Temporal dynamics of Tenacibaculum maritimum and Tenacibaculum dicentrarchi in a land-based Atlantic salmon (Salmo salar) facility in Norway

The salmonid aquaculture industry faces increasing challenges associated with the expansion of intensive production systems, particularly infectious diseases. To mitigate these issues, the Norwegian industry is progressively shifting towards land-based production systems, which reduce the grow-out phase at sea and associated pathogen exposure. However, such infrastructures provide extensive surfaces that may facilitate biofouling and bacterial persistence. In this study, we investigated the occurrence and diversity of Tenacibaculum spp. in a Norwegian land-based Atlantic salmon ( Salmo salar ) facility from September to December 2024, by screening both fish and environment samples. Bacterial isolation and multilocus sequence analysis (MLSA) revealed a temporal shift in Tenacibaculum species composition with opposite dynamics of T. maritimum and T. dicentrarchi . T. maritimum was predominantly detected in September and was associated with mortality during an ulcerative disease outbreak, whereas T. dicentrarchi was mainly isolated in December from fish ulcers and structural elements of the facility. The repeated isolation of Tenacibaculum strains from both fish and infrastructure suggests that land-based systems may enable persistence and reinfection by biofilm-forming bacteria, highlighting the need for systematic surveillance. Furthermore, recurring summer heat waves in Norway in association with global warming may help the expansion of T. maritimum . This study provides the first documented evidence of T. maritimum infecting Atlantic salmon in Norway. • Seasonal shift in Tenacibaculum species in a Norwegian land-based salmon facility • Opposite seasonal dynamics of T. maritimum and T. dicentrarchi • Land-based systems can facilitate biofilm-forming bacteria persistence • First report of T. maritimum infecting Atlantic salmon in Norway