Impact of temperature and dietary replacement of fishmeal on cardiovascular remodelling and growth performance of adult Atlantic salmon (Salmo salar L.)

The development of pathophysiological cardiac alterations in farmed salmonids is a recurrent and widespread welfare issue. Although the aetiology of such cardiac alterations is still unclear, several factors, including rearing temperature, selective breeding programs, and feeding strategies, could induce a morpho-physiological remodelling of cardiac structure. This maladaptive cardiac remodelling could impair cardiac function and health of farmed salmonids leading to premature mortality and production losses. In this study, we aimed to better understand the impact of suboptimal elevated temperature, resembling a typical summer temperature in Tasmania, and different feed formulations with low inclusion of fishmeal on cardiovascular remodelling and growth performance of adult Tasmanian Atlantic salmon. Fish were reared in a seawater recirculating aquaculture system and fed one of three diets with different fishmeal contents (6, 12, and 18%), while exposed to a temperature profile characterised by an incremental increase (1 °C per day) from 15 to 19.5 °C, 84 days at 19.5 °C (elevated temperature), and finally 34 days at 15 °C (optimal temperature). In terms of cardiac morphometric parameters, ventricular roundness (measured as height:width ratio) was not affected by temperature or dietary treatment. Conversely, the angle between the bulbus arteriosus and the ventricle significantly increased after the elevated temperature phase, along with relative ventricular mass, showing signs of maladaptive remodelling compared to initial conditions. However, these cardiac parameters were not different between the elevated and optimal temperature phase, which was associated with no changes in transcript abundance of molecular markers of pathological hypertrophy and remodelling, such as, atrial natriuretic peptide (anp) and b-type natriuretic peptide (bnp). Collectively, standard growth rate (SGR) and temperature growth coefficient (TGC), along with feed conversion ratio (FCR), were significantly elevated following exposure to suboptimum elevated temperature relative to optimal temperature. However, no differences in growth rate were observed in fish fed distinct dietary treatments at both elevated and optimal temperatures. These results imply that feeds with low levels of fishmeal can be used under challenging Tasmanian summer conditions without detrimental effect on salmon growth performance. Furthermore, our findings suggest that suboptimal elevated temperatures may induce remodelling of cardiac structure in adult Atlantic salmon, thereby potentially compromising cardiac function and overall health. Future efforts to manage Atlantic salmon susceptibility to climate change will require further research on the effect of challenging environmental conditions on cardiac physiology and performance in an attempt to prevent cardiac pathology, thereby improving health and welfare of farmed salmonids.