## 4 Discussion
The presence of similar bacterial taxa in the gut microbiota of multiple fish species suggests that these bacteria are valuable for the host and could play important roles in digestion, nutrient absorption, and immune responses [51]. These phyla have been found in the intestines of many marine and freshwater fish species [52; 53; 54; 55; 56]. However, the proportions of these phyla present in our study were different to those in other studies [57; 33; 58]. In studies on eight freshwater fish species [55], marine Atlantic cod [59], and Fine flounder [60], _Proteobacteria_ has been described as the predominant phylum. On the other hand, _Firmicutes_ predominate in the intestinal content of aquacultured Siberian sturgeon [61], grass carp [62], and Atlantic salmon [63]. Our study showed that the mean abundance of _Proteobacteria_ of a group might be a predominant phylum (Figure 3A). However, looking at individual fish, the proportions of the percent abundance of main phyla were not similar. Sometimes, _Actinobacteria_ predominate over _Proteobacteria_. This observation was noticed in control and salinity tanks, so salinity does not have a significant influence on the gut structure at the phylum level. This may be related to fish development. It appears that the diversity of bacteria increases as fish develop [64]. [PERSON] and [PERSON] [65], in their review, summarized 24 studies on the microbiome of fish larvae and fry and showed changes during fish development. Furthermore, relatively stable gut microbiota are established within the first 50 days of life for many species [66]. Therefore, gut microbiota of pike might just have been forming during the experiment. Despite these facts, our results suggest that _Proteobacteria_, _Firmicutes_, _Bacteroidetes_, and _Actinobacteria_, which dominate in the fish gut, may play important roles in the gut microbiome of pike at an early life stage. Various salinity concentrations can lead to osmotic stress in fish. Physiological changes in the host that occur during osmotic stress force its gut microbiome to adapt to the new conditions. Therefore, new species may develop in the gut microbiome. Although significant differences in the abundance of taxa between control and salinity groups (Table S1) were not found, there were small changes in the percent abundance of some groups. In some fish reared in 7%o salinity, a higher percent abundance of _Plantomycetes_ were observed, whereas in the other groups, these bacteria were absent. _Plantomyces_ were found in yellow grouper, which is a marine fish [67]. Another study reported that _Plantomyces_ were found in macroalgae biofilm [68]. This suggests that _Plantomyces_ like salty environments and could multiply under the influence of this stress factor. In most fish, at all salinity concentrations, _Proteobacteria_ usually predominate over other phyla. In contrast to our results, a study on _Oreochromis niloticus_ (Nile tilapia) showed that the percent abundance of _Proteobacteria_ changed and was higher in fish from a marine environment than in those from a freshwater environment [15]. Furthermore, the study found that _Actinobacteria_ and _Bacteroidetes_ were more abundant in fish from freshwater than in those from marine water, whereas our study found similar levels of these phyla at all salinity levels. The reason for these results is not yet completely understood, but the results might depend on many conditions, such as the salinity concentration, fish species, length of exposition, diet, age of the fish, and others. Although significant changes in phyla were not observed, differences in the percent abundance of _Proteobacteria_ classes between salinity levels were observed. _Alphaproteobacteria_ predominated in 7%o salinity, whereas in 3%o and control tanks, _Betaproteobacteria_ were more abundant. An effect of salinity on the microbiome structure was also found in black molly (_Poecilia sphenops_), where an increase in salinity induced changes in the dominant bacterial taxa in the microbiomes [32].