Infectious Salmon Anemia (ISA) is one of the most notorious and economically devastating diseases in the salmon aquaculture industry. Despite control measures implemented for decades, Norway experienced a concerning resurgence of cases in 2020 and 2021, underscoring the urgent need to better understand its propagation pathways. While waterborne transmission between nearby farms is a known factor, the role of human activities, especially the incessant traffic of vessels, has been more challenging to quantify.
A recent study published in Preventive Veterinary Medicine by researchers from the Université de Lyon, Université Clermont Auvergne, and the Royal Veterinary College has shed light on this issue. Using a network analysis approach, they mapped the maritime “highways” connecting Norwegian salmon farms. The research assessed whether vessel movements and corporate connections could explain the spatiotemporal distribution of outbreaks of the pathogenic variant of the virus (ISAV-HPRA). The findings confirm that these operational networks are not only relevant but could be a key factor in the disease’s dispersal, even between very distant regions.
Key findings
- Salmon farms in Norway are highly connected through a dense network of vessel movements, which facilitates the potential spread of pathogens.
- Although movements are more frequent within production zones, vessels connect distant regions, creating plausible pathways for ISA virus transmission across hundreds of kilometers.
- The study found a statistically significant association between the distribution of ISA outbreaks and the patterns of vessel movement and corporate affiliations.
- Both vessel movements and the exchange of equipment or personnel between farms under the same company are potential viral transmission pathways that require greater scrutiny and reinforced biosecurity measures.
Tracing the virus’s routes: A complex contact network
To understand the transmission dynamics, researchers used public data from Norway’s Barents Watch system, which records vessel movements between salmon farms using the Automatic Identification System (AIS). They analyzed data from 2021 to 2023, constructing a network where each farm is a node and a “link” is created when a vessel visits one farm and then another within a specific time frame (Delta of 1, 8, and 15 days).
In addition to vessel movement, the study built a second network based on corporate affiliation, as farms belonging to the same company may share personnel and equipment, creating an indirect pathway for virus transmission.
Salmon farms: Closer than they appear
The analysis revealed a dense and highly connected naval contact network. The most significant results were:
- High connectivity: The network demonstrated “small-world” properties, where most farms (over 72%) were interconnected in one large component. This means a pathogen could theoretically travel from one farm to nearly any other via direct or indirect routes.
- Structure by production zones: Vessel movements are strongly influenced by Norway’s 13 salmon production zones. As expected, the density of visits was significantly higher within the same zone than between different zones, suggesting that these zones act as communities that promote internal transmission.
- The role of companies: Affiliation with the same company is a powerful predictor of movement. Sixty-one percent of vessel movements occurred between farms affiliated with the same company, a proportion far greater than what would be expected by chance.
Connecting the outbreaks: Evidence of vessel-mediated transmission
The study’s crucial point was comparing this static network with the temporal data of 58 actual ISAV-HPRA outbreaks that occurred during the study period. By reconstructing the infectious periods and virus introduction windows for each case, the researchers looked for vessel movements that could have acted as plausible transmission events.
The results showed a statistically significant association: the number of cases that could be linked to a “potential infector” via a vessel was much higher than expected by chance. Of the 58 cases, 38 (65%) could be connected to at least one possible infector through the vessel network. The corporate affiliation network also showed a significant association, identifying 27 cases with a potential infector through this pathway.
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One of the most concerning findings is the potential for long-distance dispersal. Although 83% of potential transmission events occurred within the same production zone, 60% of the infector-infected farm pairs were in different zones.
This demonstrates that while local movements are more frequent, longer journeys are highly relevant from an epidemiological standpoint. The study identified one potential transmission event that spanned a maritime distance of nearly 1,700 km. This fact highlights a critical risk: poor biosecurity on a vessel, such as inadequate disinfection, could allow the virus to survive long enough to be transported across the entire country.
Implications for the salmon industry
This study does not claim that the identified vessels caused the outbreaks, but it presents strong evidence that vessel movement and corporate networks are plausible and significant transmission pathways for ISAV-HPRA. The results have direct implications for biosecurity management in salmon aquaculture.
Disease control cannot focus solely on geographical proximity; it is essential to consider the operational networks connecting the farms. The authors emphasize the need to further investigate the role of vessels and fomites (contaminated equipment) in the spread of pathogens.
The next step, they suggest, would be to combine this type of network analysis with genetic data from the viruses. This would allow confirmation of whether the virus from one farm is genetically related to that of its “potential infector,” thereby transforming a plausible connection into a proven transmission route.
In conclusion, this work provides a solid foundation for understanding how routine human activities in aquaculture facilitate the dispersal of diseases. For the salmon industry, it is a crucial reminder that in an interconnected world, no farm is an island.
Contact
Hélène Duault
Université de Lyon, INRAE, VetAgro Sup, UMR EPIA
69280 Marcy l’Etoile, France.
Email: duault.hel@gmail.com
Reference (open access)
Duault, H., Bailly, M., Zhao, M., Hill, S. C., & Fournié, G. (2025). “Network of ship movements and its potential in spreading Pathogenic Infectious Salmon Anemia Virus among Norwegian fish farms”. Preventive Veterinary Medicine, 106667. https://doi.org/10.1016/j.prevetmed.2025.106667
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
