The susceptibility of salmonid fish to an atypical strain of Aeromonas salmonicida that infects goldfish, Carassius auratus (L.), in Australia

Abstract. An enzootic, Australian, atypical strain of Aeromonas salmonicida isolated from diseased goldfish, Carassius auratus (L.), was inoculated into Atlantic salmon, Salmo salar L., brown trout, S. trutta L., rainbow trout, S. gairdneri Richardson, and brook trout, Salvelinus fontinalis (Mitchill), fingerlings by intraperitoneal injection (i.p.) and by bath challenge, the latter with and without prior abrasion of skin. The 10-day LD₅₀ (i.p.) was estimated to be 7·4 × 10^-3 colony forming units (cfu) for Atlantic salmon, 3·0 × 10^-2 cfu for brown trout, 3·7 × 10² cfu for brook trout and 6·4 × 10³ cfu for rainbow trout. Brown, rainbow and brook trout succumbed to bath challenges with between 10⁵–10⁶ cfu/ml, developing ulcers of the skin and septicaemia. The organism was trasmitted from inoculated fish to five of 195 within-tank control fish via water and established a carrier state in one of 14 Atlantic salmon. It was concluded that the organism poses a significant threat to the salmonid farming industry and wild salmonid fisheries in Australia.     

Urea in feeds for sea water farmed Atlantic salmon: effect on growth, carcass quality and outbreaks of winter ulcer

The main objective of the present study was to evaluate whether dietary supplementation of urea might reduce the incidence of winter ulcer in sea water-farmed Atlantic salmon. Salmon destined to be S0 smolt were fed with a urea-supplemented diet (0 or 20 g kg⁻¹ urea) over an 8-week period prior to sea water transfer and were then fed supplemented diet (0, 5, 10 or 20 g kg⁻¹ urea) during the first and second winters in the sea. During the first winter positive relationships between dietary urea and plasma urea and between plasma urea and plasma osmolality were observed. Further, plasma osmolality displayed a negative relationship to mortality. Of the salmon that died during the first winter in the sea 90% had one or more skin ulcers. Both during the first and second winter there were fewer salmon with ulcer among fish fed with the diets supplemented with urea. Salmon fed with 20 g kg⁻¹ urea tended to have higher percentage water in the muscle. Mortality and incidence of salmon with ulcer seemed to relate to plasma osmolality amongst fish fed on diets that differed in levels of urea supplementation, suggesting that an osmotic imbalance may contribute to the development of winter ulcer in farmed salmon. Salmon fed with 20 g kg⁻¹ urea showed significantly greater body weight during the second winter in sea. Fish killed without prior starvation had significantly higher level of muscle urea in the 20 g kg⁻¹ urea group compared with fish fed with the unsupplemented diet. However, a 13-day starvation period reduced urea content in the muscle to the level of the control. No effects of dietary urea supplementation on the sensory quality of market size Atlantic salmon were observed.     

A bath challenge model for furunculosis in rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L.

Abstract. A natural bath challenge method has been developed for furunculosis using Atlantic salmon, Salmo salar L., and rainbow trout, Salmo gairdneri Richardson. Fish were placed in an enclosed, continuously circulating tank system, supplemented with additional oxygen, the temperature raised gradually (overnight) to 15–16°C, a low dose of Aeromonas salmonicida (strain 184/186) introduced into the tank and the challenge maintained for 14 days. The challenge strain was characterized with respect to possible virulence factors and possessed an A-layer, ability to auto-agglutinate, haemagglutinate, adhere to Atlantic salmon cells and resist destruction by serum. No caseinase activity was detected. LD₅₀ for salmon using this method was 1.8×10³ cells per millilitre while trout had an LD₅₀ of 9.5×10⁴ cells per millilitre. Onset of the disease appeared to depend on fish size with larger trout (50 g) taking up to 10 days to show signs of the disease while mortalities in smaller trout (8.5g) commenced on day 1 post-challenge.     

Variation in semen production of farmed Atlantic salmon and rainbow trout

The variation in semen production among farmed Atlantic salmon (Salmo salar) and rainbow trout (Salmo gairdneri) has been studied. Both species were stripped at weekly intervals, the Atlantic salmon four times and the rainbow trout three times.The individual variation in volume of semen was very high, particularly in rainbow trout. The total volume of semen obtained was 137 ml (20 ml/kg body weight) in Atlantic salmon and 23 ml (5 ml/kg body weight) in rainbow trout. The intraclass correlation for volume of semen was estimated at 0.73 in Atlantic salmon and at 0.59 in rainbow trout. The correlations between volume of semen and body size (weight and length) were all positive. They were all significant and medium in Atlantic salmon whereas in rainbow trout they were all low and significant only for volume of semen at first stripping.The number of males needed to supply the Norwegian fish farming industry with semen is discussed. It is concluded that the possibility of disseminating genetic improvement throughout the whole population of farmed Atlantic salmon and rainbow trout by transport of semen from selected males is considerable.     

The Induction of Tetraploidy in Salmonids

The induction of tetraploidy was attempted in three intraspecies crosses (autotetraploid) of sal-monid: chinook salmon ( Oncorhynchus tshawytscha ), coho salmon ( O. kisutch ) and rainbow trout ( Salmo gairdneri ), and in the interspecies hybrid (allotetraploid) coho × Atlantic salmon ( Salmo salar ).
Pressure treatments of 6.2 × 10⁴ kPa (9,000 psi) were applied for six minutes at intervals from 50% of the time to cleavage to beyond first cleavage. Analysis of eyed stage embryos via flow cytometry indicated two periods of tetraploid induction existed corresponding to 55–75% and 100–110% of the first cleavage interval (FCI).
The effects of different pressures and the times of application were analyzed in a study with rainbow trout. Treatments of 4.8 × 10⁴ kPa (7,000 psi) for eight minutes, and 5.5 and 6.2 × 10⁴ kPa (8,000 and 9,000 psi) for four minutes, were applied at 65, 78, and 100% FCI. Tetraploids were found in the 5.5 and 6.2 × 10⁴ kPa group treatments only when applied at 65% FCI.
The results indicate that, while tetraploidy is inducible in salmonids, further experimentation is necessary to discern the appropriate time and treatment for optimal tetraploid induction in each species.     

Swimming performance of wild and F1-hatchery-reared Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts

Abstract –  The swimming performance of wild and hatchery-reared smolts of two salmonid species was investigated. Wild Atlantic salmon smolts (WS) and brown trout smolts (WT) of equal size were caught in fish traps during migration. Hatchery-reared smolts of both species (HS and HT for salmon and trout respectively) were first generation offspring from wild broodstock. The swimming performance of individual smolts from the four groups (WS, HS, WT, HT) was tested three consecutive times using a swimming flume with water flowing at a start rate of 0.16 m·s⁻¹ and a constant acceleration rate of 0.167 cm·s⁻² (10 cm·s⁻¹·min⁻¹). Wild caught smolts of both species performed significantly better than those reared in hatchery conditions. The WS group were observed to maintain an average swimming speed ( U _burst) that was 30% faster than the HS group, whereas the wild trout smolts were superior to HT by approximately 25%. Repeated measures revealed species-specific exhaustion patterns. Brown trout smolts maintained consecutive U _burst indicating significant stamina compared with Atlantic salmon smolts that were found to be exhausted by the initial trial.     

An experimental study of the susceptibility of Atlantic salmon fry, Salmo salar L., to viral haemorrhagic septicaemia

Abstract. Infection trials using two serotypes of VHS viruses (type 1 and 23/75) demonstrated that Atlantic salmon fry were susceptible to the disease when injected intraperitoneally (i.p.) with 10³ pfu of virus/fish but resistant to infection by a bath method when exposed for 3 h in water containing 5 × 10⁴ pfu of virus/ml. In the i.p.-infected fish, mortality reached 78 and 67% within 13 days with VHSV₁ nad 23/75 serotypes, respectively. High virus yields were recovered from infected fish and virus shedding was demonstrated by the onset of VHS in rainbow trout kept in the outflow water from the aquaria containing infected salmon. Neither mortality nor virus shedding occurred in salmon infected by the water route but virus multiplication was demonstrated in 2 of 60 fish with VHSV₁ and 3 of 60 fish with virus 23/75. On day 79 post-infection the sera from surviving salmon of both i.p. and bath infection trials exhibited good neutralizing titres (around 1000) against the homologous viruses.     

Olfactory nerve response of masu salmon (Oncorhynchus masou Brevoort) and rainbow trout (O. mykiss Walbaum) to clove oil and MS-222

Two anaesthetics, clove oil and methane sulphonate (MS-222), were examined for their effects on the olfactory nerve response of masu salmon ( Oncorhynchus masou Brevoort) and rainbow trout ( O. mykiss Walbaum). Exposing both species to clove oil for 3 min at concentrations of 50 and 100 mg L⁻¹, or for 10 min at 50 mg L⁻¹, did not significantly reduce their olfactory response. Directly applying clove oil anaesthesia to the olfactory epithelium significantly reduced olfactory response though after 20 min, olfactory response recovered to 70% and 52% of pre-treatment levels in masu salmon and rainbow trout respectively. Compared with the post-anaesthetic recovery of responses after clove oil (50 mg L⁻¹), buffered MS-222 (100 mg L⁻¹) with NaHCO₃ (100 mg L⁻¹), and unbuffered MS-222 (100 mg L⁻¹) treatment for 3 min, the response after MS-222 treatment declined gradually and significantly, but not after clove oil and MS-222+NaHCO₃ treatments. Clove oil appears to be an effective and relatively safe anaesthetic for salmonids with little long-term impact on their olfactory response, which plays a crucial role in their life history.     

Susceptibility of selected freshwater fish species to a UK Lactococcus garvieae isolate

Gram-positive cocci recovered from diseased rainbow trout from a farm in England were characterized by different methods, including pulsed field gel electrophoresis, as virulent Lactococcus garvieae serogroup 2 (pulsotype A1). Groups of rainbow trout were kept at a range of temperatures and injected intraperitoneally (i.p.) with one of the UK isolates, L. garvieae 00021. The 18 °C and 16 °C groups showed 67% and 28% mortality, respectively, by day 27 post-injection. Fish kept at 14 °C or lower were less susceptible (≤3% mortality). Raising the temperature of all groups to 18 °C at day 27 post-injection did not result in recurrence of the disease, even though viable bacteria were recovered from all groups 42 days later. Grayling were highly susceptible, with 65% mortalities when challenged with 200 colony forming unit fish⁻¹ by i.p. injection and 37% mortalities when exposed to effluent water from tanks containing affected rainbow trout. Other fish species tested, Atlantic salmon, brown trout and seven cyprinid species, were less susceptible. Viable L. garvieae was isolated from the internal organs of all species tested at the end of the trials, suggesting that they may pose a threat as possible carriers to susceptible farmed and wild fish.     

Comparative sensitivity of carp, Cyprinus carpio L. and rainbow trout, Salmo gairdneri Richardson, to Renibacterium salmoninarum

Abstract. The pathogenicity of Renibacterium salmoninarum to carp, Cyprinus carpio L., and rainbow trout, Salmo gairdneri Richardson, was investigated. All carp injected with 4·8 × 10⁸ cells/fish, or 4·8 × 10⁷ cells/fish survived for 38 days. R. salmoninarum was isolated from all moribund fish, but not from the kidney of surviving fish, although R. salmoninarum antigen was detected in several of these fish by the dot blot assay. On the other hand, mortality in rainbow trout was 95% in the fish injected with 4·8 × 10⁸ cells/fish, and 15% in those which received 4·8 × 10⁷ cells/fish. R. salmoninarum antigen was detected by the dot blot assay in all surviving rainbow trout. The number of R. salmoninarum cells was immediately decreased by carp or rainbow trout serum, and the serum bactericidal activity of carp was higher than that of rainbow trout. Carp blood leucocytes had higher phagocytic activity than those of rainbow trout.     

Pancreas disease in farmed Atlantic salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), in Norway

The present paper describes, for the first time, clinical signs and pathological findings of pancreas disease (PD) in farmed Atlantic salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), in sea water in Norway. Similarities and differences with reports of PD from Ireland and Scotland are discussed. Samples of 68 rainbow trout from disease outbreaks on 14 farms and from 155 Atlantic salmon from outbreaks on 20 farms collected from 1996 to 2004 were included in the present study. The histopathological findings of PD in Atlantic salmon and rainbow trout in sea water were similar. Acute PD, characterized by acute necrosis of exocrine pancreatic tissues, was detected in nine Atlantic salmon and three rainbow trout. Salmonid alphavirus (SAV) was identified in acute pancreatic necroses by immunohistochemistry. Most fish showed severe loss of exocrine pancreatic tissue combined with chronic myositis. Myocarditis was often but not consistently found. Kidneys from 40% and 64% of the rainbow trout and Atlantic salmon, respectively, had cells along the sinusoids that were packed with cytoplasmic eosinophilic granules. These cells resembled hypertrophied endothelial cells or elongated mast cell analogues. Histochemical staining properties and electron microscopy of these cells are presented. SAV was identified by RT-PCR and neutralizing antibodies against SAV were detected in blood samples.     

Furunculosis in Atlantic salmon (Salmo salar L.) is not readily controllable by bacteriophage therapy

The potential of bacteriophage therapy to control bacterial disease in farmed fish was tested using, as an example, furunculosis of Atlantic salmon, caused by Aeromonas salmonicida subsp. salmonicida.

In vivo testing with Atlantic salmon and rainbow trout (Oncorhynchus mykiss Walbaum) showed no adverse effects, with bacteriophage generally cleared within 96 h of administration by either intraperitoneal (i.p.) injection or oral in-feed.

Juvenile Atlantic salmon were administered a combination of bacteriophage O, R and B (1.9 × 10⁸ pfu fish^− 1) by i.p. injection, after they had been challenged with A. salmonicida subsp. salmonicida 78027, also by i.p. injection. The fish that were injected with bacteriophage immediately after challenge died at a significantly slower rate then those that were either not treated with bacteriophage, or treated 24 h post-challenge. However, the end result (100% mortality) was not affected.

In further experiments the effects of oral (1.88 × 10⁵ pfu g^− 1 fish^− 1 daily for 30 days), bath (1.04 × 10⁵ ml^− 1 daily for 30 days) and i.p. (6.25 × 10⁷ pfu fish^− 1) phage treatment to control furunculosis in experimentally infected Atlantic salmon were compared with antibiotherapy (treatment with 10 mg kg^− 1 bw^− 1 day^− 1 oxolinic acid for 10 days), using an indirect cohabitation challenge. No protection was offered by any of the bacteriophage treatments, compared to the positive challenge group, although significant protection was offered by the oxolinic acid treatment. Analysis of samples taken from the trials demonstrated that bacteriophage were correctly administered to the fish and, on occasion, were isolated from fish that had succumbed to furunculosis. It was also shown that bacteriophage resistant A. salmonicida subsp. salmonicida isolates could be recovered from mortalities in all the treatment groups.

The results suggest that, although there were no safety problems associated with the approach, furunculosis in Atlantic salmon is not readily controllable by application of bacteriophage.     

Occurrence of viral haemorrhagic septicaemia in rainbow trout Salmo gairdneri Richardson reared in sea-water

Abstract. Viral haemorrhagic septicaemia (VHS) is reported for the first time in sea-water cultured rainbow trout. Heavy mortalities with typical signs and lesions A VHS virus (serotype 1) was isolated from the diseased fish. The mortalities were caused only by the VHS virus and 80 days post transfer of trout to sea-water the mortalities reached 85%, of the initial population.
The disease was experimentally transmitted to rainbow trout, both in sea-water 3·10⁴ pfu/ml of virus or by intramuscular injection of various doses of VHS₁ (7·10¹ 7·10⁴ or 7·10⁴ pfu per fish). Death occurred in all infected groups and started earlier in sea-water. Typical signs of VHS were observed in moribund fish. Viral multiplication was demonstrated to have occurred in fish organs.     

Liver damage in brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), following administration of the cyanobacterial hepatotoxin microcystin-LR via the dorsal aorta

Purified microcystin-LR (MC-LR) was administered via the dorsal aorta to brown trout, Salmo trutta L., or rainbow trout, Oncorhynchus mykiss (Walbaum), and, within 24 h, a dose of 300 μ g MC-LR kg^–1 caused increased activities in the blood by enzymes originating mainly from the liver, i.e. lactate dehydrogenase (LDH) and alanine transaminase (ALT). A dose of 75 μ g MC-LR kg^–1 significantly increased liver enzyme activities in the blood of brown trout at 24 h, but was without effect on rainbow trout, whereas 25 μ g MC-LR kg^–1 had no effect on blood LDH or ALT activities in either species. However, histopathological analysis of liver from both species following administration of the lowest toxin dose showed hepatocyte swelling and necrosis. Liver damage was more severe in brown trout compared to rainbow trout following administration of 300 μ g MC-LR kg^–1, showing disruption of the parenchymal architecture. After 48 h, there was a dose-dependent increase in the hepatosomatic index in both species. It is concluded that brown trout are less tolerant to MC-LR than rainbow trout.     

Branchial lesions associated with intermittent formalin bath treatment of Atlantic salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum)

Two trials were conducted to assess the effects of repeated prophylactic formalin treatments on the gill structure of salmonids. In trial 1, which involved Atlantic salmon, Salmo salar L., approaching smoltification in a commercial facility, fish were treated with either 167 or 250 mg l^-1 formalin for 90 min every 2 weeks for 12 weeks. Formalin-treated salmon had slight, but not significant, increases in the frequency of lamellar fusion, numbers of lamellar mucous cells, and numbers of an endemic gill ciliate, Trichophyra piscium, after 6 and 12 weeks of treatment. In trial 2, which involved juvenile rainbow trout, Oncorhynchus mykiss (Walbaum), fish were treated with 200 mg l^-1 formalin for 60 min twice weekly for 12 weeks. Significant effects were limited to an increase in the numbers of mucous cells present on gill lamellae. In both trials, there was no evidence of lamellar oedema or necrosis of lamellar epithelial cells.     

Differences in performance, flesh composition and water output quality in relation to density and feeding levels in rainbow trout, Oncorhynchus mykiss (Walbaum), farming

Abstract. The aim of this research was lo evaluate the effect of density and feeding level (initially 8 or 16kg/m³ and 1·3% or 2% of present biomass respectively) on productive traits, meat composition and water quality output in farmed rainbow trout, Oncorhynchus mykiss (Walbaum). Individual fish growth shows significant differences in relation to density and feeding level; feed conversion rate is positively Influenced by the lower feeding level and low rearing density. At low density, specific growth rate, protein efficiency ratio and coefficient of variation of individual final weights are more favourable than at high density. Dry matter and lipid composition of meat are positively affected by feeding level; whereas no differences are recorded on the amino acid and fatty acid profiles. Water quality shows differences due to density (NO₃ and NH₄⁺) and feeding level (NO₂, NH₄⁺ and P).     

Total calcium and inorganic phosphate in the blood plasma of farmed rainbow trout, Oncorhynchus mykiss

Reference (physiological) value intervals determined by the lower 2.5% and upper 97.5% quantiles were calculated for total calcium (Ca _t ) and inorganic phosphate (P) in farmed rainbow trout, Oncorhynchus mykiss , at an age of 10–12 months in raceway culture. Total calcium levels were significantly ( P ≤0.0009) greater in males (2.63–4 mmol L⁻¹, n =47, mean weight 359±142 g) than in immature females (2.48–3.97 mmol L⁻¹, n =410, mean weight 400±145 g). The reference range for P in immature females and males was 3.09–9.32 mmol L⁻¹ ( n =467, mean weight 401±144 g). The distribution and density of the quantiles in the tested reference group were made possible by the use of histograms, which showed normal distributions for Ca _t in males but not for Ca _t and P in immature females and males, in which a sinistral asymmetry was found.     

Comparison of NADH-dependent cytochrome b5 reductase activity and in vitro methemoglobin induction by sodium nitrite in Oncorhynchus mykiss, Salmo salar, and Salvelinus fontinalis

Methemoglobin is hemoglobin containing ferric iron. Methemoglobin cannot bind to oxygen and at high concentrations causes tissue hypoxia. Brook trout (Salvelinus fontinalis) develop significantly greater methemoglobinemia than Atlantic salmon (Salmo salar) or rainbow trout (Oncorhynchus mykiss) following general anesthesia with benzocaine or tricaine methanesulfonate. The objective of this study was to compare the activity of the major methemoglobin reducing enzyme, NADH-dependent cytochrome b5 reductase (CB5R), in brook trout erythrocytes to the activity of CB5R in Atlantic salmon and rainbow trout erythrocytes. Methemoglobin levels were compared using co-oximetry following in vitro incubation of erythrocytes with sodium nitrite (NaNO₂). The CB5R activity was measured using a ferricyanide assay. There was significantly greater methemoglobin at time 0 in brook trout erythrocytes than in rainbow trout or Atlantic salmon erythrocytes (2.79 ± 0.29 %, 2.19 ± 0.23 %, 2.08 ± 0.14 %), (P < 0.001). There was significantly greater methemoglobin induction by NaNO₂ in brook trout erythrocytes (33.14 ± 3.32 %) than in rainbow trout or Atlantic salmon erythrocytes (28.73 ± 2.92 % and 24.85 ± 1.40 %, respectively), (P < 0.001). The CB5R activity was significantly less in brook trout erythrocytes (median of 3.05 μmol/min/μl) than in rainbow trout erythrocytes (median of 6.73 μmol/min/μl). The CB5R activity in Atlantic salmon erythrocytes (median 4.09 μmol/min/μl) was not significantly different than in brook or rainbow trout erythrocytes. Total methemoglobin at any one time is a balance between induction by oxidants and reduction by antioxidants. Lower CB5R activity in brook trout erythrocytes may contribute to a species-specific sensitivity to methemoglobin induction; however, there are likely additional factors.     

Alphavirus infections in salmonids--a review

The first alphavirus to be isolated from fish was recorded in 1995 with the isolation of salmon pancreas disease virus from Atlantic salmon, Salmo salar L., in Ireland. Subsequently, the closely related sleeping disease virus was isolated from rainbow trout, Oncorhynchus mykiss (Walbaum), in France. More recently Norwegian salmonid alphavirus (SAV) has been isolated from marine phase production of Atlantic salmon and rainbow trout in Norway. These three viruses are closely related and are now considered to represent three subtypes of SAV, a new member of the genus Alphavirus within the family Togaviridae. SAVs are recognized as serious pathogens of farmed Atlantic salmon and rainbow trout in Europe. This paper aims to draw together both historical and current knowledge of the diseases caused by SAVs, the viruses, their diagnosis and control, and to discuss the differential diagnosis of similar pathologies seen in cardiomyopathy syndrome and heart and skeletal muscle inflammation of Atlantic salmon.     

Behaviour of rainbow trout, Salmo gairdneri Richardson, in marine cages

Abstract. Underwater television was used to observe the distribution and swimming behaviour of groups of farmed rainbow trout, Salmo gairdneri Richardson, in marine net cages. Undisturbed rainbow trout (mean total length 240 mm) aggregated near the surface, remaining motionless in slack water and formed polarized shoals at higher current velocities. Larger fish (mean total length 317 mm) occupied deeper parts of the cage and were more active during periods of slack water. The results are compared with previous work on farmed Atlantic salmon, Salmo salar L., and discussed in relation to the biology of wild rainbow trout and recent work on exercise, growth and condition in salmonids.