The primary function of the coronary circulation is to supply oxygen and nutrients to the cardiac muscle. Blockages, such as those that can result from coronary arteriosclerosis, can lead to myocardial ischemia, causing cardiac damage and potentially death. One-third of teleost species have a coronary circulation, predominantly in highly active species like Atlantic salmon, facilitating an active aerobic lifestyle and demanding swimming behaviors.1 Despite the necessity of the additional energy resource provided by the coronary circulation, coronary arteriosclerosis is highly prevalent in Atlantic salmon, with ubiquitous lesions in sexually mature individuals.1–3
Even though coronary arteriosclerosis is considered an inevitable aspect of their physiology, its severity may impact their health, welfare, and performance. Experimental studies have demonstrated that salmonids with surgically occluded main coronaries exhibit reduced tolerance to warming, hypoxia, impaired aerobic performance, and increased susceptibility to crowding stress.4-7 These findings have significant implications for aquaculture practices involving stressful handling such as crowding, pumping, transport, and different treatments.
The pathogenesis of coronary arteriosclerosis in salmonids is not completely clear, but studies have shown that farmed salmonids bred at elevated temperatures, resulting in increased growth rate exhibit a higher prevalence and severity of arteriosclerosis compared to wild and farmed fish with slower growth rates.1,3,4 The most recent comprehensive study on the prevalence and severity of coronary arteriosclerosis in wild and farmed Atlantic salmon was conducted 30 years ago.3 Since then, smolt breeding programs have intensified, potentially exacerbating the issue due to increased growth rates. Therefore, we assessed the prevalence and severity of coronary arteriosclerosis in commercially farmed Atlantic salmon to assess its status in relation to today’s farming practices.
Hearts from Faroese farmed Atlantic salmon were sampled at a range of different farming sites to ensure comprehensive representation. We sampled one month after sea transfer, and again ~10 months later, just prior to slaughter. From each fish, 18 serial sections of the main coronary artery were examined, and lesions were scored from 0-5 according to their severity, with 0 representing no lesions and 5 representing severe lesions.
One month after sea transfer, lesions were found in 23-37.7% of the all the fish examined at the different sites. One site, however, had a much higher prevalence where 66.6 % of all the fish contained lesions. This same site also exhibited more severe lesions, with a mean lesion score of 0.80, compared to the other sites, which had a mean lesion score between 0.12-0.46.
Just prior to slaughter the lesion prevalence was significantly higher with lesions found in 89.5% to 100% of all the fish examined. The lesions were also more severe with a mean lesion score between 3.54 and 4.14. Variations in lesion prevalence and severity may be attributed to differences in size and growth rates among sites, as fish that where larger or had a higher growth rate displayed higher prevalence and severity. Our findings highlight significant concerns regarding both the prevalence and severity of coronary arteriosclerosis in commercially farmed Atlantic salmon. The observed variability among farming sites underscores the importance of critically evaluating current aquaculture practices. As smolt breeding programs intensify and farming techniques aimed at maximizing growth are implemented, it is crucial to also consider the cardiovascular health and welfare of the fish.
References
1. Farrell, A. P. Coronary arteriosclerosis in salmon: Growing old or growing fast? Comp. Biochem. Physiol. - A Mol. Integr. Physiol. 132, 723–735 (2002).
2. Farrell, A. P., Johansen, J. A. & Saunders, R. L. Coronary lesions in Pacific salmonids. J. Fish Dis. 13, 97–100 (1990).
3. Saunders, R. L., Farrell, A. P. & Knox, D. E. Progression of Coronary Arterial Lesions in Atlantic Salmon ( Salmo salar ) as a Function of Growth Rate . Can. J. Fish. Aquat. Sci. 49, 878–884 (1992).
4. Brijs, J. et al. Prevalence and severity of cardiac abnormalities and arteriosclerosis in farmed rainbow trout (Oncorhynchus mykiss). Aquaculture 526, 735417 (2020).
5. Ekström, A., Axelsson, M., Gräns, A., Brijs, J. & Sandblom, E. Influence of the coronary circulation on thermal tolerance and cardiac performance during warming in rainbow trout. Am. J. Physiol. - Regul. Integr. Comp. Physiol. 312, R549–R558 (2017).
6. Ekstrom, A., Axelsson, M., Grans, A., Brijs, J. & Sandblom, E. Importance of the coronary circulation for cardiac and metabolic performance in rainbow trout (Oncorhynchus mykiss). Biol. Lett. 14, (2018).
7. Morgenroth, D. et al. Coronary blood flow influences tolerance to environmental extremes in fish. J. Exp. Biol. 224, (2021).