Annual report on health monitoring of wild anadromous salmonids in Norway 2025

Infectious diseases in fish farming in Norway is a problem which often leads to serious economic losses, reduced fish welfare and increased mortality (Table 1) [1]. The most frequently reported viral diseases in salmon farming are heart and skeletal muscle inflammation (HSMI), caused by piscine orthoreovirus 1 (PRV1) and cardiomyopathy syndrome (CMS), caused by piscine myocarditis virus (PMCV).
Heart and skeletal muscle inflammation (HSMI) is the most abundant viral disease in all salmon farming production areas (PO). The disease is an increasing problem in fish farming in Norway with 115–188 annual registered cases in recent years (Table 1) [1,2]. High PRV1 viral loads are found in fish developing HSMI but may also occur in healthy fish.
Cardiomyopathy syndrome (CMS) is a growing problem in Norwegian salmon farming with 72 to 155 annual outbreaks in the last few years [1,3]. The disease has a significant economic impact, as it typically affects large and harvest-ready fish.
Bacterial kidney disease (BKD) is a serious chronic systemic infection in salmonids that is caused by Renibacterium salmoninarum (R. salmoninarum). It is a notifiable disease (category F) and has been sporadically reported in farmed and wild salmonids in Norway in the last 20 years. However, 12, 8 and 2 BKD-outbreaks were reported in fish farms in 2023, 2024 and 2025 respectively (Table 1) [1]. The sudden increase in outbreaks is alarming and needs further attention from the fish farming industry as well as the management authorities. Establishing the baseline prevalence of R. salmoninarum in wild salmon populations is therefore needed to assess the potential pathogen exchange between farmed and wild fish [4,5].

Table 1: The number of registered disease outbreaks in fish farming in the years 2021-2025 [1]

The effect of fish farming on the infection status of wild salmon stocks can be evaluated by comparing pathogen prevalence in wild fish populations originating from areas with different fish farming intensities and disease outbreak profiles.
Wild salmon can be infected by pathogens prevalent in salmon farming; in rivers as fry or parr by pathogen-infected farmed escapees or to less extent by spawning wild salmon, or from salmon farms in the fjord when migrating as post-smolts or returning as adults. Therefore, infection status in migrating post-smolts may represent a direct indicator of infection pressure from salmon farming during their migration routes. However, the study of pathogen infection during all the life stages of salmon is necessary to assess the overall impact of diseases in fish farming on the wild salmon populations.
Since 2012, the Institute of Marine Research (IMR) has been commissioned by the Norwegian Food Safety Authority (NFSA) to carry out an annual health monitoring of wild anadromous salmonids in Norway. The current monitoring activities are financed by both NFSA and the Norwegian Ministry of Trade, Industry and Fisheries (NFD). The activities lie within a prioritized research area at IMR which addresses the environmental impact of disease transmission from Norwegian fish farming to wild fish. The surveillance activities aim to evaluate the pathogen transmission from farmed fish to wild salmonids by monitoring and identifying changes in the prevalence of selected pathogens in wild salmonids as a result of fish farming activities. In addition, the surveillance aims to increase the knowledgebase about pathogens in wild salmonids in general, as well as establish a biobank that can be used when new disease challenges arise. Furthermore, the surveillance consolidates with the other activities in the larger strategic research effort on diseases and disease transmission in wild fish.
The current monitoring program aims to investigate the occurrence of pathogen infections in wild salmonids captured from different Norwegian coastal areas with different farming intensities and disease outbreak frequencies. Each year selected sets of fish are analysed in order to complement or complete our data and time series. Part of the results from pathogen screening is used in an annual health monitoring of wild anadromous salmonids in Norway commissioned by NFSA. The generated knowledge from the program contributes to the institute's main goal/strategy in providing advice and further development of sustainable management of aquaculture and is utilized in the IMR's annual risk assessment of Norwegian fish farming [6,7].

The aim of the current study was to investigate the occurrence of PRV1, PMCV and R. salmoninarum infections in migrating wild Atlantic salmon post-smolt.

To provide data about the prevalence of different pathogens in different geographical regions, we tested migrating post-smolts (N=200) captured in the outer parts of the Boknafjorden (PO2), Hardangerfjorden (PO3) and Sognefjorden (PO4) and Altafjorden (PO12) by trawling in May-July 2025 (Fig. 1), as part of the national salmon lice monitoring program [8]. Weight and length of all fish were recorded and the post-smolts were then frozen (-20 ^oC) as soon as possible. At autopsy, tissues from the gills, head kidney and heart were taken from the fish while still frozen and stored at -80 ^oC until tested. Samples for analysis were sent on dry ice to an accredited commercial laboratory (Pharmaq Analytic AS) for RNA extraction and pathogen testing using real-time RT-PCR assays. The migrating post-smolts were tested for the PRV1, PMCV and R. salmoninarum (Table 2). Samples with Ct (cycle threshold) value below 37.0 were considered positive. A total of 500 analyses were performed on samples from 200 fish and were used in the current report.

Fig. 1: Map showing post-smolt collection fjords and fish farms (yellow circle) in these fjords. PO refers to production area.

PRV1 was not detected in any of the tested fish (Table 2). Low concentrations of PMCV (Ct-values: 35, 35) were detected in two post-smolts from Sognefjorden. None of the tested fish from Sognefjorden or Altafjorden were positive for R. salmoninarum.

Table 2: The numbers (N) and the collection sites of tested fish and the numbers (percentage) of pathogen-positive post-smolt.

* R. salmoninarum. NT: Not tested

PRV1 and PMCV infections are abundant in fish farming in all production areas PO2-13. It is likely that migrating post-smolts were exposed to pathogens released from the fish farms before (subclinical infections) and during disease outbreaks. The absence of PRV1 and the low prevalence of PMCV infection in the tested migrating post-smolts are consistent with previous findings in wild salmonids [4, 5, 9-11]. Our earlier reports have shown low prevalence of PRV1 and PMCV in sea trout, returning adult salmon, post-smolt and juveniles from Norwegian rivers [9-14]. Previous reports have also shown that there was no regional pattern in PRV1 genotypes isolated from wild and farmed salmon, suggesting prolonged and extensive spread due to aquaculture activities (fish transport) and frequent exchange of the virus types between farmed and wild fish [13,15]. However, little is known about the mechanism of transmission of the virus.
There was one confirmed BKD-outbreak in PO5 and one unconfirmed outbreak in PO6 during 2025. There were no available wild fish from either PO5 or PO6 in 2025. However, R. salmoninarum was not detected in any of the tested fish collected from PO4 and PO12. Our previous reports and unpublished results did not reveal any R. salmoninarum infection in wild salmonids collected from areas with or without BKD-outbreaks [4, 5]. Previous findings suggest that despite the increased number of BKD-outbreaks in PO6 in 2023 and 2024, R. salmoninarum was not detected in wild salmonids collected from that area. The results from the current and previous reports suggest that the occurrence of R. salmoninarum infection in wild salmon is rare and therefore it is unlikely that sporadic outbreaks in fish farms may occur due to introduction of the infection from wild salmonids [4, 5]. 
The farmed fish production in 2025 in PO2, PO3, PO4 and PO12 were approximately 108 402, 189 749, 178 869 and 124 537 tonnes respectively [7]. Hardangerfjorden is located in PO3 which is one of the areas with the highest fish farming intensity in Norway. Our results indicate that the prevalence of pathogen infections in post-smolt collected from these production areas was not associated with high fish farming intensity. The current findings are in line with our previous reports that showed no apparent relationship between the prevalence of pathogen infection in wild salmon and the fish farming intensity or the frequency of disease outbreaks in collection areas [4, 5, 9-15, 16, 17]. These observations may indicate that wild salmon are exposed to a low infection pressure from fish farming. However, the possibility that infection may lead to rapid disappearance or altered behaviour of the infected fish, and therefore potentially affect the results, cannot be ruled out. Other explanations for the low prevalence of pathogens in post-smolts is the time needed after pathogen exposure (incubation time) before the pathogen can be detected in tissues of fish.
These findings complement and corroborate our previously reported data and suggest that it is unlikely that wild salmonids are the main reservoir for pathogens that are common in fish farming and therefore an unlikely source of spill over for these pathogens to fish farming. There are still gaps in our knowledge about diseases in wild fish and the interaction between farmed and wild fish. Time series of samples of all life stages of wild salmonids from areas with different salmon farming intensity are necessary to better evaluate and understand the long-term effect of infection pressure from aquaculture on the pathogen prevalence in wild salmon populations. Such series will also enable us to assess changes in the prevalence due to increased fish farming activities, increased pathogen virulence, the emergence of new diseases and importantly climate change.

1.    Moldal, T., et al.  (2026). Norwegian Fish Health Report 2025, Norwegian Veterinary Institute report 5a/2026, Norwegian Veterinary Institute.
2.    Løvoll, M., et al. (2012). Quantification of piscine reovirus (PRV) at different stages of Atlantic salmon Salmo salar production. Diseases of Aquatic Organisms, 99: 7-U5.
3.    Jensen, B. B., et al. (2013). Risk factors for cardiomyopathy syndrome (CMS) in Norwegian salmon farming. Diseases of Aquatic Organisms, 107: 141-150.
4.    Madhun, A. S., et al. (2024). “Annual report on health monitoring of wild anadromous salmonids in Norway 2023; Rapport fra Havforskningen 2024-13: Institute of Marine Research, Bergen. 15 pp. Available at https://www.hi.no/hi/nettrapporter/rapport-fra-havforskningen-en-2025-26
5.    Madhun, A. S., et al. (2025). “Annual report on health monitoring of wild anadromous salmonids in Norway 2024; Rapport fra Havforskningen 2025-26: Institute of Marine Research, Bergen. 13 pp. Available at https://www.hi.no/hi/nettrapporter/rapport-fra-havforskningen-en-2025-26
6.    Taranger, G. L., et al. (2015). “Risk assessment of the environmental impact of Norwegian Atlantic salmon farming”. ICES Journal of Marine Science, 72: 997-1021.
7.    Grefsrud, E. S., et al. (2026). “Risk assessment of Norwegian fish farming” (In Norwegian). Rapport fra havforskningen 2026-10: Institute of Marine Research, Bergen. Available at https://www.hi.no/hi/nettrapporter/rapport-fra-havforskningen-2026-10
8.    Myksvoll MS, et al. (2018) Evaluation of a national operational salmon lice monitoring system-From physics to fish. Plos One 13. e0201338
9.    Madhun, A. S., et al. (2018). “Annual report on health monitoring of wild anadromous salmonids in Norway 2017; Health monitoring of returning adult salmon from river Etne, western Norway ”. Rapport fra Havforskningen 26-2018: Institute of Marine Research, Bergen. 13 pp. Available at https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2020-16
10.    Madhun, A. S., et al. (2019). Annual report on health monitoring of wild anadromous salmonids in Norway 2018; Screening of migrating Atlantic salmon (Salmo salar) postsmolts from the Trondheim fjord for viral infections. Rapport fra Havforskningen 2019-28: Institute of Marine Research, Bergen. 9 pp. Available at https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2019-28
11.    Madhun, A. S., et al. (2023). Annual report on health monitoring of wild anadromous salmonids in Norway 2022. Screening of Atlantic salmon (Salmo salar) postsmolts from Boknafjorden, Hardangerfjorden and Romsdalsfjorden for viral infections. Rapport fra Havforskningen 2023-41. Institute of Marine Research, Bergen. 10 pp. Available at https://www.hi.no/hi/nettrapporter/rapport-fra-havforskningen-en-2023-41
12.    Madhun, A. S., et al. (2016). Occurrence of salmonid alphavirus (SAV) and piscine orthoreovirus (PRV) infections in wild sea trout Salmo trutta in Norway. Diseases of Aquatic Organisms, 120:109-113.
13.    Madhun, A. S., et al. (2018). “Prevalence of piscine orthoreovirus and salmonid alphavirus in sea-caught returning adult Atlantic salmon (Salmo salar L.) in northern Norway”. Journal of Fish Diseases, 41: 797-803.
14.    Madhun, A. S., et al. (2024). "Occurrence of salmonid alphavirus and piscine orthoreovirus-1 infections in migrating salmon (Salmo salar L.) post-smolt in western Norway." Journal of Fish Diseases 47: e13874.
15.    Garseth, Å. H., et al. (2013). “Phylogenetic evidence of long distance dispersal and transmission of piscine reovirus (PRV) between farmed and wild Atlantic salmon”. PloS One, 8: e82202.
16.    Garseth, Å. H., et al. (2013). “Piscine reovirus (PRV) in wild Atlantic salmon, Salmo salar L., and sea-trout, Salmo trutta L., in Norway”. Journal of Fish Diseases, 36: 483-493.
17.    Garseth, Å. H., et al. (2012). “Piscine myocarditis virus (PMCV) in wild Atlantic salmon Salmo salar”. Diseases of Aquatic Organisms, 102: 157-161.