Bent out of shape: The impact of deviating heart morphology on cardiac function in Atlantic salmon (Salmo salar L.)

Farmed Atlantic salmon are subject to high mortality, particularly during handling and delousing, and cardiac morphological deviations are frequently observed among affected individuals. Yet, the direct functional consequences of these deviations remain poorly understood. In the present study, we sought to link heart morphology and function in farmed Atlantic salmon ( Salmo salar L.) by combining in vivo echocardiography and intraventricular pressure measurements with ex vivo qualitative morphological assessment. Hearts with two or more morphological deviations (DT2) displayed clear functional alterations at rest, including increased stroke volume, cardiac output, myocardial strain and accelerated late diastolic inflow, along with focal thickening of the compact myocardium. Functional outcomes depended strongly on deviation type: fish with strictly ventricular deviations (DT2V) exhibited enhanced volumetric filling and output, characterised by larger atria, greater bulbo-ventricular valve diameter and lower end-diastolic pressure, whereas fish with combined ventricular and bulbar deviations (DT2VB) showed increased myocardial strain without corresponding increases in stroke volume or cardiac output. Notably, these group differences were most evident in small and medium-sized fish and diminished in the largest individuals, suggesting size-dependent limits to compensatory capacity. Overall, our findings indicate that cardiac morphological deviations elicit compensatory hyperfunction at rest, but that this compensation may be unsustainable as fish approach harvest size. This provides a potential mechanistic link between heart morphology and the elevated mortality observed during stressful operational events in aquaculture. • Fish with morphologically deviating hearts exhibit augmented cardiac function. • Most functional augmentation occurs in fish with ≥2 deviating cardiac traits. • Functional changes include increased stroke volume, contractility and diastolic flow. • Type and combination of morphology trait have differing effects on cardiac function.