RAS vs Open Flow: discover differences in antiviral response of fish

Recently, norwegian scientists described various differences in sea performance between fish farmed in open flow systems (FTS) vs RAS systems. The conclusion was that fish farmed in FTS showed a faster adaptation to the seawater environment, with greater osmoregulatory capacity and growth. 

Now, a new study conducted by researchers from the Universities of Aberdeen and Stirling, Scotland, examined how Atlantic salmon farmed in these two types of systems respond immunologically after being transferred to seawater systems.

After two weeks of being transferred to seawater ponds, researchers injected the fish with poly I:C, to simulate a viral infection for 24 hours, and thus evaluate their immune response through a comprehensive transcriptome analysis of the gill tissue.

Differentiated Immune Response

Among the findings, it was found that the smolts which were not stimulated with poly I:C and were cultured in FTS showed a higher expression of immune genes compared to those raised in RAS. However, after stimulation with poly I:C, the RAS smolts showed a more intense immune response, with a greater induction of immune genes.

"The fish from both systems developed a strong transcriptomic immune response after stimulation with poly I:C as expected, without highlighting any severe immune dysfunction due to the culture environment. However, the intensity of the response to stimulation was stronger in RAS fish compared to LOCH", the authors detailed.

As an explanation, scientists pointed out that possibly RAS fish have a reduced (or suppressed) immune system in terms of initial mRNA expression, but are capable of immunologically compensating by mounting a broader response when faced with a pathogen, "reaching their FTS counterparts. Alternatively, those FTS fish have a higher steady-state immune function and are more prepared for the immune attack".

With these results, experts concluded that further research is needed to determine if creating a larger immune response negatively impacts other physiological aspects of RAS fish, and how this would be maintained or not over time.

Read the full study titled "Impact of freshwater rearing history on Atlantic salmon gill response to viral stimulation post seawater transfer", here.