Atlantic salmon aquaculture increasingly relies on recirculating aquaculture systems (RAS). These closed-loop systems offer several advantages, including higher growth rates, stricter environmental control, and potentially reduced water usage. However, the impact of RAS farming on juvenile salmon, specifically their immune function after transfer to seawater (SWT), remains an enigma.
Scientists from the Scottish Fish Immunology Research Centre at the University of Aberdeen (UK) and the Institute of Aquaculture at the University of Stirling (UK) used RNA sequencing to identify a panel of genes with different expression profiles in the gills of salmon raised in RAS or a traditional flow-through system (LOCH) two weeks after transfer to SWT.
The puzzle: immune system development in RAS
While RAS offers several advantages, a key question remains: how does this controlled environment affect the immune system of a salmon, particularly in the gills?
Atlantic salmon have a robust immune system with both innate and adaptive defenses. The gills, constantly exposed to the environment, play a crucial role in this defense system. They host specialized immune tissues (GiALT) packed with immune cells, proteins, and signaling molecules to combat pathogens.
Interestingly, research suggests that smoltification itself might inhibit the immune system. This could be a trade-off, allowing the fish to focus their energy on the dramatic physiological changes necessary for adaptation to saltwater.
RAS versus traditional systems: impact on immune response
This study investigated how the farming system during the freshwater stage affects the immune response of a smolt after transfer to seawater. Parr were raised in a RAS or a traditional flow-through system (LOCH) until smoltification. After SWT in a shared marine environment, researchers assessed the fish’s ability to generate an immune response by stimulating their immune system with a viral mimic.
The researchers compared the gill gene expression of fish raised in RAS and LOCH after their transfer to SWT.
Intriguing findings: lower baseline, stronger response in RAS
Interestingly, the study revealed that unstimulated smolts raised in the LOCH system had higher levels of immune gene expression compared to those raised in RAS. This suggests that the constant, controlled environment of RAS could lead to a lower “baseline” immune activity in smolts.
However, the story does not end there. When stimulated with the viral mimic, RAS-raised smolts showed a much stronger immune response. Their immune genes exhibited a greater increase in activity compared to the LOCH smolts.
“Our hypothesis is that in the first few weeks following transfer to SWT, an early immune response develops in the gills of fish raised in LOCH, stimulated by the transition to a new environment, which is absent or suppressed in RAS-raised fish,” the study authors reported.
Two possible interpretations: catching up or overreacting?
The researchers propose two possible explanations for this finding:
- Catch-up mechanism: The RAS environment might “down-regulate” the immune system due to the lack of constant exposure to pathogens and environmental variations. When stimulated, RAS smolts experience a stronger response as they “catch up” to the immune activity level of LOCH smolts.
- Overreaction: Alternatively, the intense immune response in RAS smolts might be an overreaction to stimulation, potentially leading to unnecessary energy expenditure.
Conclusion
“Fish raised in RAS had a lower steady-state transcript level of immune-related genes in the gills two weeks after SWT compared to fish raised in a LOCH system,” the scientists concluded.
This study highlights the need for further research to understand the long-term consequences of RAS farming on immune function and the health of salmon after SWT. By resolving these complexities, we can optimize farming practices to ensure the well-being and survival of Atlantic salmon smolts during this critical transition.
The research was funded under the ROBUSTSMOLT project by UKRI and the Scottish Aquaculture Innovation Centre.
Contact
Samuel A.M. Martin
Scottish Fish Immunology Research Centre, School of Biological Sciences, University of Aberdeen
Aberdeen, AB24 2TZ, UK
Email: sam.martin@abdn.ac.uk
Reference (open access)
Lorgen-Ritchie, M., Chalmers, L., Clarkson, M., Taylor, J. F., MacKenzie, S., Migaud, H., & Martin, S. A. (2024). Impact of freshwater rearing history on Atlantic salmon gill response to viral stimulation post seawater transfer. Fish & Shellfish Immunology, 150, 109653. https://doi.org/10.1016/j.fsi.2024.109653
