Ventricular hypoplasia in farmed Atlantic salmon Salmo salar

Atlantic salmon Salmo salar L. parr and pre-smolts from 2 Norwegian hatcheries 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 normal in affected fish. Histology showed poor development or absence of the outer, compact myocardium, large numbers of fat cells and melanomacrophages in the epicardium, fibrosis, and inflammation of the compactum/spongiosum interphase. Nuclei of the inner spongious myocardium showed signs of compensatory hypertrophy. The cause(s) of this malformation is(are) unknown, but a high prevalence of other malformations in fish from the same population indicates high temperature during incubation of the eggs as a possible aetiology.