Mortality and skin pathology of farmed Atlantic salmon (Salmo salar) caused by exposure to the jellyfish Physalia physalis in Ireland.

In October 2019, during Tropical Storm Lorenzo, an unusual bloom of Physalia physalis (commonly known as the Portuguese man o' war) appeared on the west coast of Ireland. The jellyfish were observed aggregating in high numbers surrounding salmon pens on two Atlantic salmon farms with tentacles extending underwater through the nets into the pens. Acute skin lesions and mortality were reported as a consequence of exposure to P. physalis on both sites. The impact of noxious zooplankton species such as P. physalis may pose a significant and more frequent threat in future if increased adverse weather events continue to transport these and other jellyfish outside their normal range. This is the first published record of Physalia physalis causing mortality in cultured fish.