Cranial nodules in farmed Atlantic salmon, Salmo salar L, fry

Cranial nodules are described from Atlantic salmon, Salmo salar fry hatched from certain egg batches. Conspicuous, smooth, mostly unilateral nodules were recorded on the cranium. Histologically, the cerebellum appeared normal, but was displaced dorsally. Karyorhectic malpighian cells were absent from the epidermis and the meninges appeared normal. There was no apparent contact of the cerebellum with the water, or evidence of fungal, bacterial or parasitic infection. Mortality of up to 15% as recorded. The aetiology of this condition is currently unknown.