## 4 Materials and Methods
### Data Preprocessing for Analysis and Annotation  
The NMR measurement outcomes were normalized according to the DSS signal and peaks were annotated by comparison with premeasured standards from our database of metabolites acquired under the same conditions (Figure S1, Table 2). Datasets from NMR  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Qualitative Parameters** & **Observed Number** \\ \hline Ecology4 (Demersal fish) & 26 \\ Food1 (Plankton eaten) & 42 \\ Food2 (Herbivore) & 41 \\ Food3 (Polycheeta eater) & 150 \\ Food4 (Crustacea eater) & 259 \\ Food5 (Molluske eater) & 82 \\ Food6 (Fish eater) & 162 \\ Habitat1 (Freshwater) & 24 \\ Habitat2 (Brackish water) & 22 \\ Habitat3 (Coast) & 136 \\ Habitat4 (Ofshore) & 92 \\ Habitat5 (Deep sea) & 48 \\ Length1 (50–150 mm) & 94 \\ Length2 (150–200 mm) & 97 \\ Length3 (200–300 mm) & 88 \\ Length4 (Over 300 mm) & 58 \\ NMR Class 1 (from i-means) & 192 \\ NMR Class 2 & 99 \\ NMR Class 3 & 7 \\ NMR Class 4 & 11 \\ Scales1 (Matte) & 52 \\ Scales2 (Glossy) & 133 \\ Scales3 (Shiny) & 131 \\ Season1 (March-May) & 90 \\ Season2 (June–August) & 68 \\ Season3 (September–November) & 115 \\ Season4 (December–February) & 46 \\ Tail1 (Two distinct) & 122 \\ Tail2 (Others) & 191 \\ \hline \hline \end{tabular} The sample had a single category; if there were overlaps and missing values, the number did not always reflect the sample number.  
\end{table}
Table 1: Cont.  
and gut microbe analysis (MiSeq) were processed to obtain the composition ratio to the sum of the measurement in fish intestines for each sample. Subjective qualitative information was assigned to samples by observation, as mentioned above (Table 1). These observations were made by multiple persons subjectively. The classification of a sample was difficult if qualitative information belonged to multiple categories.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Peak Number** & \(\Delta^{1}\)**H (ppm)** & \(\Delta^{3}\)**C (ppm)** & **Annotation** \\ \hline
1 & 0.93 & 13.9 & Ile \\
2 & 0.946 & 23.7 & Leu \\
3 & 0.97 & 19.4 & Val \\
4 & 1.001 & 20.6 & Ile \\
5 & 1.034 & 20.6 & Val \\
6 & 1.319 & 22.2 & Lactate, Thr \\
7 & 1.471 & 18.9 & Ala \\
8 & 1.701 & 26.7 & Leu \\
9 & 1.72 & 29 & Lys \\
10 & 1.909 & 25.9 & Acetate \\
11 & 1.973 & 38.6 & Ile \\
12 & 1.999 & 31.9 & Pro \\
13 & 2.045 & 29.7 & Glu \\
14 & 2.124 & 29 & Gln \\
15 & 2.261 & 31.9 & Val \\
16 & 2.339 & 36.2 & Glu \\
17 & 2.394 & 36.9 & succinate \\
18 & 2.636 & 31.5 & Met \\
19 & 2.677 & 39.4 & Asp \\
20 & 2.711 & 37.2 & DMA \\
21 & 2.807 & 39.4 & Asp \\
22 & 2.876 & 47.4 & TMA \\
23 & 3.025 & 39.7 & Creatine \\
24 & 3.196 & 56.6 & Choline \\
25 & 3.233 & 43.3 & Arg \\
26 & 3.223 & 76.9 & Glucose \\
27 & 3.258 & 62.3 & TMAO, Taurine \\
28 & 3.414 & 38.1 & Taurine \\
29 & 3.501 & 70 & Choline \\
30 & 3.548 & 44.2 & Gly \\
31 & 3.645 & 65.3 & Glycerol \\
32 & 3.921 & 56.5 & Creatine \\
33 & 3.955 & 63 & Ser \\
34 & 4.103 & 71.2 & Lactate \\
35 & 4.241 & 68.7 & Thr \\
36 & 4.642 & 98.6 & Glucose \\
37 & 5.224 & 94.8 & Glucose \\
38 & 6.091 & 91.1 & inosine \\
39 & 6.887 & 118.6 & Tyr \\
40 & 7.108 & 119.8 & His \\
41 & 7.181 & 133.4 & Tyr \\
42 & 7.265 & 124.8 & Trp \\
43 & 7.318 & 131.9 & Phe \\
44 & 7.365 & 130.5 & Phe \\
45 & 7.412 & 132 & Phe \\
46 & 7.518 & 114.6 & Trp \\
47 & 7.721 & 121.2 & Trp \\
48 & 7.999 & 138.6 & His \\
49 & 8.178 & 148.3 & Inosine \\
50 & 8.223 & 148.9 & Inosine \\
51 & 8.338 & 143 & Inosine \\ \hline \hline \end{tabular}
\end{table}
Table 2: NMR signal annotation of gut contents of fish extracts using KPi/D\({}_{2}\)O.