Intensive smolt production is associated with deviating cardiac morphology in atlantic salmon

High pre-slaughter mortality rates of Atlantic salmon (Salmo salar L.) are a reoccurring welfare issue and economic burden in aquaculture. Sudden death immediately prior to slaughter is particularly problematic given the considerable resources invested to reach this stage. Although the underlying causes of such mortality are largely unknown, cardiac deformities and diseases have become increasingly prevalent observations in deceased fish. The factors leading to this pathology remains to be revealed. Thus, we presently examined if intensive smolt production and concordant fast growth rates in young fish is associated with altered cardiac morphology at later production stages in Atlantic salmon. The observed alterations were subsequently related to mortality risk following de-lousing in a facility with a cardiomyopathy syndrome (CMS) outbreak. We observed that intensive smolt production is indeed associated with slower growth rates at sea, distinct pathological cardiac morphological alterations, and expression of cardiac pathology markers. Moreover, the observed cardiac alterations co-occurred with CMS-related cardiac rupture at a different production facility. The present study demonstrates a clear link between pace of growth at early rearing stages and cardiac deformities later in life. Furthermore, these cardiac deformities are associated with cardiac rupture and mortality in individuals with CMS during delousing. We therefore believe that a slower pace of smolt production improves cardiac health and reduces the risk of mortality during CMS outbreaks.