Clinical Presentation and Pathological Effects of a Hydrozoan Bloom on Farmed Atlantic Salmon.

Largely unpredictable, gelatinous plankton blooms are a growing concern for the ever-expanding mariculture industry. Recently, a bloom of a colonial hydrozoan known as Apolemia sp. was detected via a citizen-science initiative, Nye.dugnadforhavet.no, in mid-Norway in July 2023. By November 2023, the bloom spanned the entire 2500 km Norwegian coast, adversely impacting the aquaculture industry nationwide. This report describes the clinical presentation and pathology on farmed Atlantic salmon (Salmo salar) caused by an unprecedented bloom of Apolemia sp. at the Austevoll station of the Institute of Marine Research in Norway. Shortly after the Apolemia sp. arrived, many fish showed impaired swimming capacity and erratic behaviour. Over the following days, several fish displayed wounds on their eyes and skin, while daily mortality in some cages exceeded 17%. Histopathological analyses on moribund fish revealed acute gill damage and secondary complications, including necrosis and bacterial infections, in addition to ongoing amoebic gill disease (AGD). Blood biomarker profiles indicated severe physiological stress and organ dysfunction. Ultimately, all affected fish on site were euthanized due to welfare concerns, as was also the case for several other Norwegian salmon farms. This underscores the necessity for enhanced monitoring and mitigation strategies to protect farmed fish from such harmful events. The events of 2023 in Norway highlight the vulnerability of aquaculture to gelatinous plankton blooms and the necessity for research and innovation to develop effective monitoring and management practices.