Gill Health in Fish Farmed in Recirculating Aquaculture Systems (<scp>RAS</scp>): A Review

Recirculating Aquaculture Systems (RAS) have been proposed as the future of aquaculture, because they can be used anywhere regardless of access to water, they offer high level of control over farming environment, including biosecurity, and are considered to be sustainable. However, despite of continuous development, there can be still issues with water quality affecting gill health of fish farmed in these systems. This review provides an overview of fish gill structure and gill immune response, and discusses the known impacts of RAS on gill health. Several experimental studies have inadequately reported conditions, particularly water quality, making it difficult to determine if the observed effects were due to water quality issues or RAS system itself. It is crucial for studies investigating the impact of RAS on fish to report water quality during the study. Furthermore, assessments of RAS effects on gill health should include sufficient independent replicates and flow through controls using a common water source. Various methods have been used to assess gill health in RAS, including gill histology, presence of pathogens, gene expression in the gills and gill microbiome analysis. Differences in gill health in fish from RAS and a flow through system have been shown for a number of freshwater and marine fish species. However, these results have been inconsistent across studies, and some results have been challenging to interpret as indicators of gill health. Holistic studies including a number of different methods to assess fish gills would give more conclusive results. More research is needed, in particular, on brackish and marine RAS, to fully understand their impacts on gill health.