Ventricular hypoplasia in farmed Atlantic salmon Salmo salar

Atlantic salmon Salmo salar L. parr and pre-smelts from 2 Norwegian hatcher ies showed reduced weight gain, abnormal behaviour and signs of circulatory disturbances. Necropsy revealed conspicuous fat deposits around the heart to be the most consistent finding. Furthermore, the ventricle/atrium ratio was altered, with the size of the ventricle significantly smaller than norm al in affected fish. Histology showed poor development or absence of the ou ter, compact myocardium, large numbers of fat cells and melanomacorophages in the epicardium, fibrosis, and inflammation of the compactum/spongiosum i nterphase. Nuclei of the inner spongious myocardium showed signs of compens atory hypertrophy. The cause(s) of this malformation is(are) unknown, but a high prevalence of other malformations in fish from the same population in dicates high temperature during incubation of the eggs as a possible aetiol ogy.