Prevalence and severity of cardiac abnormalities and arteriosclerosis in farmed rainbow trout (Oncorhynchus mykiss)

Cardiovascular disease may pose a major threat to the health and welfare of farmed fish. By investigating a range of established cardiovascular disease indicators, we aimed to determine the prevalence, severity and consequences of this affliction in farmed rainbow trout (Oncorhynchus mykiss) from an open cage farm in the Baltic Sea, an open cage farm in a freshwater lake, and a land-based recirculating aquaculture system. We also aimed to identify environmental, anthropogenic and physiological factors contributing towards the development of the disease. The majority of trout possessed enlarged hearts with rounded ventricles (mean height:width ratios of 1.0–1.1 c.f. ~1.3 in wild fish) and a high degree of vessel misalignment (mean angles between the longitudinal ventricular axis and the axis of the bulbus arteriosus of 28–31 °c.f. ~23° in wild fish). The prevalence and severity of coronary arteriosclerosis was also high, as 92–100% of fish from the different aquaculture facilities exhibited coronary lesions. Mean lesion incidence and severity indices were 67–95% and 3.1–3.9, respectively, which resulted in mean coronary arterial blockages of 19–32%. To evaluate the functional significance of these findings, we modelled the effects of arterial blockages on coronary blood flow and experimentally tested the effects of coronary occlusion in a sub-sample of fish. The observed coronary blockages were estimated to reduce coronary blood flow by 34–54% while experimental coronary occlusion adversely affected the electrocardiogram of trout. Across a range of environmental (water current, predation), anthropogenic (boat traffic intensity, hatchery of origin, brand of feed pellets) and physiological factors (condition factor, haematological and plasma indices), the hatchery of origin was the main factor contributing towards the observed variation in the development of cardiovascular disease. Therefore, further research on the effects of selective breeding programs and rearing strategies on the development of cardiovascular disease is needed to improve the welfare and health of farmed fish. •Cardiovascular disease is highly prevalent and severe in farmed rainbow trout.•High growth rate is linked to the development of cardiovascular disease.•Observed levels of arteriosclerosis may reduce coronary blood flow by up to 34–54%.•Coronary occlusion results in symptoms of myocardial infarction.•Cardiovascular disease may impair the ability of rainbow trout to cope with stress.