Annual report on health monitoring of wild anadromous salmonids in Norway 2020 - Screening of wild Atlantic salmon (Salmo salar) postsmolts for viral infections

Abstract

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 2019 in three fjord systems located in three aquaculture production areas (PO2-4). These areas have had a sporadic case of ISA and a stable high incidence rate of PD, as well as a very high aquaculture intensity. The fish were collected as part of the national salmon lice monitoring program in the outer parts of the Bokna (N=132), Hardanger (N=110) and Sogne (N=110) fjords by trawling during the period May-June. ISAV was detected in two postsmolts from Hardanger fjord and SAV in one smolt from Sogne fjord. The Ct-value of SAV-positive fish was very high (38.9) which may suggest a false positive result. 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.