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Annual report on health monitoring of wild anadromous salmonids in Norway 2021 Screening of Atlantic salmon (Salmo salar) postsmolts for viral infections

The Institute of Marine Research has investigated the prevalence of infectious salmon anaemia virus (ISAV) and salmonid alphavirus (SAV, PD-virus) infections in migrating wild Atlantic salmon postsmolts captured in 2020 in three fjord systems located in three aquaculture production areas (PO3, 5, 12). The fish were collected as part of the national salmon lice monitoring program in the outer parts of Hardangerfjord (N = 100), Romsdalsfjord (N = 100) and Altafjord (N = 100) by trawling during the period May-June. SAV was detected in one postsmolts from Hardangerfjord and ISAV in one smolt from Altafjord. The Ct-value of SAV-positive fish was however very high (37.6), which may suggest a false positive result. ISAV detected in the positive fish is likely to be avirulent HPR0 variant. The findings from the current report indicate a very low prevalence of these viruses in wild migrating postsmolts. These findings complement and corroborate our previously reported data and may suggest that prevalence of ISAV or SAV infections in wild salmon postsmolts are not significantly influenced by the occurrence of these infections in fish farming.