Microbial community dynamics in semi-commercial RAS for production of Atlantic salmon post-smolts at different salinities

- The major findings in this study were:
- The microbial community of the water was similar between the compartments in each RAS.
- The microbial communities of the water underwent large changes over time within each system.
- The biofilm microbiota differed significantly from the water microbiota within each system.
- Biofilm community profiles were more stable over time and between systems than the water communities.
- Salinity appeared to structure the microbial communities.

Fish live in close contact with microbes in their surrounding water. For cultivated marine fish larvae, stable microbial environments, characterized by slow-growing, K-selected bacteria, have been found to improve growth and survival. Compared to flow-through systems, recirculating aquaculture systems (RAS) offer a better possibility for obtaining microbial stability in the rearing water. However, the microbial ecology in RAS is complex and little studied, and to optimize the microbial water quality more knowledge is needed on microbial community dynamics and interactions between microbes and fish. In the present study we investigated microbial community dynamics in three large-scale RAS, operated with different salinities (12, 22 and 32 ppt), for production of Atlantic salmon post-smolts. We focused on the stability of microbial communities over time in the bioreactors and the fish tanks, and compared the microbial communities within and between the three RAS. The study showed that: 1) The microbial community of the water was similar between the compartments of each RAS at the same time point, 2) The microbial communities of the water (and to a lesser extent the biofilm) within each system underwent large changes over time, 3) The microbiota of biofilm and water differed significantly within each system, 4) Biofilm community profiles were more similar between RAS than the water community profiles, and 5) Salinity structured the microbial community composition. The present study provides new knowledge on how the microbiota in RAS varies with time and how it is influenced by environmental factors.