Introduction
Recirculating aquaculture systems (RAS) have emerged as a promising alternative to the traditional flow-through (FT) system for land-based rearing of Atlantic salmon, offering advantages regarding water usage, biosecurity and control of environmental parameters. However, RAS is often associated with higher concentrations of metabolites and residues in the system, which lowers water quality and potentially increases the vulnerability of fish to multifactorial stressors encountered later in the marine phase. Climate change has become a significant concern in sea-based production, as it can intensify adverse environmental conditions. Elevated water temperatures, reduced oxygen levels, and exposure to jellyfish blooms have been shown to negatively affect fish growth and welfare, yet limited research has focused on the combined effects of multiple stressors. The study examines how production environments, particularly RAS and FT, influence the cardiac health and function of Atlantic salmon, and whether these systems affect susceptibility to climate-related stressors during the marine phase. The findings provide insight for developing climate adaptation strategies and reducing fish morbidity and mortality in aquaculture.
Materials and methods
For the Main Trial, Atlantic salmon parr were divided to be reared in FT or RAS. Fish were subjected to 5 weeks of continuous illumination and a gradual salinity increase to induce smoltification, with heart samples being collected 1 day and 14 days after. Seawater transfer was simulated, and the fish were divided into Control and Test groups. The Test groups were subjected to a stress test that included a gradual increase in water temperature from 12ºC to 17ºC (1ºC per day), followed by a decrease in dissolved oxygen (DO) to 60% for 24 hours, and, lastly, exposure to macerated jellyfish. Hearts were sampled 1 day and 5 days post-exposure. Additionally, 9 fish from each group were selected for a 10-minute electrocardiogram (ECG) assessment, during which hypoxic bradycardia was induced by ceasing artificial gill ventilation in anaesthetised fish. Heart samples were sectioned into atrium, bulbus arteriosus, and ventricle, and differences in the expression of genes related to inflammation, thermal stress, angiogenesis, cardiac growth/remodelling, and hypoxia were analysed by RT-qPCR. Histology and immunohistochemistry were also conducted.
Results
The gene expression profile of the Main Trial showed statistically significant differences between the FT and RAS production environments in atrium and bulbus arteriosus samples, specifically in ctgf and cyr61, both markers of angiogenesis, with higher relative expression in the RAS groups. In contrast, ventricle samples showed no significant upregulations related to the production systems. Multiple significant differences between the two sampling points were identified across all heart compartments in cardiac growth and remodelling-related genes, including col1a and anp, as well as the previously mentioned angiogenesis markers. In the climate change stress challenge, only one statistically significant upregulation in the RAS group was noted in heat shock protein 70, a thermal stress-related gene, within bulbus arteriosus samples. Atrium and ventricle samples showed similar expression patterns in both the FT and RAS environments. Significant differences between sampling points or between the Control and Test groups were frequent across all heart compartments for angiogenesis, thermal stress, and cardiac growth/remodelling markers. The relative expression patterns were maintained throughout the three heart regions, especially in genes associated with inflammation, thermal stress and hypoxia. Results from the ECG assessment revealed no differences between the FT and RAS groups. Induced hypoxic bradycardia was also comparable between FT and RAS, however, in the Test group, the bradycardic response was attenuated one day following jellyfish exposure, with signs of recovery evident 5 days post-exposure. Histology and immunohistochemistry samples are currently being analysed.
Discussion
Results from the gene expression analysis showed only minor differences between the FT and RAS production environments, with similar trends in relative expression observed across all three heart compartments and in both trials. This consistency was particularly evident for genes related to inflammation, thermal stress, cardiac growth and remodelling, and hypoxia. These preliminary findings suggest that land-based RAS, under the conditions in the experimental set-up, does not compromise cardiac health in Atlantic salmon, nor does it increase susceptibility to climate-related stressors encountered during the marine phase, compared with FT rearing.
Acknowledgment
This project was financed by the Norwegian Seafood Research Fund (GillHealthAct #901911).