FISH SKIN MICROBIOME AS A TOOL TOWARD UNDERSTANDING AND IMPROVING FISH WELFARE

The welfare of farmed fish is influenced by environmental and management factors. Fish skin is one of the sites of fish immune system and a major route by which infections can be acquired. The bacterial variability of gilthead seabream (Sparus aurata) skin in healthy and diseased conditions was characterized, using a high-throughput DNA sequencing method. The fish skin microbiome in both healthy and diseased conditions was compared in a controlled environment, using UV-treated water and non UV-treated water. Swab samples were collected from fish lateral lines, abdomen area and gills, before (T₀), and 24 hr. (T₁), one week (T₂) and three weeks (T₃) after an experimentally induced infection by exposure to Vibrio harveyi followed by netting fish out of the water (handling stress). The results showed a similar bacterial composition present initially (T₀) in the different sites sampled, which included six main constant bacterial communities (families). These bacteria families were mostly similar to those present in the surrounding environment (water). The skin bacterial composition changed when the fish were confronted with bacterial infection followed by a handling stress (T₁), resulting in variations in fish skin bacterial communities. Such changes presumably played an important role in fish health deterioration. Furthermore, when skin microbiome occurring prior to fish challenge, was not re-established, fish mortality was observed in both non UV-treated and UV-treated water treatments (60% and 100%, respectively). The importance of a balanced bacterial community appears quite evident when non UV-treated water was compared with UV-treated water (general practice in aquaculture). 30% of the total initial communities (OTU's) present on fish skin were unique to fish that eventually survived the challenge in comparison to the bacterial communities in those that did not survive (Fig 1). These results suggests the importance of a balanced epithelial microbiome on fish health status and the need to preserve this balance.