## 3 Results
### Characterization of the Pike Gut Microbiome
The bacterial community composition of the gut of an individual fish tended to be more similar to that of other fish in the same group than to that of fish in other groups. _Rhodobacterales_ were present in all fish microbiomes, but in TS_7%o, they were more abundant in five fish (5.8%-25.2%) (Figure 5B). Interestingly, in the 3% salinity group, _Aeromonadales_ were more abundant in four fish than in the other fish (5.97%-79.4% vs. \(\leq\)1.57%). One-way ANOVA was calculated and differences were considered significant at \(p<0.05\) after Benjamini-Hochberg FDR correction for multiple comparisons. In addition, the corresponding effect size (ETA-Squared) was calculated. This analysis showed no statistically significant differences between groups in terms of the abundance of each taxa of bacteria. Statistical analyses of all taxa and their relative abundance (%) are reported in Table S1. Although any significant differences in relative abundance (%) of all taxa between groups were not found, it is interesting to note that the abundance of some bacteria was higher for specific salinity concentrations in most of the fish gut microbiome group. At 7%o salinity, _Plantomycetes_, _Rhodobacterales_, and _Alphaproteobacteria_ were more abundant than at other levels of salinity; at 3%o, in contrast, _Betaproteobacteria_ and _Burholderiales_ were more abundant; and in the control tanks, _Aeromonadales_ were more abundant (Table S1). Permutation multivariate analysis (PERMANOVA) indicated that, overall, there was a significant divergence between groups only in terms of unweighted UniFrac distance matrices (\(p\) = 0.001; Pseudo-F = 1.63) (Table 2). The beta-diversity pairwise test on the unweighted UniFrac data showed that the gut microbiomes of fish in 7%o salinity were significantly different to those of the 3%o salinity group. Weighted Unifrac did not show a significant overall difference in the abundance of bacteria. QIIME 2 was used to compute microbial beta diversity metrics. Analyses were performed using weighted and unweighted UniFrac distances. Data from UniFrac metrics were used to prepare three-dimensional plots using principal coordinates analysis (PCoA) (Figure 6). PCoA reflected the PERMANOVA results. Weighted Unifrac showed that the 0%o and 3%o salinity groups clustered together. Fish from 7%o salinity were separated from the rest of the samples. PCoA of the unweighted Unifrac distance matrix showed that samples from the 3%o salinity group clustered together and samples from the other groups were scattered. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**PERMANOVA Analysis**} & \multicolumn{2}{c}{**Unweighted Unifrac**} & \multicolumn{2}{c}{**Weighted Unifrac**} \\ \cline{2-5}  & \(p\)**-Value** & **Pseudo-F** & \(p\)**-Value** & **Pseudo-F** \\ \hline One-way & & & & \\ All groups & 0.001 & 1.63 & 0.052 & 1.69 \\ \hline PERMANOVA pairwise test: & & & & \\
0\%o vs. 3‰ & 0.002 & & 0.241 & \\
0\%o vs. 7‰ & 0.137 & & 0.146 & \\
3\%o vs. 7‰ & 0.001 & & 0.031 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Permutation multivariate analysis (PERMANOVA) of weighted and unweighted Unifrac data of intestinal microbiomes of pike living in different environments.