Compromised gill health remains a critical welfare and production concern in the Norwegian aquaculture industry, where
Atlantic salmon (Salmo salar) are increasingly reared in land-based recirculating aquaculture systems (RAS) during smolt
production, before transfer to seawater pens. The shift in production technology has raised questions regarding the potential
long-term effects of RAS environments on the gills, and subsequent susceptibility to gill-related stressors in the marine
phase. The present study investigated the effects on gill health of rearing Atlantic salmon in 3 different smolt-production
systems: flow-through (FT), recirculating aquaculture system with low water-exchange (RAS-L) and recirculating
aquaculture system with high water-exchange (RAS-H). Moreover, the influence of these rearing environments on the
susceptibility of salmon to gill health-related challenges in sea was investigated. The central hypothesis proposed that the
use of recirculation technology would impact gill mucosal integrity, increasing vulnerability to gill-related stressors
following seawater transfer.
Gill condition was assessed through histological examination, clinical scoring and transcriptional analysis of gene expression
related to immune regulation, remodelling and antioxidant defence. Fish were monitored at multiple time points throughout
the production phase, with particular attention to pathological indicators commonly associated with compromised gills, and
challenged by Neoparamoeba perurans, the causative agent of amoebic gill disease (AGD) before treatment with hydrogen
peroxide (H2O2). No significant differences were observed in disease susceptibility, and limited differences were detected
regarding histopathological changes or gene expression of selected genes among the three production groups. These findings
suggest that the use of recirculation technology in land-based smolt-production does not inherently compromise gill barrier
function or increase the risk of gill-related health issues compared to FT, under the conditions examined. However, the trial
resulted in a low development of the disease, which limited the potential to investigate system-dependent differences in
disease outcomes.
This study contributes to a growing body of evidence supporting the viability of RAS technology within sustainable
aquaculture production models, while also highlighting the need for continued monitoring of gill health in evolving
production environments.