## 3 Results
### Characterization of the Pike Gut Microbiome  
The gut microbiome of 20 fish in three groups that were kept at different levels of salinity was examined to characterize its structure and to reveal the differences between groups and individual fish. The microbiome structures of each investigated group of fish at the phylum, class, order, and family level were successfully described. In our study, 15 phyla, 32 classes, 53 orders, and 88 families were classified. The gut microbial community in the groups as a whole and in individual fish are presented at the phylum (Figure 3), class (Figure 4), and order (Figure 5) levels. The figures present taxa with a  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Group/Tank** & **Number of** & **Reads** & **Observed** & **Shannon** & **Good’s** & **Chao1** \\
**Salinity** & **Analyzed Fish** & **Specs** & **Specs** & **Coverage** & **Chao1** \\ \hline TS\_0\%o & 7 & 9278 \(\pm\) 5067 & 172 \(\pm\) 34 & 5.1 \(\pm\) 1.9 & 0.99 \(\pm\) 0.0 & 174.3 \(\pm\) 34.0 \\ TS\_3\%o & 6  
relative abundance \(\geq\)0.5% (up to the class level) and \(\geq\)2.5% at the order level. All taxa are shown in Table S1. Figure 3: Relative abundance (%) of the most prevalent bacterial phyla in three groups kept at different levels of salinity (**A**) and in individual pike (**B**). Figures show all phyla with an abundance \(>\)0.5%. Phyla with an abundance \(\leq\)0.5% are pooled and labeled as “Others”. Figure 4: Relative abundance (%) of the most prevalent bacterial classes in three groups kept at different levels of salinity (**A**) and in individual pike (**B**). Figures show all classes with an abundance >0.5%. Classes with an abundance \(\leq\)0.5% are pooled and labeled as “Others”. At the phylum level, _Proteobacteria_ predominated in each group (mean abundance of 40%-53%), followed by _Actinobacteria_, _Firmicutes_, and _Bacteroidetes_ (Figure 3A, Table S1). In individual fish, the composition of the bacterial community was generally similar to that in the groups as a whole, with four exceptions. In fish S3-3, _Firmicutes_ predominated (42.7%) (Figure 3B), whereas in fish S3-0, S6-0, and S9-7, _Actinobacteria_ predominated (43%-72%). Interestingly, in the 7% salinity group, _Plantomycetes_ were more abundant in three fish than in the other fish (5.3%-30.2% vs. \(\leq\)0.5%)  
At the class level, in almost all fish in the investigated groups, the gut microbiome was dominated by members of _Gammaproteobacteriaia_ (17.7%-37.5%) and _Actionobacteria_ (15.6%-22.2%) (Figure 4A, Table S1). _Alphaproteobacteria_ were third in terms of the percent abundance in all groups. Some of the gut microbiome profiles of individual fish were different from those of other individuals. For example, the gut microbiome of fish S6-0 was dominated by _Acidimicrobiia_ (48.64%), whereas in  
Figure 5: Relative abundance (%) of the most prevalent bacterial orders in three groups kept at different levels of salinity (**A**) and in individual pile (**B**). Figures show all orders with an abundance >2.5%. Orders with an abundance \(\leq\)2.5% are pooled and labeled as “Others”. other fish, its abundance was less or even absent (Figure 4B). The gut microbiome of fish S3-0 was completely dominated by _Gammaproteobacteria_ (93.8%), whereas in fish S9-7, the gut was dominated by _Actionobacteria_ (71.4%). _Bacili_ dominated the gut microbiome of fish S3-3 (40.6%), whereas _Alphaproteobacteria_ dominated that of fish S2-7. At the order level, a higher percent abundance of unclassified bacteria (23.8-33.7%) was observed than at higher taxonomic levels. In all groups, there was a similar predominance of _Actinomycetes_ (21.8%-22.8%) (Figure 5A). In TS_7%, the mean abundance of _Rhodobacterales_ was greater (10.84%) than in the rest of the groups (0.97%-2.44%). _Burkholderiales_ were present in all groups, but more abundant in TS_3%o (12.66%) (Figure 5A).