Introduction
Compromised cardiac health negatively impacts the welfare of fish and could also lead to pre-slaughter losses. Pathogenic infections such as cardio myopathy syndrome (CMS) and non-infectious factors, such as deformities can both lead to adverse cardiac outcomes. The altered morphology such as round ventricles and misalignment of bulbus, that occur more often in farmed salmon than wild ones (Poppe et al. 2003), could be also associated with decreased cardiac performance and reduced stress resistance resulting in arrhytmias and cardiac episodes (Brijs et al. 2020). Farmed samonids also have high prevalence of atherosclerosis, which can lead to lower oxygenation of cardiac cells during stressful events requiring higher cardiac performance (Brijs et al. 2020). Rearing practices promoting fast growth could increase the severity of cardiac deviations and coronary lesions (Frisk et al. 2020). However, there is still lack of knowledge of factors contributing in formation of morphological deformities.
Visual classification has been proposed as a superior tool for describing morpholocigal deviations compared to commonly used morphometric rations, which are not able to describe vast diversity of morphological variations (Engdal et al 2024). In this study we applied visual classification method for rainbow trout along with morphometrics and histology to get a comperehensive overview of heart health of commercially farmed slaughter sized rainbow trout in Finland. Our aim was to get an overview of cardiac deviations, prevalence and severity of coronary atherosclerosis, cardiac injuries and infections and to compare the differences between different farming sites and the origins of fish.
Materials and methods
Altogether 180 hearts of slaughter size rainbow trouts were collected and fixed in formalin from three net cage farms having fish of three origins. Two commercially reared breeding lines were also included in samples. Hearts were photographed following protocol by Engdal et al. (2024). Six hearts from each sampling group were randomly selected fo histology. Bulbuses of these hearts were cut in to three cross sections and ventricles cut half sagittally. Visual classification of morphological deviations was done according by Engdal et al.'s (2024) protocol established for Atlantic salmon. The presence of deviation was marked either as present or absent. Morphometrical measurements were analysed with ImageJ according Poppe (2003) and Brijs (2020). Coronary lesion severity was scored on a scale 0-5 with small modification on Brijs et al. (2020) protocol. Sections of bulbuses and ventricles were examined for having epicarditis, endocarditis and signs of cardiac injuries. Histopathological findings were classified either present or absent.
Results
Out of 43 different deviation categories average amount of deviations per heart was 10 (range 2-19). Number of all morphological deformities did not differ between origin or farming site, nor did it correlate with body mass or relative heart size. However, certain categories had significant differences between farming environment and origin of fish. Most common ventricle deviations were low left dorsal apex (47 %) along with extended right dorsal ventricular apex (40 %), therefore hearts appeared in major part asymmetrical. 38 % of right ventricular sides were also distinctly enlarged compared to left side. 38 % hearts hand ventroventricular ridge curving inwards, which has been previously seen in higher prevalence in moribund and dead fish (Engdal et al 2024). From deviations of bulbus most common findings were ballooning and bulging bulbus (42 %) and misaligned bulbus (40 %). Extensive fat was present in 26 % of hearts. Prevalence of extensive fattiness did not differ among origin or farming site. From histology samples 93 % fish had coronary lesions. Severity of atherosclerosis did not differ between origin of hatchery or farming site. Epicarditis was detected in 46 % and endocarditis in 32 % of bulbus samples. Significantly more fish from lake farm had epicardial (76%) inflammation. All of the ventricles had signs of recent hypoxic cardiac injury.
Discussion
All the analysed hearts had morphological deviations, therefore altered morphology seems to be more rule than exeption. Distinct finding of our study was high prevalence of asymmetrical shape of ventricles. The asymmetry of ventricle has previously been associated with impaired cardiac performance. Previous methods measuring symmetry were not sufficient, since symmetry axis is commonly measured from the left lateral projection, which has usually been extended side in Atlantic salmon. In our study right side of ventricle was found to be extended and enlarged one, usually combined with low left dorso-ventricular apex, which narrows down the symmetry angle and thus seemingly resembles more of morphometrics of wild type. Cardiac inflammation was also common finding in our study, although causes remain unclear and need further investigation. Most of the deviations found in this study were not related to farming site or origin of fish, which indicates that altered heart shape is universal challenge in commercial aquaculture. High prevalence of deviations, asymmetry, inflammation and atherosclerosis may indicate fish being in higher risk of mortality in future, especially if heat waves and handling of fish mid season becomes frequent.
References
Brijs J., Hjelmstedt P., Berg C., Johansen I., Sundh H., Roques J., Ekstr��m A., Sandblom E., Sundell K., Olsson C. & Axelsson M. (2020). Prevalence and severity of cardiac abnormalities and arteriosclerosis in farmed rainbow trout (Oncorhynchus mykiss). Aquaculture, Vol (526), 735417. https://doi.org/10.1016/j.aquaculture.2020.735417
Engdal V., Dalum A, Kryvi H., Frisk M., Torsvik H., Hodne K., Romstad H. & Johansen I. (2024). State of the heart: Anatomical annotation and assessment of morphological cardiac variation in Atlantic salmon (Salmo salar L.), Aquaculture, Vol (578), 740046, https://doi.org/10.1016/j.aquaculture.2023.740046
Frisk, M., H��yland, M., Zhang, L., Vindas, M.A., ��verli, ��. & Johansen I.B. (2020). Intensive smolt production is associated with deviating cardiac morphology in Atlantic salmon (Salmo salar L.). Aquaculture, Vol (529), 735615, https://doi.org/10.1016/j.aquaculture.2020.735615
Poppe T., Johansen R., Gunnes G. & T��rud B. (2003) Heart morphology in wild and farmed Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss. Dis Aquat Org, Vol (57), 103-108 https://doi.org/10.3354/dao057103