Muscular and myocardial degeneration in cultured Atlantic salmon, Salmo salar L., suffering from 'Hitra disease'

Abstract. In this investigation of the condition known currently as 'Hitra disease', anaemia was usually seen and transudates to the peritoneal cavity and pericardial sac were often found. Muscular and myocardial degeneration were constant features and deposits of ceroid were sometimes found in the livers. Compared to similar conditions in fish and in higher animals it is reasonable to suggest that the main cause of the disease may be related to vitamin E/selenium deficiency and diets high in polyunsaturated fatty acids. The levels of unsaturated fatty acids in particular would appear of great importance in the formulation of diets for cultured salmon.     

Antibiotic resistance of Aeromonas salmonicida isolated from Atlantic salmon, Salmo salar L., in Scotland

Abstract. Antibiotic sensitivity patterns of 304 isolates of Aeromonas salmonicida from 229 outbreaks of furunculosis among salmon in Scotland between 1988 and 1990 were investigated. Fifty-five per cent were resistant to oxytetracycline and 37% resistant to oxolinic acid. Multiple resistance was common (52%) and 18 out of 19 antibiograms which were found in the first year recurred in the succeeding year. More than a quarter of the outbreaks were associated with two or more A. salmonicida variants distinguishable by their antibiotic sensitivity patterns. The implications of these findings in the control of furunculosis are considered.     

Experimental transmission of a salmonid Rhodococcus sp. isolate to juvenile Atlantic salmon, Salmo salar L.

Abstract. This paper provides further characterization of a 'Rhodocoecus' -like pathogen of Atlantic salmon, Salmo salar L., and describes its experimental transmission. An isolate from naturally infected fish was determined to be 'Rhodococcus' -like based on its biochemical profile and from gas liquid chromatography assessment of its cellular fatty acid methyl esterase profile in comparison to a data base. This isolate was used in an infectivity trial in which Atlantic salmon juveniles received an intraperitoneal injection of known concentrations of the bacteria. Fish receiving 0.5 ml of a 10⁹bacteria ml⁻¹ solution developed severe peritoneal granulomatous reactions, and bacteria were recovered from the peritoneum and the kidney. In contrast to the natural disease, in which pathological changes are often most dramatic in the renal interstitium, in our study, the lesions were restricted to the injection site—the peritoneal cavity. However, in this location, the large colonies of bacteria and the nature of the host response were both characteristic of the changes noted in the renal interstitium of naturally infected fish. Lesser concentrations of injected bacteria resulted in either no or minimal pathological change, and subsequent recovery of bacteria from injected fish was poor. Horizontal transmission to non-injected co-habitants was not achieved.     

Genetic characterization of Streptococcus phocae strains isolated from Atlantic salmon, Salmo salar L., in Chile

Streptococcus phocae is a beta-haemolytic bacterium frequently involved in disease outbreaks in seals causing pneumonia or respiratory infection. Since 1999, this pathogen has been isolated from diseased Atlantic salmon, Salmo salar , causing serious economic losses in the salmon industry in Chile. In this study, we used different molecular typing methods, such as pulsed-field gel electrophoresis (PFGE), randomly amplified polymorphic DNA (RAPD), enterobacterial repetitive intergenic consensus sequence PCR (ERIC-PCR), repetitive extragenic palindromic PCR (REP-PCR) and restriction of 16S–23S rDNA intergenic spacer regions to evaluate the genetic diversity in S. phocae . Thirty-four strains isolated in different years were analysed. The S. phocae type strain ATCC 51973^T was included for comparative purposes. The results demonstrated genetic homogeneity within the S. phocae strains isolated in Chile over several years, suggesting the existence of clonal relationships among S. phocae isolated from Atlantic salmon. The type strain ATCC 51973^T presented a different genetic pattern with the PFGE, RAPD, ERIC-PCR and REP-PCR methods. However, the fingerprint patterns of two seal isolates were distinct from those of the type strain.     

Antibody specificities in Atlantic salmon, Salmo salar L., against the fish pathogens Vibrio salmonicida and Vibrio anguillarum

Abstract. Specificities of polyclonal salmon antisera made against the fish pathogens Vibrio salmonicida and Vibrio anguillarum were studied. Using ELISA and Western blot techniques, antisera made against V. salmonicida or V. anguillarum serovar 1 demonstrated high responses against the homologous bacterium or its isolated LPS. In contrast, antisera obtained after immunization with V. anguillarum serovar 2 displayed low antibody titres against homologous antigens. Elcctrophoretic transfer of SDS-PAGE separated V. salmonicida LPS antigen to nitrocellulose strips and subsequent immunostaining with salmon antisera revealed a strong reaction exclusively in the low molecular weight region (<14kD). On the other hand, immunoblots of V. anguillarum LPS preparations using salmon immunesera raised against this species showed a heterogenous staining pattern ranging from high to medium LPS-size. In addition, most of the salmon antisera made against V. anguillarum serovar 2 also reacted with a low molecular weight LPS antigen band.     

In situ adherence of Vibrio spp. to cryosections of Atlantic salmon, Salmo salar L., tissue.

Pathogenic and presumed non-pathogenic bacteria isolated from fish were tested for their adhesion to cryosections from different mucosal surfaces of Atlantic salmon, Salmo salar L. Adhered bacteria were detected by immunohistochemistry. Mucus was stained and fixed with Alcian blue after incubation of bacteria. The majority of the bacteria tested, i.e. Vibrio anguillarum serotype O1 , Vibrio salmonicida , Vibrio viscosus, Flexibacter maritimus and 'gut vibrios', i.e. Vibrio iliopiscarius and intestinal isolates of V. salmonicida , all adhered to mucus on all salmon epithelial surfaces tested, including sections from the foregut, hindgut, pyloric caeca, gills and skin. In contrast, V. anguillarum serotype O2, including both serotypes O2a and O2b, did not adhere to mucus, but did adhere to all other components of the tissues. As a positive control for adhesion of bacteria on cryosections, Escherichia coli was bound to piglet ileal mucosal lining, and as a negative control for adhesion, Staphylococcus aureus was found not to bind to any of the tissues tested. The present study shows that adhesion to mucus was not restricted to pathogenic bacteria, and furthermore, that not all pathogenic bacteria studied adhered to mucus. Hence, on the basis of these findings, the present authors suggest that V. anguillarum O2 may have an invasion strategy which does not involve adhesion to mucus, and thus, differs from the other pathogenic bacteria in the present study, which all bound to salmon mucus.     

Plasmid-mediated antibiotic resistance in Aeromonas salmonicida isolated from Atlantic salmon, Salmo salar L., in Scotland

Abstract. Forty oxytetracycline-resistant isolates of Aeromonas salmonicida obtained from outbreaks of furunculosis in Atlantic salmon in Scotland were tested for their susceptibility to 12 antibacterial agents. There were 10 resistance patterns with multiple resistance to two to six antibacterial agents. Transferable R-plasmids encoding oxytetracycline resistance were demonstrated in 11 out of the 40 isolates. The resistance transferred was multiple; to oxytetracycline, streptomycin, sulphamethoxine and trimethoprim, or to oxytetracycline and one or two of these in combination. Oxytetracycline resistance was transferred in a single, large step, >250-fold increase, and the MICs for individual transconjugants from mating populations were the same.     

Cross-protection after immunization of Atlantic salmon, Salmo salar L., against different strains of Vibrio salmonicida

Abstract. Atlantic salmon were immunized with one or other of two bacterins made from different strains of Vibrio salmonicida. The two strains differed both in geographic origin and plasmid profile. One of the strains had given rise to disease outbreaks in salmon and also in cod which were kept in net-pens. However, by using a virulent challenge, it appeared that salmon, immunized with any of the two strains, were also protected against infection with the other. This result indicates that the two strains are serologically similar.     

Experimental infection of Atlantic salmon, Salmo salar L., with infectious salmon anaemia virus: a histopathological study

Recent reports of the isolation of infectious salmon anaemia virus (ISAV) from Atlantic salmon, Salmo salar L., affected by haemorrhagic kidney syndrome (HKS) suggest that ISAV can cause severe renal haemorrhage and necrosis in addition to well-known pathognomonic hepatocellular necrosis and haemorrhage. The prevalence of ISAV-induced pathognomonic renal HKS lesions and their correlation to pathognomonic hepatic lesions of infectious salmon anaemia (ISA) is not known. The present experimental infection of Atlantic salmon with a Canadian isolate of ISAV found that pathognomonic hepatic ISA lesions were present in 90.6% and pathognomonic renal HKS lesions in 78.1% of fish which died after the experimental challenge. Both pathognomonic hepatic ISA lesions and pathognomonic renal HKS lesions were found together in 65.6% of fish which died after ISAV challenge. The present study clearly demonstrates that ISAV can cause a very high prevalence of both HKS and ISA pathognomonic lesions.     

Atlantic salmon, Salmo salar L., feed in Scottish coastal waters

Abstract. The stomach contents of 256 Atlantic salmon, Salmo salar L., of fork lengths 53–66cm, caught in bag nets on the Scottish west coast near Ullapool, were examined between June 1983 and July 1986. A total of 61 fish contained fish in the stomach or had faecal pellets containing fish bones in the gut. All recognizable whole fish were sandeels, Ammodytes marinus Raitt, ranging in size from 4.5 to 15cm. No evidence for crustacean or other non-fish prey items was found. Results indicate that feeding salmon were caught up to a certain cut-off point in June or early July, after which all salmon sampled were not feeding. It is suggested that either there is a local feeding stock of salmon or that fish feed during migration from the Faroe Isles or other possible distant water origins.     

Screamer disease in Atlantic salmon, Salmo salar L., in Chile

Investigation of cranial and spinal deformities in Atlantic salmon smolts from Chile immediately after sea transfer, over two successive crops, demonstrated ankylosis of the mandibular articulation, spinal foreshortening, fracture of vertebrae and rarefaction of osseous and cartilaginous tissues including the operculum. As a result the mouth was permanently agape and opercula could be folded back on themselves. All affected fish had been transferred to sea water at 20 °C plus and fed on particular commercial diets. Fish in cooler areas, or on diets high in vitamin C and phosphorus in the high temperature zone, were clinically normal. The condition has not recurred following dietary adjustment.     

Pathogenicity of Saprolegnia spp. to Atlantic salmon, Salmo salar L., eggs

Live and dead Atlantic salmon eyed eggs were challenged with eight different Saprolegnia isolates, selected because of their varied origins, known morphological characteristics and growth/germination pattern. Some isolates were also tested for pathogenicity to Atlantic salmon parr. Challenge of eggs was performed by exposure to spores in suspension or by co-incubation of live eggs with infected dead eggs. The phenotypic characteristics of the isolates were evaluated in relation to their observed pathogenicity from the challenge experiment, to identify possible virulence factors leading to egg-infection by Saprolegnia. The results from the experiments confirm that live eggs are refractory to infection with Saprolegnia spores in suspension and that an infection of live eggs can only occur from an infection nucleus represented by dead eggs or debris. It was observed that strains pathogenic to salmon parr were not particularly infective towards eggs, and the isolates that gave the highest infection rates to eggs were species considered to be saprotrophs.     

Reproductive strategies in Atlantic salmon, Salmo salar L.

Abstract. Life-history strategies are means by which animals solve the problems of successful reproduction in varying environments. Their development patterns are consequences of responses to the opportunities the environment offers them. Understanding them requires an understanding of the way they evolved, their ontogenetic development, their physiological control, and their adaptive value. The present paper views the salmonids as marine fishes, which have radiated into fresh water through using river beds as protected spawning grounds. It also takes the view that the maturation process has priority over somatic growth in fish, and that it has already been initiated by the time of first feeding. Its completion is environmentally dependent, and can be arrested annually. Whether or not it will be arrested depends on the status of the energy stores of the individual at particular critical times of year. This mechanism has adaptive value both for immediate reproductive success — adequate energy to provision the next generation — and for later overwinter survival, ensuring that if energy stores are inadequate for reproduction they are spared. Atlantic salmon show variation in their reproductive patterns, and examples are given from laboratory and aquaculture experiments to demonstrate some environmental controls which result in these variations. A hypothetical model is presented to account for the operation of these controls.     

Swimming height of Atlantic salmon, Salmo salar L., crossing a Crump weir

Abstract. A closed-circuit television system was used to determine the height in the water column at which salmon swam when ascending and descending over a Crump weir. The swimming depth distribution of fish ascending the weir was very skewed with 88.1% of the fish seen swimming within 10cm of the weir surface while the descending fish seen were scattered throughout the available water column. The distribution of water velocities over the downstream face of the weir showed that the velocity was lowest near the weir surface and highest at the water-air interface except at the weir crest where high velocity water was found at the weir surface, often causing fish to rise as they crossed the crest. This has important implications in choosing the position of electrode arrays in open-channel counters as the upstream electrode must not be too close to the weir crest or fish may be missed as a result of being too far from the electrode to produce a large enough signal.