THE USE OF FISH MICROBIOME AS A DIAGNOSTIC AND THERAPEUTIC TOOL TOWARD INCREASED FISH HARVEST

The welfare of farmed fish is influenced by environmental and management factors, their skin being an important site for immunity and a major route by which infections are acquired. The objective of this study was to characterize skin bacterial variability in healthy and sick gilthead seabream (Sparus aurata), using a high-throughput DNA sequencing method. We compared the fish skin microbiome in a controlled environment with UV-treated water and non UV-treated water in both healthy and sick conditions. Swabs samples were collected from fish lateral lines, abdomen and gills, before stress (T₀), and 24 hr (T₁), one week (T₂) and three weeks (T₃) after stress. The results showed a similar bacterial composition to be present naturally (T₀) in different sites on fish skin (lateral line, abdomen and gills), which were comprised of a consistent bacterial community (65% Proteobacteria, 16% Firmicutes and 13% Actinobacteria), and was different from surrounding environments, ie. water (47% Proteobacteria, 25% Cyanobacteria and 14% Actinobacteria). The skin bacterial composition was also changed when the fish were confronted with handling stress followed by a bacterial pathogen infection (T₁), resulting in changes of fish skin bacterial community, causing fish sickness. More so, when the skin natural occurring microbiome did not recover it led to fish mortality (60% death in our experiment). We further confirmed the importance of bacterial communities when comparing non UV-treated water to UV-treated water (general practice in fisheries) in which all fish died at T₁. These results shows the importance and the need to change current fisheries practice towards the preservation of the natural microbial community in order to increase yield and minimize fish loss.