Polyculture in aquaculture systems could lead to a major improvement in the efficiency of such breeding. Indeed, associations between complementary species can enhance feed allocation, welfare or immunity status (Hisano et al., 2019). Considering polyculture in fish production and its benefits, little attention was given to the importance of the global microbiome. However, it is known that these microbial communities can have a significant impact in modulating health and disease (de Bruijn et al., 2018). Pikeperch (Sander lucioperca) is a very promising species for the development of the European inland aquaculture, however its high sensitivity to stress remains a major bottleneck (Dalsgaard et al., 2013). In the ancient literature, tench (Tinca tinca) is often considered a “doctor fish”. Thus, a biculture tench-pikeperch in recirculated aquaculture systems (RAS) could be an opportunity for improving its breeding compared to monoculture (Thomas et al., 2020). Indeed, associations between complementary species can lead to an improvement in feed allocation, welfare or immunity status. Furthermore, the association of different fish species could change the structure of water and fish-associated microbial communities and therefore influences fish welfare and health. This study aims to assess the effect of tench on pikeperch when reared together in RAS on fish health and water bacterial communities. To achieve this objective, an experiment in RAS was performed, where pikerperch were reared with or without (control group) tench. In order to describe the response of pikeperch, several indicators were measured at the beginning of the experiment and 84 days later. Stress (cortisol and glucose rates in plasma and percentage of hematocrits), immunity (lysozyme and complement activity) and zootechnical indicators (growth parameters, feed conversion ratio, between others) were investigated. To better explore the influence of biculture with tench, bacterial communities structure in the tank water were characterized using an Illumina Miseq metabarcoding approach. As each species of fish might bring its own and particular microbiota, the study of bacterial communities in RAS could be a major feature in understanding the effects of polyculture on target species. Poorly described in the actual literature, the relationship between fish and microbial diversity remains an important knowledge gap in aquaculture field. To complete these investigations and obtain a comprehensive dataset, mucus of both species was also characterized, in biculture and control group. The approach we are developing is intended to both find new ways to improve pikeperch rearing and improve aquaculture practices. In addition, highlighting the importance of fish diversity and its impact on microbial communities is part of the broader search for new ways to improve aquaculture practices.