Does the manipulation of light conditions affect the gut microbiome of Eurasian perch ( <scp> <i>Perca fluviatilis</i> </scp> L.) in a recirculating aquaculture system?

An important element of modern aquaculture is the creation of optimal environmental conditions for economically profitable fish farming, as only such conditions can guarantee a high growth rate and the welfare of the fish. One of the most important factors is the photoperiod, which has a substantial influence on the circadian rhythm, as it is a direct modulator of physiological functions. It has been reported that the photoperiod affects the microbiome of fish, which could potentially serve as an indicator of their welfare. Therefore, the main aim of this study was to determine the possible effects of shortening and lengthening daylight on the gut microbiome of Eurasian perch (Perca fluviatilis L.) in a recirculating aquaculture system. Our study indicates that rearing the fish in very short daylight for 2 months has no significant effects on their biomass and length; however, long daylight reduced the specific growth rate in comparison to the control group. Furthermore, no significant differences in the structure of the gut microbiome were observed. Importantly, potential differences in the structure of the gut microbiome of fish kept under different light conditions could be masked by the possibility of bacterial transfer between tanks within individual recirculation systems. Nevertheless, discriminant analysis showed that the abundance of Flavobacterium sp. and members of the Microbacteriaceae family was significantly increased in the group of fish reared in the dark for 22 h compared to the control group. Similarly, Pseudopropionibacterium sp. was identified as a significant biomarker between the group of fish reared in the light for 24 h and the control group. These results expand our understanding of the influence of the gut microbiome on the welfare of fish in aquaculture.