Functional and structural cardiac remodelling in farmed Atlantic salmon (Salmo salar)

Cardiac performance is key for survival and fitness, particularly during energetically demanding and stressful events such as migration in the wild and stressful handling and crowding in aquaculture. Heart morphology is well-known to be altered in farmed salmon, yet the functional consequences remain unknown. Cardiac morphology, structure and function were compared between seawater-ready wild and farmed Atlantic salmon ( Salmo salar ) smolts by morphological and histological assessment, and echocardiography. In line with previous research, farmed hearts had rounder and less symmetrical ventricles with enlarged bulbi. Morphological deviations were present in a staggering 76 % of farmed smolts but were completely absent in the wild population. Echocardiography revealed reduced atrial area and E/A ratio in farmed individuals, indicating larger reliance on atrial contraction under rest. Histological analyses showed a relatively thicker compact myocardium, due to cardiomyocyte widening in farmed fish — features consistent with hypertrophic remodelling. Collectively, these findings confirm consistent structural remodelling in farmed salmon hearts and reveal functional adaptations to altered cardiac morphology. Most likely, these physiological alterations are shaped by selective breeding and rearing conditions and may influence cardiac performance under routine aquaculture conditions. • Wild and farmed smolts differ in heart morphology, structure, and function. • Cardiac deviations are common in farmed smolts (76 %) and absent in wild (0 %). • Echo shows smaller atrial area and lower E/A ratio, implying more atrial reliance. • Farmed smolts have thicker compact myocardium and larger cardiomyocytes.