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.     

Vaccine development for winter ulcer disease, Vibrio viscosus, in Atlantic salmon, Salmo salar L.

Coldwater Vibrio species isolated from Atlantic salmon, Salmo salar L., during winter ulcer disease outbreaks at saltwater sites in Norway and Iceland were characterized phenotypically, tested for virulence, and used to evaluate the efficacy of multivalent, oil-adjuvanted vaccines. The intraperitoneal (i.p.) injection of rainbow trout, Oncorhynchus mykiss (Walbaum), in fresh water with one bacteria species isolated during winter ulcer outbreaks, V. ‘viscosus’, produced rapid mortality and disease signs which resembled those observed during natural outbreaks [10⁵ colony-forming units (cfu) fish^??1]. Another species, V. ‘wodanis’, was not virulent to rainbow trout (10³–10⁶ cfu fish^??1). Although vaccination of rainbow trout with a mineral-oil-adjuvanted, injectable vaccine containing V. anguillarum (serotypes 01 and 02), V. salmonicida and Aeromonas salmonicida did not provide protection against injection challenge with V. viscosus, vaccines which included V. viscosus produced significant protection in Atlantic salmon and rainbow trout. Atlantic salmon vaccinated with an oil-adjuvanted vaccine containing V. viscosus, V. wodanis and atypical A. salmonicida produced a relative percentage survival (RPS) of 97% when challenged i.p. with V. viscosus, demonstrating cross-protection between strains from Iceland and Norway. Short-term efficacy was demonstrated in rainbow trout by injection challenge at 21 and 43 days post-vaccination with an oil-adjuvanted vaccine containing V. viscosus, V. anguillarum (01/02), V. salmonicida and A. salmonicida, which produced an RPS of 96–99%. Rainbow trout challenged with V. viscosus at 52 and 362 days post-vaccination produced an RPS of 93% and 79%, indicating that vaccination provided long-term protection. In a similar manner, rainbow trout injected i.p. with 0.2 mL of a vaccine containing the five bacteria species and infectious pancreatic necrosis virus produced a 90% RPS when challenged with V. viscosus 66 days later. The high RPS under a severe challenge burden, along with disease signs in experimental freshwater challenges which resembled the saltwater disease condition, indicated that V. viscosus is a contributing factor to winter ulcer and that vaccination will protect against the disease.     

Reduction of brown trout, Salmo trutta L., salmon, S. salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), smolt bycatches in eel pound nets

Two types of bycatch reduction devices (BRDs) were tested in the Randers Fjord, Denmark, aimed at deflecting the surface-oriented smolts of brown trout, Salmo trutta L., salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), before they became trapped, while retaining the catches of eel, Anguilla anguilla (L.). The methods tested were: a) placing a floating guard net between the wings; and b) submerging the pot net (i.e. last enclosure before fyke net) 55–100 cm below mean sea level. After each haul the pound net-setups were changed from the control to one of the types of BRD, and vice versa. Both methods significantly reduced the bycatch of brown trout smolts, while catches of legal-sized eels were not affected. Submerging the pot net reduced smolt catches of all species; mean reductions were 91.1% for brown trout, 74.5% for rainbow trout, and 86.1% for salmon.     

Piscine reovirus (PRV) in wild Atlantic salmon,Salmo salar L., and sea‐trout,Salmo trutta L., in Norway

This is the first comprehensive study on the occurrence and distribution of piscine reovirus (PRV) in Atlantic salmon, Salmo salar L., caught in Norwegian rivers. PRV is a newly discovered reovirus associated with heart and skeletal muscle inflammation (HSMI), a serious and commercially important disease affecting farmed Atlantic salmon in Norway. A cross‐sectional survey based on real‐time RT‐PCR screening of head kidney samples from wild, cultivated and escaped farmed Atlantic salmon caught from 2007 to 2009 in Norwegian rivers has been conducted. In addition, anadromous trout (sea‐trout), Salmo trutta L., caught from 2007 to 2010, and anadromous Arctic char, Salvelinus alpinus (L.), caught from 2007 to 2009, were tested. PRV was detected in Atlantic salmon from all counties included in the study and in 31 of 36 examined rivers. PRV was also detected in sea‐trout but not in anadromous Arctic char. In this study, the mean proportion of PRV positives was 13.4% in wild Atlantic salmon, 24.0% in salmon released for stock enhancement purposes and 55.2% in escaped farmed salmon. Histopathological examination of hearts from 21 PRV‐positive wild and one cultivated salmon (Ct values ranging from 17.0 to 39.8) revealed no HSMI‐related lesions. Thus, it seems that PRV is widespread in Atlantic salmon returning to Norwegian rivers, and that the virus can be present in high titres without causing lesions traditionally associated with HSMI.     

The occurrence of proliferative kidney disease (PKD) in cultured and wild fish: further investigations

Abstract. In an investigation of the occurrence of proliferative kidney disease (PKD) in freshwater fish other than rainbow trout, 18 species of wild fish and seven species of fish raised in cultivation wore sampled from waters where the disease occurred annually in rainbow trout, Oncorhynchus mykiss (Richardson). Results revealed that certain wild stocks of brown trout. Salmo trutta L., grayling, Thymallus thymallus L., and pike, Esox lucius L., were infected with PKD, as were cultivated Atlantic salmon, Salmo salar L., parr, brown trout and char, Salvelinus alpinus (L.). Microscopical examination revealed the presence of the PKX cell in these species and also intraluminal protozoa possibly related to the PKX cell, which were not found in the rainbow trout. Other species of freshwater fish had myxosporidan infections but, unlike PKD infection, there was little host/parasite tissue response. The PKX cell as a myxosporidan stage is discussed and the presence of the disease in wild fish is reported.     

Incidence of hybridization between Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in Ireland

The substantial growth of the farmed salmon industry in Europe since the 1970s has highlighted concerns regarding the genetic impact of escaped farmed salmon on wild salmonid stocks. High incidences of salmon × trout hybrids have been recorded in rivers situated near intensive salmon farming in Norway and Scotland, which may be indicative of a breakdown in reproductive isolation between salmon, Salmo salar L., and brown trout, Salmo trutta L. In the present study, salmonid fry and 0+ parr were collected from rivers in western Ireland. Allozyme and minisatellite DNA analysis were carried out on fry to determine the frequency of F1 hybrids from 10 rivers located within 38 km of salmon farms and three rivers at least 80 km from salmon farms. A total of 49 hybrids were recorded from 4135 salmonid fry (frequency = 1.2%). Mitochondrial DNA analysis showed that all hybrids arose from Atlantic salmon female × brown trout male crosses. Hybrid parr were recorded from one of the low-risk rivers (1.0%), but were present in seven out of the 10 catchments located within 38 km of salmon farms, with frequencies ranging from 0.7% to 3.1%. The results of the present survey, which represents the first extensive record of the levels of salmon-trout hybridization in Ireland, are discussed in relation to the continued growth of salmon farming in this country.     

Infectious pancreatic necrosis in Atlantic salmon, Salmo salar L

A clinical and histopathological review was carried out of 21 outbreaks of acute infectious pancreatic necrosis (IPN) in Scottish Atlantic salmon, Salmo salar L., farms (13 marine and eight fresh water) during 1991-2004. A distinctive syndrome was evident in both post-smolts in sea water and fry in fresh water, where liver lesions, which had not previously been associated with IPN, became a consistent finding in addition to the more typical pancreatic and intestinal changes. Initial cases were described in post-smolts in Shetland, but by the end of the period of investigation this type of pathology had extended down the West coast of Scotland and into Ireland. Limited viral strain analysis suggested that similar strains were involved in both fresh water and sea water and that these differed from earlier isolates from rainbow trout, Oncorhynchus mykiss (Walbaum). In fresh water, recovered fish frequently developed a greatly distended intestine associated with accumulation of undigested food. In sea water, after the initial, often significant (50% or more), losses, there were many fish which failed to grow and became chronically emaciated and prone to sea louse infection. Although use of transfer diets containing immune enhancers and the selection of IPN resistant broodstock has reduced losses the disease remains a serious cause of economic loss.     

Morphometric and meristic characters in salmon, Salmo salar L., trout, Salmo trutta L., and their hybrids

Abstract. Atlantic salmon, Salmo salar L., brown trout, S. trutta L. and their hybrids are normally identified in the field by empirical differences in maxilla length, thickness of the caudal peduncle, degree of forking of the tail and overall body conformation. This study quantifies these characters and analyses their variations in electrophoretically identified hatchery-reared individuals. Means and variances of the various measures are presented for salmon, trout and hybrids separately. The morphometric characters do not satisfactorily distinguish hybrids from the pure species, and even within samples of the pure species, some individuals will be misclassified as hybrids, or as members of the opposite species, if single characters are used on their own. Hybrids often resemble one or other parent species in one or more characters and are less often intermediate in phenotype. Triploidized hybrids are more like salmon than diploid hybrids are, and triploidized salmon are not different from diploid salmon. The results confirm that frequencies of hybrids of these species cannot be reliably assessed by morphological characters alone, and even for individuals of the pure parental species, independent confirmation of species status is advisable. Early reports of hybrid frequencies in wild stocks should be treated with caution, and apparently higher levels of hybridization in more recent studies compared with older, traditional surveys may simply reflect the greater precision of electrophoretic identification.     

Vibriosis in rainbow trout, Salmo gairdneri Richardson, in fresh water in north-eastern Italy

Abstract. Vibriosis is a significant problem in rainbow trout, Salmo gairdneri Richardson, culture in fresh water in north-eastern Italy. Furunculosis is the main cause of disease in brown trout, Salmo trutta L., and brook trout, Salvelinus fontinalis (Mitchill). Spontaneous outbreaks over the period 1975–1978 are described together with relevant husbandry and environmental parameters associated with them. Biological, chemical and physiological characteristics of 40 bacterial strains isolated from outbreaks are described.     

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.     

Influence of water quality on the outbreak of proliferative kidney disease – field studies and exposure experiments

This paper presents data from field studies and exposure experiments and the possible association of limno- and physicochemical parameters with outbreaks of proliferative kidney disease (PKD) in rainbow and brown trout. The investigations were carried out at Singold Brook in southern Germany. Exposure experiments and sampling of wild fish were performed in Singold at least twice a year in 1992, 1994, 1996 and 2000. Both wild fish collected from the specific sites and experimentally exposed rainbow trout were investigated histologically for the occurrence of PKD. At the time of sampling, various water parameters at the respective river sites were measured. The results indicate strongly the existence of a correlation between organic pollution of water, the presence of Bryozoa and the outbreak of PKD.     

Assessment of a semi-quantitative electric fishing sampling technique for juvenile Atlantic salmon, Salmo salar L., and trout, Salmo trutta L., in small streams

Abstract. A semi-quantitative technique of assessing trout, Salmo trutta L., and juvenile Atlantic salmon, Salmo salar L., stocks in small rivers is described and evaluated against a commonly used quantitative method. The results indicate that for 0+ and >0+ juvenile salmon and trout a log₁₀ n+1 transformed linear regression accounted for between 68.6% and 90.2% of the total variance in the relationship between the semi-quantitative and the quantitative result. The use of a categorization system is recommended and the applicability of the technique to an extensive juvenile salmonid monitoring programme is described.     

Triacylglycerol-, wax ester- and sterol ester-hydrolases in midgut of Atlantic salmon (Salmo salar)

Bile salt‐dependent lipase (BSDL) is assumed to be the predominant lipid hydrolase in fish digestive tracts where it hydrolyses dietary triacylglycerols (TAG), sterol esters (SE) and wax esters (WE). BSDL is known to hydrolyse TAG at much faster rates than SE and WE in both fish and mammals. An assay for BSDL has previously been developed for rainbow trout (Oncorhynchus mykiss). However, this setup may not be valid in other fish species. Accordingly, the present study aimed at optimizing previous assays in rainbow trout for use on intestinal luminal contents of Atlantic salmon (Salmo salar L.). Crude intestinal extracts from midgut were desalted before the assay and concentrated bile salts supplemented. In general, the rank order for the degree of hydrolysis in Atlantic salmon was TAG > WE > SE. The optimal assay conditions were determined as being 100 μg protein, 125 μm lipid substrate and 20 mM bile salt (taurocholate) during the 4 h of incubation. Atlantic salmon and rainbow trout of 1500 g showed similar lipolytic activity, while salmon smolts of 300 g showed a significantly lower activity. Furthermore, the inhibition of intestinal lipase activities, especially triacylglycerol hydrolase and sterol ester hydrolase, observed in trout intestinal extracts at bile salt concentrations around 10 mm , was not observed in salmon. This could indicate that the activities in these two salmonids may display different enzyme biochemistry.     

Dispersion of stocked brown trout, Salmo trutta L., and landlocked salmon, Salmo salar L., from stocking sites in Lake Oulujärvi, Finland

Movement and recaptures of two hatchery-reared brown trout, Salmo trutta L., stocks and landlocked salmon, Salmo salar L., released at different sites in regulated Lake Oulujärvi, were studied in relation to release site. Five groups of fish from each stock were released in approximately equal numbers. Most of the fish released in June and July were recaptured within 3 months, whereas the majority of the fish released in early winter (October and November) were caught the following spring, about 7–9 months after stocking. The release site had a significant effect on recapture rate. The results showed that fishing restrictions targeted mainly at gill net fishing are needed to preserve the stocked fish from overfishing. Significantly fewer recaptures were observed from the landlocked salmon stocking compared with brown trout. The recaptures from the landlocked salmon stocking indicated more active movement and less clumping compared with the two brown trout stocks.     

Haematological assessment of rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L., infected with Renibacterium salmoninarum

Abstract. Groups of rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, S. salar L., were infected with Renibacterium salmoninarum , injected with killed bacteria or starved. Selected red and white cell parameters were measured over a 35–day period. As a result of successful infection a typical granulomatous disease developed during which haematocrit, red cell count, haemoglobin, red cell diameter and the ratio of mature to immature erythrocytes all decreased. Red cell depletion was shown to be caused by their retention in the spleen and to be the cause of the observed splenomegaly. Increases in monocytes, thrombocytes and neutrophils were recorded in both infected groups.     

Changes in serum parameters of rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L., infected with Renibacterium salmoninarum

Abstract. Bacterial kidney disease was induced experimentally in rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, S. salar L., and selected serum parameters measured over a 35-day period. In those fish which became infected a significant increase in the erythrocyte sedimentation rate and in the serum levels of bilirubin, blood urea nitrogen and potassium were recorded. A significant decrease in total serum protein, cholesterol and sodium also occurred in these fish. Electrophore-tic analysis of serum proteins showed the fast migrating fraction was reduced before the slower migrating material in infected fish.     

River bed construction: impact and habitat restoration for juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

The River Soya, Norway, was canalized for agricultural purposes. In order to compensate for damage to the Atlantic salmon, salmo salar L. and brown trout, Salmo trutta L. populations, different weirs were built. The aims of this study were firstly to analyse the effects of weirs covering the river bank and entire river bottom with blasted stones and secondly, to analyse the effects of sediments transported by freshets on the downstream salmon and trout populations after canalization. Restoration of the river bottom with blasted stones provided salmon with more substrate spaces. Densities of trout increased after the river bank was covered with stones. Sediments transported downstream from the canalized river stretch decreased the densities of juvenile salmon and trout.     

Proliferative kidney disease in rainbow trout Salmo gairdneri Richardson

Abstract. Two entirely separate outbreaks of PKD are described in an attempt to properly define the condition and enable it to be differentiated from other diseases of a similar nature. Losses of up to 75% of fingerling rainbow trout were recorded in one outbreak, illustrating the potentially serious nature of the disease. Although the kidneys showed the most obvious pathological changes, other organs were also affected. The host response was granulomatous in type, the foci of each cellular reaction being 5-20 μm eosinophilic cells considered to be parasitic. The cytoplasm of these cells contained numerous electron dense bodies some with a tail, but each with an electron lucent bar always orientated at right angles to the plasmalemma whenever they were near it. The most obvious cytoplasmic inclusion of these 'parasites' were other cells numbering up to five or six, each with its own nucleus and nucleolus, but devoid of any of the small electron dense bodies. The significance of either of these inclusions is unknown.
A limited differential diagnosis is given, and the argument presented that the condition PKD is the result of infection by a parasite, possibly an amoeba, in combination with poor quality water.     

Geographical distribution of salmonid alphavirus subtypes in marine farmed Atlantic salmon, Salmo salar L., in Scotland and Ireland

Sequence data from salmonid alphavirus (SAV) strains obtained from farmed marine Atlantic salmon, Salmo salar L. , over a 20-year period between 1991 and 2011 was reviewed to examine the geographical distribution of the genetically defined SAV subtypes in twelve regions across Ireland and Scotland. Of 160 different Atlantic salmon SAV strains examined, 62 belonged to subtype 1, 28 to subtype 2, 34 to subtype 4, 35 to subtype 5 and 1 to subtype 6. SAV subtypes 1, 4 and 6 were found in Ireland, while subtypes 1, 2, 4 and 5 were found in Scotland. In the majority of regions, there was a clear clustering of subtypes, with SAV subtype 1 being the dominant subtype in Ireland overall, as well as in Argyll and Bute in Scotland. SAV subtype 2 predominated in the Shetland and Orkney Islands. The emergence in Atlantic salmon of subtype 2 strains typically associated with sleeping disease in rainbow trout in Argyll and Bute, strongly suggesting transmission of infection between these species, was noted for the first time. SAV subtype 4 was the most common subtype found in the southern Western Isles, while SAV subtype 5 predominated in the northern Western Isles and north-west mainland Scotland. No single strain was dominant on sites in the western Highlands, with a number of sites in this region in particular having more than one subtype detected in different submissions. The significance of these results in relation to aspects of the epidemiology of infection, including transmission, biosecurity and wildlife reservoirs are discussed and knowledge gaps identified.