Five years of data show winter ulcer remains persistent threat to Norwegian salmon farms.
New analysis from aquaculture data firm Manolin indicates that winter ulcer, caused by Moritella viscosa, continues to be one of the most consistent and damaging seasonal challenges in Norwegian salmon farming.
Based on five years of mortality data, Manolin found that disease-related losses follow a clear annual pattern: mortality begins to rise in January, peaks sharply through February and March, and declines by early summer. The trend has remained stable in timing but has intensified in severity.
In the 2020–2021 production years, mortality linked to M. viscosa accounted for less than 12 percent of total losses at its seasonal peak. By contrast, the 2023–2025 winters have seen sustained peaks of 20–30 percent, with the highest levels recorded in February 2023. The 2025 season has followed a similar curve, with an earlier decline heading into spring.
Manolin said the data suggest that while the industry has improved in tracking and anticipating outbreaks, the increasing intensity of winter ulcer events shows that predictability has not yet led to effective control.
Winter ulcer, caused by M. viscosa, typically occurs in cold seawater conditions and remains a significant cause of mortality and downgraded fish quality in Norwegian aquaculture.
