Heritability of mortality in response to a natural pancreas disease (SPDV) challenge in Atlantic salmon,Salmo salar L., post‐smolts on a West of Ireland sea site

Pancreas disease (PD) is an economically important disease of European farmed Atlantic salmon. It can cause significant losses because of morbidity, mortality and reduced production. The disease is caused by an alphavirus, known as salmon PD virus (SPDV) or salmonid alphavirus subtype 1 in Ireland. To examine whether it is possible to improve the natural resistance of Atlantic salmon to SPDV by selective breeding, 6000 genotyped, tagged, pedigreed fish from 150 full‐sib families were exposed to a natural challenge during 2005 in a sea cage on a commercial salmon farm in the West of Ireland. Histopathological and serological examination was performed weekly on a proportion of all moribund fish to determine the onset of the infection and the likely cause of death. Heritabilities and genetic correlations are presented for resistance to a natural PD challenge and smolt input weight. The results indicate that the susceptibility of salmon to SPDV could be reduced by selective breeding based on the survival in a natural challenge to the virus.     

Seasonal increase in sea temperature triggers pancreas disease outbreaks in Norwegian salmon farms

Pancreas disease (PD) is a viral disease causing negative impacts on economy of salmon farms and fish welfare. Its transmission route is horizontal, and water transport by ocean currents is an important factor for transmission. In this study, the effect of temperature changes on PD dynamics in the field has been analysed for the first time. To identify the potential time of exposure to the virus causing PD, a hydrodynamic current model was used. A cohort of salmon was assumed to be infected the month it was exposed to virus from other infective cohorts by estimated water contact. The number of months from exposure to outbreak defined the incubation period, which was used in this investigation to explore the relationship between temperature changes and PD dynamics. The time of outbreak was identified by peak in mortality based on monthly records from active sites. Survival analysis demonstrated that cohorts exposed to virus at decreasing sea temperature had a significantly longer incubation period than cohorts infected when the sea temperature was increasing. Hydrodynamic models can provide information on the risk of being exposed to pathogens from neighbouring farms. With the knowledge of temperature‐dependent outbreak probability, the farmers can emphasize prophylactic management, avoid stressful operations until the sea temperature is decreasing and consider removal of cohorts at risk, if possible.     

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.     

Evaluation of sea lice abundance levels on farmed Atlantic salmon (Salmo salar L.) located in the Broughton Archipelago of British Columbia from 2003 to 2005

Salmon farming began in British Columbia (BC) in the 1970s and in 2006, aquaculture represented BC's largest agricultural export. Along with this growth in production has been a growth in controversy, including the concern that sea lice originating from Atlantic salmon farms negatively impact wild juvenile pink salmon in the Broughton Archipelago. To understand the dynamic interaction between wild and farmed fish, data for on‐farm abundance of sea lice are required. In this study, 33 000 Atlantic salmon from 20 active farms were examined over 3 years. Two species of lice were found: Lepeophtheirus salmonis and Caligus clemensi. Inter‐annual and seasonal variations in abundance levels occurred with lower levels of L. salmonis in 2003 compared with 2004 and 2005, while C. clemensi levels were highest in 2003. The abundance of L. salmonis was greater on older farmed fish. The findings are compared with European and eastern Canadian sea lice reports, and possible sources of sea lice on farmed salmon are discussed.     

Mortality and weight loss of Atlantic salmon,Salmon salar L., experimentally infected with salmonid alphavirus subtype 2 and subtype 3 isolates from Norway

Pancreas disease (PD) caused by salmonid alphavirus (SAV) has a significant negative economic impact in the salmonid fish farming industry in northern Europe. Until recently, only SAV subtype 3 was present in Norwegian fish farms. However, in 2011, a marine SAV 2 subtype was detected in a fish farm outside the PD‐endemic zone. This subtype has spread rapidly among fish farms in mid‐Norway. The PD mortality in several farms has been lower than expected, although high mortality has also been reported. In this situation, the industry and the authorities needed scientific‐based information about the virulence of the marine SAV 2 strain in Norway to decide how to handle this new situation. Atlantic salmon post‐smolts were experimentally infected with SAV 2 and SAV 3 strains from six different PD cases in Norway. SAV 3‐infected fish showed higher mortality than SAV 2‐infected fish. Among the SAV 3 isolates, two isolates gave higher mortality than the third one. At the end of the experiment, fish in all SAV‐infected groups had significantly lower weight than the uninfected control fish. This is the first published paper on PD to document that waterborne infection produced significantly higher mortality than intraperitoneal injection.     

Observations on open-water densities of sea lice larvae in Loch Shieldaig, Western Scotland

Sea trout (Salmo trutta L.) stocks on the West Coast of Scotland and Ireland have decreased due to reduced survival during the marine phase of their life cycle. Lice‐infested sea trout returning to rivers could indicate that parasite burdens are contributing to the decline. Salmon farms represent a potential source of substantial quantities of sea lice, leading to a conjectured link between parasites on salmon fish farms and infestations on wild sea trout. To investigate the potential infective pressure on sea trout in Loch Shieldaig, offshore and sub‐littoral plankton samples were collected and analysed for sea lice nauplius and copepodid stages. During the plankton survey, numbers of sea lice on a fish farm in the loch reached a maximum in November 2001. Soon after, numbers of sea lice larvae peaked in open‐water samples and then in sub‐littoral samples. Nauplii were found adjacent to the farm and occurred less elsewhere. This study reports a concentration of sea lice larvae at the head of Loch Shieldaig and indicates a possible relationship between sea lice numbers on the fish farm and lice larvae densities in the open‐water of the loch and in the sub‐littoral zone.     

Longitudinal serological surveys of Atlantic salmon, Salmo salar L., using a rapid immunoperoxidase-based neutralization assay for salmonid alphavirus

Longitudinal serological surveys for salmon pancreas disease virus (SPDV), the causal agent of pancreas disease (PD), were conducted on multiple caged populations of Atlantic salmon, Salmo salar L., on two farms over a 77-week period (farm 1, freshwater and marine stages) and a 36-week period (farm 2, marine stage only), using a microtitre-based virus neutralization (VN) assay. Collected sera were also screened for viraemia with SPDV, and pancreas, heart and muscle tissues were examined for lesions consistent with PD. Outbreaks of PD occurred during the marine phase on both farms, as demonstrated by seroconversion, the isolation of virus and progressive histopathological changes consistent with a PD outbreak. All populations monitored showed a progressive increase in seroprevalence of 90-100%, typically accompanied by rises in geometric mean antibody titres. With the exception of one caged population, which showed a marked biphasic seroprevalence pattern, the seroprevalence figures in the remaining four monitored populations remained high (> or =70%) until the end of the study period. Peak VN titres of > or =1/1280 were detected on both farms. The results provide essential baseline information for the interpretation of SPDV VN serology results, and indicate that this methodology is suited to both the diagnosis and seroepidemiology of SPDV infections.     

Assessment of infectious salmon anaemia virus prevalence for different groups of farmed Atlantic salmon, Salmo salar L., in New Brunswick

Abstract Infectious salmon anaemia (ISA) virus (ISAV) has been causing disease in New Brunswick since 1996. As a control measure, all fish in an outbreak cage are killed. The objective of this study was to compare ISAV prevalence in cages experiencing an outbreak with healthy cages from the same farm, neighbouring farms and distant farms. Atlantic salmon from five different groups were tested using an RT-PCR test. Groups included moribund fish from a cage experiencing an outbreak (A), healthy fish from an outbreak cage (B), healthy fish from a negative cage from a farm experiencing an outbreak in a different cage (C), healthy fish from a negative farm near an outbreak farm (D), and healthy fish sampled at a negative farm located in an area with only negative farms (E). Apparent prevalences (standard error) for the different groups (A-E) were 0.94 (+/-0.026), 0.41 (+/-0.062), 0.29 (+/-0.040), 0.08 (+/-0.037) and 0.08 (+/-0.037), respectively. All groups were significantly different (P < 0.002) from each other except for groups B and C and groups D and E. Because the prevalence of the virus was significantly higher in the outbreak cage (B) compared with other sites, early harvest of outbreak cages will remove one source of virus. However, ISA negative cages (C) that remain on the positive farm may potentially act as a viral reservoir.     

Differences in Lepeophtheirus salmonis abundance levels on Atlantic salmon farms in the Broughton Archipelago, British Columbia, Canada

Sea lice data collected from Atlantic salmon farms in the Broughton Archipelago between 2003 and 2005 were examined for inter-regional differences in mobile Lepeophtheirus salmonis (Krøyer) abundance using the generalized linear model procedure. Factors such as age of the salmon populations, location of farms and time of year had a significant effect on the abundance of the mobile stages of L. salmonis whereas water temperature and salinity did not. Separate evaluation of SLICE® treatment data found no significant difference in treatment frequency among the areas but did show that there were significantly lower numbers of farm treatments during the summer months when compared with other seasons. The role of environment and wild fish in influencing sea lice abundance on the farmed salmon is discussed. The findings suggest that effective management programmes for sea lice should not only be based on geographical location but should take into account other factors which could influence lice abundance levels.     

Aeromonas salmonicida in Finland: pathological problems associated with atypical and typical strains

Abstract. Aeromonas salmonicida was studied at fish farms producing salmonid smolts in northern and central Finland from 1982 onwards. Aeromonas salmonicida subsp. salmonicida has been causing epizootics in salmon, Salmo salar L., and trout, S. trutta L., at two coastal farms in northern Finland sinee 1986, involving 1–29% mortality in the fish-rearing units affceted. The disease causes more serious losses of sea trout yearlings and brood fish than of salmon. The achromogenic atypical A. salmonicida proved to be the most common bacterial disease in brown and sea trout at one farm in northern Finland throughout the period, causing constant heavy losses, mainly of fingerlings, especially in 1982–1986. It was found only occasionally in central Finland. Pigment-producing atypical A_- salmonicida caused mortality among brown trout at one farm in northern Finland and one in central Finland.     

Risk factors for furunculosis, infectious pancreatic necrosis and mortality in post-smolt of Atlantic salmon, Salmo solar L.

Abstract An epidemiological longitudinal study comprised information obtained from 124 randomly selected Norwegian sea-sites housing post-molts of Atlantic salmon, Salmo salar L. Information on crude cumulative mortality, fish characteristics and farm factors was gathered by the veterinary practitioners servicing the farms. The observation period lasted from the time of sea transfer of the smolts in the spring until 1 October 1991. On average, 116480 smolts were transferred to each sea-site. The mean crude cumulative mortality of post-smolts during the period concerned was 16.8%. The cumulative farm-level incidence was 54.0% for furunculosis, 39.5% for infectious pancreatic necrosis (IPN) and 10.5% for vibriosis. Occurrence of any of these specific infectious diseases in the post-smolt was significantly associated with mortality. The risk of furunculosis was significantly related to the location of the sea site. Mixing of smolt from many freshwater hatcheries in the sea farm increased the risk of IPN significantly. In addition, the risk of IPN was related to age of site and geographic location. The mean cumulative mortality in 1-year-old smolt groups was 17.5%, significantly greater than in two-year-old fish (7.9%). In addition, the method used to transport fish from the freshwater hatchery to the sea site was significantly associated with mortality. Neither the weight of smolts at sea transfer nor the date of transfer were significantly related to mortality.     

Development of the use of wrasse to control sea lice on salmon farms in Ireland

Abstract. Developments in the use of wrasse as cleaner-fish in the control of sea lice ( Lepeophtheirus salmonis, Caligus elongatus ) in Ireland are reviewed. Corkwing, Symphodus melops , and goldsinny, Ctenolabrus rupestris , have controlled L. salmonis (<1 louse per salmon) on commercial farms. C. rupestris and rockcook, Centrolabrus exoletus , removed both adult and chalimus C. elongatus in aquarium trials. No egg-bearing lice were ever found on salmon in cages with wrasse, and numbers of mobile lice were maintained at <1 per salmon smolt.
Wrasse are fished using shrimp pots, optimally set for about 2 h. The fishery is seasonal and limited to >10cm wrasse occurring close to the farm, and distant from other farms. Localized reductions in the body length and numbers of wrasse have been detected in the second year of fishing in two areas respectively. Due to the fishery limitations and high fecundity of wrasse, impacts on wrasse populations should remain localized. All wrasse populations fished are screened for microbial diseases, and protozoan and metazoan parasites. Results are typical for wild marine fish but metazoan skin parasites are rare. No pathogens, including tests with an Aeromonas sp. found in wrasse, have been found to be transferable to salmon.
Wrasse are the only effective and continuous method of sea lice control legally available. Research efforts should be directed towards improving the availability, pathogen-free status, and husbandry of wrasse, to enable cleaner-fish technology to be more widely and efficiently utilized.     

Distributions of planktonic sea lice larvae Lepeophtheirus salmonis in the inter-tidal zone in Loch Torridon, Western Scotland in relation to salmon farm production cycles

This study examines the relative density of sea lice, Lepeophtheirus salmonis (Krøyer), larvae in the inter‐tidal areas of Loch Torridon, a fjordic sea loch in Western Scotland and the site of several commercial salmon farms. Samples of planktonic sea lice were collected in each spring from 1999 to 2003 near the mouth of the River Shieldaig, where sea trout have been shown to congregate in the first few days after going to sea, and from 2001 to 2003 near the mouth of the River Balgy. No larval sea lice were found near the mouth of the River Shieldaig in spring 2000 or 2002, when gravid female sea lice were absent on local fish farms. The same finding was observed in 2002 at the mouth of the River Balgy. In springs of 1999, 2001 and 2003, peak densities of larval sea lice of 33, 94 and 143 m^?3, respectively, were observed at the mouth of the river Shieldaig. At the mouth of the River Balgy, peak densities of 74 and 78 lice m^?3 were observed in 2001 and 2003 respectively. At the time these samples were collected, the two local salmon farms were in the second year of their production cycle and gravid female sea lice were present. Samples of larval sea lice were also obtained year round, at approximately weekly intervals, from a 50‐m transect at the mouth of the River Shieldaig, from March 2001 to June 2003, and compared with frequencies of gravid female sea lice on the two local salmon farms. High levels were found in the winter. No planktonic sea lice were found in this transect when gravid females were not present on the local fish farms.     

Prevalence of piscine orthoreovirus and salmonid alphavirus in sea‐caught returning adult Atlantic salmon (Salmo salar L.) in northern Norway

Heart and skeletal muscle inflammation (HSMI) caused by piscine orthoreovirus (PRV) and pancreas disease (PD) caused by salmonid alphavirus (SAV) are among the most prevalent viral diseases of Atlantic salmon farmed in Norway. There are limited data about the impact of disease in farmed salmon on wild salmon populations. Therefore, the prevalence of PRV and SAV in returning salmon caught in six sea sites was determined using real‐time RT‐PCR analyses. Of 419 salmon tested, 15.8% tested positive for PRV, while none were positive for SAV. However, scale reading revealed that 10% of the salmon had escaped from farms. The prevalence of PRV in wild salmon (8%) was significantly lower than in farm escapees (86%), and increased with fish length (proxy for age). Sequencing of the S1 gene of PRV from 39 infected fish revealed a mix of genotypes. The observed increase in PRV prevalence with fish age and the lack of phylogeographic structure of the virus could be explained by virus transmission in the feeding areas. Our results highlight the need for studies about the prevalence of PRV and other pathogens in Atlantic salmon in its oceanic phase.     

Relationship between sea lice levels on sea trout and fish farm activity in western Scotland

The relationship between aquaculture and infestations of sea lice on sea trout, Salmo trutta L., is controversial. Here, the association between sea lice infestations on wild sea trout and characteristics of local Atlantic salmon, Salmo salar L., farms were investigated using data collected on the Scottish west coast. The proportion of sea trout with louse burdens above a critical level was positively related to the fork length of the sea trout and the mean weight of salmon on the nearest fish farm, and negatively related to the distance to that farm. The distance to the nearest fish farm did not influence the probability of infestations above the critical level beyond 31 km although there was considerable uncertainty around this cut‐off distance (95% limits: 13–149 km). The results support a link between Atlantic salmon farms and sea lice burdens on sea trout in the west of Scotland and provide the type of information required for marine spatial planning.     

Longitudinal study of a natural outbreak of heart and skeletal muscle inflammation in Atlantic salmon, Salmo salar L

Heart and skeletal muscle inflammation (HSMI) is a transmissible disease of farmed Atlantic salmon, Salmo salar L. It is characterized by significant epi-, endo- and myocarditis, as well as myositis, particularly involving red skeletal muscle. The aetiology of HSMI is currently unresolved, though a viral cause is suspected. Since its discovery in 1999, HSMI has become an increasing problem for the Norwegian farming industry, with some farms experiencing yearly outbreaks and subsequent economic losses. In the present study an Atlantic salmon farm was studied from December 2003 to April 2005. Samples from apparently healthy as well as clinically diseased fish were collected monthly and examined histopathologically. The first fish to be diagnosed with HSMI was sampled in May, 8 months after transfer to sea. A clinical outbreak of HSMI followed in June, when all fish in the sample had lesions consistent with HSMI. Subsequent samples revealed that cardiac lesions decreased in severity 2 months after the start of the outbreak, but that multiple foci of cellular infiltration and necrosis persisted throughout the year. There appeared to be a shift in lesion location from being most severe in the compact myocardium in early stages of disease to a greater involvement of the atrium and spongy layer of the ventricle in later samples. Late samples also showed increased fibrosis of cardiac tissue. In conclusion, HSMI appears to be a severe disease with elevated mortality, morbidity close to 100% and prolonged duration.     

Quantitative estimation of degree of silvering displayed by Atlantic salmon (Salmo salar L.) juveniles originating from natural populations and from fish-rearing farms

An investigation was made of the degree of silvering of Atlantic salmon juveniles reared in fish-rearing farms and released into rivers and that of juveniles from natural populations at the time of migration to the sea. The silvering intensity was determined by reflection of light from the body surface of the fish. The data indicated a correlation between the rate of silvering and fish body weight and illumination in tensity during the rearing period. Further detailed investigation is needed of the intensity and dynamics of silvering taken as factors characterizing the degree of adaptability of catadromous salmon juveniles to the pelagic way of life.     

An assessment of salmon farms and wild salmonids as sources of Lepeophtheirus salmonis (Krøyer) copepodids in the water column in Loch Torridon, Scotland

Wild salmonids and farmed salmon can both be sources of Lepeophtheirus salmonis (Krøyer, 1838) larvae . Farmed salmon smolts free of L. salmonis infections are stocked in sea cages and may subsequently contract L. salmonis infections, probably from wild fish. The contribution of gravid L. salmonis at Atlantic salmon, Salmo salar L., farms to populations of L. salmonis larvae in the water column has in the past been based on estimated parameters, such as louse fecundity. This present study augments these calculations by combining empirical data on densities of infective L. salmonis copepodids in the field with estimates of the number of gravid L. salmonis on farmed and wild salmonids in Loch Torridon. Data collected between 2002 and 2007 show a significant correlation between mean densities of L. salmonis copepodids recovered in the water column and the numbers of gravid L. salmonis at the local salmon farms. Generally, the farms with greatest numbers of salmon were observed to have stronger correlations with densities of copepodids in the water than the farms with fewer fish. The study suggests that louse management approaches, e.g. treatment trigger levels, need to take account of individual farm biomass, or numbers of fish. This study highlights the importance of control of L. salmonis on salmon farms for the co-existence of both wild salmonid populations and the aquaculture industry.     

Differences in sea migration between wild and reared Atlantic salmon (Salmo salar L.) in the Baltic Sea

The effect of origin, smolt size and year of release on the sea migration pattern of Atlantic salmon (Salmo salar L.) in the Baltic Sea was examined by tagging experiments conducted in 1991–1993 on wild and reared smolts of the Simojoki river salmon stock. The tag recovery data analysed by log-linear models revealed significant differences in both spatial and temporal sea migrations between the wild and reared salmon; the variation was attributed to the year of release and to the origin of the fish. Grilse accounted for the majority of reared returners (76%) but for a smaller proportion (46%) of the wild fish. The effect of smolt size could be studied only in the smolt groups tagged in 1991. Wild fish were more frequently (71%) caught in the Baltic Main Basin than were reared fish (51%) during their second sea year, and the size variation between wild and reared smolts did not explain the recovery site. No such differences in spatial distribution were found during the third sea year. The tagging place (hatchery/trap) of the reared fish did not affect their later sea migration. The differences in sea migration patterns suggest that the wild salmon are more vulnerable to the intensive salmon fishery in the Baltic Main Basin than are reared fish.