) and host Atlantic salmon (Salmo salar)

Some members of the ABC-transporter superfamily, such as P-glycoprotein and the multidrug resistance associated protein, may confer resistance to the avermectin subclass of macrocyclic lactones. The aim of this study was to examine the presence of ABC transporters in both sea lice (Lepeophtheirus salmonis) and its Atlantic salmon host (Salmo salar) using monoclonal antibodies (C219 and JSB-1, with high selectivity for P-gp) and a new polyclonal antibody (SL0525) generated against a putative sea louse ABC transporter. The antibody raised to SL0525 did not react with rat P-gp, suggesting that an ABC transporter, not necessarily P-gp, was isolated. C219 was the only antibody to localize P-gp in all 3 salmon tissues (intestine, kidney and liver). American lobster (Homarus americanus) was used as a reference crustacean for L. salmonis immunostaining reactions and showed positive staining in the hepatopancreatic and intestinal tissues with all 3 antibodies. The L. salmonis showed positive staining in the intestinal epithelial lining with all antibodies. This report represents the first documented evidence for the expression of ABC transporters in L. salmonis, its Atlantic salmon host, and the American lobster.     

Microbial and pathological findings in farmed Atlantic salmon Salmo salar with proliferative gill inflammation

Proliferative gill inflammation (PGI) is an important cause of loss in seawater-farmed Atlantic salmon in Norway. Several microbes have been associated with PGI, including the commonly but not exclusively observed inclusions (epitheliocysts) within the gill lamellae related to infection with 'Candidatus Piscichlamydia salmonis'. Atlantic salmon transferred in the spring of 2004 to 12 seawater farms situated in mid- and southwest Norway were sampled throughout that year. Outbreaks of PGI, as evaluated by clinical examination, histology, and mortality data, were diagnosed in 6 of 7 farms in southwest Norway but not in the 5 farms studied in mid-Norway. Generally, mortality started 3 to 5 mo after seawater transfer and outbreaks lasted at least 1 to 3 mo. 'Ca. P. salmonis' was detected by real-time PCR only in fish from PGI-affected farms and our results indicate an association between 'Ca. P. salmonis' load and PGI severity. Likewise, although widely distributed in all 12 farms studied, epitheliocyst prevalence and number per fish as observed by histology appears associated with PGI prevalence and severity. However, the occurrence of epitheliocysts showed no association with molecular detection of 'Ca. P. salmonis', suggesting that at least 1 other organism is responsible for many of the observed inclusions. A microsporidian, Desmozoon lepeophtherii, was identified at high prevalence regardless of fish and farm PGI status, but at higher loads in fish with PGI. Our results support a multifactorial etiology for PGI in which 'Ca. P. salmonis', an unidentified epitheliocyst agent, and the microsporidian are contributing causes. No evidence for the involvement of Atlantic salmon paramyxovirus in PGI development was identified in the present study. High water temperatures and ectoparasites probably exacerbated mortality.     

A lymphosarcoma in an Atlantic salmon (Salmo salar)

A lymphosarcoma that appeared to be of thymic origin and of lymphoblastic type was found in a 3.5-yr-old Atlantic salmon (Salmo salar). The fish was from a population of 60 broodfish maintained at a research fish laboratory. A large tumor mass was found under the left operculum. Small tumor nodules were found on the swim bladder and in the abdominal adipose tissue. The location of this neoplasm differed from those of previously described tumors in this fish species.     

Characterisation of an emerging rickettsia-like organism in Tasmanian farmed Atlantic salmon Salmo salar

A rickettsia-like organism (RLO) was observed in farmed Atlantic salmon Salmo salar located in south-east Tasmania, Australia. Several assays such as immunoperoxidase, immunoelectron microscopy, polymerase chain reaction and nucleic acid sequencing, as well as phylogenetic analysis of rDNA sequences, were performed on infected fish tissues. Immunohistochemistry results suggested the presence of related antigenic determinants between the Tasmanian RLO and the type strain LF-89 of Piscirickettsia salmonis. However, sequence alignment demonstrated that the Tasmanian RLO contains a 19 bp deletion at the 3'-end of the internal transcribed spacer region of the rDNA operon, indicating a genetic divergence from P. salmonis isolates, which are exotic to Australia.     

Diseases of farmed Atlantic salmon Salmo salar associated with infections by the microsporidian Paranucleospora theridion

The microsporidian Paranucleospora theridion was discovered in Atlantic salmon Salmo salar suffering from proliferative gill disease in a marine farm in western Norway in 2008. The parasite develops in cells of the reticuloendothelial system, cells important for normal immune function. The aim of this study was to see if P. theridion could play a part in some of the diseases with unclear causes in salmon production in Norway, i.e. proliferative gill disease (PGI), pancreas disease (PD), heart and skeletal muscle inflammation (HSMI) and cardiomyopathy syndrome (CMS). P. theridion was present in all areas with salmon farming in Norway, but high prevalence and densities of the parasite in salmon and salmon lice were only seen in southern Norway. This region is also the main area for PGI and PD in Norway. Quantification of pathogens associated with PGI, PD, HSMI and CMS diagnoses showed that P. theridion levels are high in southern Norway, and may therefore play a role in susceptibility and disease development. However, among the different diagnoses, fish with PGI are particularly heavily infected with P. theridion. Therefore, P. theridion appears as a possible primary agent in cases with high mortality in connection with PGI in western Norway.     

A microsatellite linkage map for Atlantic salmon (Salmo salar)

A linkage map of the Atlantic salmon is described here consisting of 15 linkage groups containing 50 microsatellite loci with a 14 additional unlinked markers (including three allozymes). The map shows the largest sex-specific recombination rate differences so far found in any vertebrate species (3.92:1 female:male). Homologies with previous linkage mapping studies of Atlantic salmon and rainbow trout are described. An in silico search of the Genbank database carried out using the microsatellites used in the mapping process identified significant matches between the flanking regions of the microsatellite SS11 and the calcium-binding mitochondrial carrier protein, 'Aralar1'.     

Liver ultrastructure of juvenile Atlantic salmon (Salmo salar)

Light and transmission electron microscopy of the liver of juvenile Atlantic salmon (Salmo salar) reveals a tubular arrangement of parenchymal cells, with biliary passages typically located at the center of tubules. Hepatocytes generally contain a single nucleus surrounded by a cuff of rough endoplasmic reticulum (RER), with many round to elongate mitochondria associated with the perinuclear RER. Whereas glycogen deposits are common and usually lie at the cell periphery, parenchymal cells seldom contain lipid droplets. Golgi complexes and heterogeneous dense bodies also occur in many hepatocytes, often in close proximity to bile canaliculi. Numerous microvilli from hepatocytes extend into the subendothelial space of Disse, which is also the location of stellate fat-storing cells. Interhepatocytic macrophages, sometimes containing prominent phagolysosomes and residual bodies, are common in the liver. The intrahepatic biliary system consists of intercellular canaliculi, bile pre-ductules, ductules, and ducts. In contrast to some other teleosts, the liver of the Atlantic salmon contains no intracellular bile canaliculi or Kupffer cells. The hepatic endothelium, arterioles, and perivenous regions are also described.     

Mortality in Atlantic salmon (Salmo salar) associated with trichodinid ciliates

A protozoan infection (Trichodina truttae) was identified in captive Atlantic salmon (Salmo salar) kelts that died in spring of 1988 and 1989. Fish with intense infections showed signs of listlessness, erratic swimming and inappetence. The infection induced excessive mucus secretion, epithelial sloughing and lesions that probably permitted entry of opportunistic bacteria which eventually caused ulcers and death. A seawater bath for 30 min each week for 4 wk effectively controlled the parasite.     

Catabolism of circulating collagen in the Atlantic salmon (Salmo salar)

The catabolic fate of circulating collagen (Col) in the Atlantic salmon (Salmo salar) was studied. Serum t_1/2 and organ distribution of circulating Col in salmon were determined using Col conjugated with ^l25I-tyramine cellobiose (¹²⁵I-TC), a low molecular weight adduct which is trapped intralysosomally at the site of uptake. Intravenously administered ^l25I-tyramine cellobioselabelled Col type I was prepared either from salmon skin (sCol) or rat skin (rCol). Biphasic clearance kinetics of ^l25I-TC-sCol in salmon were apparent, with 78% being removed from the circulation in an initial rapid α-phase (t_1/2(α) = 2.4 min), and 22% being removed more slowly in a terminalβ-phase (t]2(β) = 25.8 min). Serum half life of ¹²⁵I-TC-rCol was found to be 5.4 min (in this type of experiment the number of data points allow the determination of only a monophasic decay slope). Approximately 90% of recovered radioactivity was found in the kidney of the fish. In comparative experiments, 74% of administered ¹²⁵I-TC-sCol was cleared from the circulation of rats during an initial rapid α-phase with t_l/2(α) = 0.8 min, and 26% was eliminated in the terminal β-phase with t_1/2(β) = 7.2 min ^l25I-sCol was endocytosed and degraded in pure cultures of ral liver endothelial cells, which are the main site of clearance of circulating Col in the rat. Moreover, Col from the two species competed for the same receptor on cultured rat liver endothelial cells, Intravenous administration of tetramethyl rhodamine isothiocyanate-labelled sCol (TRITC-sCol) in salmon, and subsequent examination of sections of kidney in the fluorescence microscope, revealed that the fluorochrome was accumulated exclusively in discrete vesicles of sinusoidal lining cells. Analyses of kidney tissue 24h after intravenous administration of a mixture of fluorescein isothiocyanate-labelled latex beads and TRITC-sCol revealed no codistribution of the two fluorochromes, suggesting that the injected Col was taken up in cells different from macrophages. Purified pronephros macrophages prepared after simultaneous injections of stained beads and Col contained only fluorescein-labelled latex particles. Interestingly, the cells which had accumulated TRITC-sCol appeared to be equally distributed in both pronephros and the part of the kidney containing tubuli. We conclude that Col which gains access to the circulation of the Atlantic salmon is cleared mainly by uptake into sinusoidal lining cells of the kidney. These cells are distinct from phagocytosing macrophages, and morphologically similar to the highly specialized scavenging endothelial cells of mammalian liver sinusoids.