## 2 Materials and Methods
### Acoustic Backscattering Strength  
The range of total length of _Gadus macrocephalus_ was 36.4-74.0 cm, and that of _Engraulis japonicus_ was 9.5-15.4 cm. In addition, the range of total length of _Todarodes pacificus_ was 16.0-27.0 cm, and that of _Clupea pallassii_ was 12.0-27.0 cm. The total weight of the collected _Gadus macrocephalus_ was 3459 g, and the total weight of _Engraulis japonicus_ was 1910 g. _Todarodes pacificus_ had a collection weight of 1235 g, and _Clupea pallasii_ had a collection weight of 4617 g.  
The frequency characteristics of each fish species for the dominant species were identified, and the results of a previous study were applied for the acoustic scattering characteristics for each frequency. Information on the acoustic scattering strength of the dominant species is presented as follows [12; 13; 14; 15]. \[\text{{Gadus macrocephalus}: TS}_{\text{38 kHz}}=24.8\text{log}\left(\text{TL} \right)-73.1\text{, TS}_{\text{120 kHz}}=17.9\text{log}\left(\text{TL}\right)-61.5\]  
\[\text{{Engraulis japonicus}: TS}_{\text{38 kHz}}=16.7\text{log}\left(\text{TL} \right)-62.8\text{, TS}_{\text{120 kHz}}=15.9\text{log}\left(\text{TL}\right)-64.7\]  
\[\text{{Todarodes pacificus}: TS}_{\text{38 kHz}}=12.9\text{log}\left(\text{TL} \right)-63.7\text{, TS}_{\text{120 kHz}}=20\text{log}\left(\text{TL}\right)-73.5\]  
\[\text{{Clupea pallasii}: TS}_{\text{38 kHz}}=21.79\text{log}\left(\text{TL} \right)-66.01\text{, TS}_{\text{120 kHz}}=25.30\text{log}\left(\text{TL}\right)-72.07\]  
For the identification of fish species, the range of total length for each species and acoustic scattering strength were used to represent acoustic scattering strength for each frequency, and the frequency range was set using the dB-difference method. There \begin{table}
\begin{tabular}{c c} \hline
**Species (Name)** & **Seq. Identity (\%)** \\ \hline Pacific cod (_Gadus macrocephalus_) & 33.92 \\ Pacific herring (_Clupea pallasii_) & 28.33 \\ Blackfin flounder (_Glytopcephalus stelleri_) & 12.73 \\ Anchovy (_Engraulis japonicus_) & 4.61 \\ Seaperch (_Lateolabrax japonicus_) & 4.27 \\ Jack mackerel (_Trachurus japonicus_) & 3.09 \\ Spanish mackerel (_Scomberronous nipinous_) & 2.58 \\ Yellow goosefish (_Lophius littlon_) & 2.22 \\ Cubed snailfish (_Liparis tessellatus_) & 1.68 \\ Chub mackerel (_Scomber japonicus_) & 1.46 \\ Torachi (_Trachipterus isihkawa_) & 1.27 \\ Butterfish (_Penopsis anomala_) & 1.26 \\ Pointhead flounder (_Clesthenes pintertum_) & 0.79 \\ Silver crooker (_Pemunita ageratta_) & 0.63 \\ Slender pongfish (_Leigonathus elongatus_) & 0.32 \\ Hatchetfish (_Maurolicus muelleri_) & 0.28 \\ Bastard halibut (_Paralalolithys olivuccus_) & 0.11 \\ Long shanny (_[PERSON]_) & 0.10 \\ Sailfin (_Arcoscopus japonicus_) & 0.09 \\ Starry flounder (_Platichys stellatus_) & 0.05 \\ Horse king fish (_Kaiaurinus equula_) & 0.04 \\ AmberJack (_Serola quinqueradida_) & 0.04 \\ \hline \end{tabular}
\end{table} 
Table 2: List of fish species for analysis. fore, the dB-difference range for _Gadus macrocephalus_ was set to be \(-0.86\) dB \(<\Delta\)MVBS 38-120 kHz \(<0.82\) dB, and the dB-difference range for _Engraulis japonicus_ was set to be 2.66 dB \(<\Delta\)MVBS\({}_{\text{38-120 kHz}}<2.84\) dB. In addition, the dB-difference range for _Todarodes pacificus_ was 0.36 dB \(<\Delta\)MVBS\({}_{\text{38-120 kHz}}<1.25\) dB, and the range for _Clupea pallasii_ was 0.88 dB \(<\Delta\)MVBS 38-120 kHz \(<2.28\) dB. These ranges were set for fish species identification. For the identification of fish species, it is necessary to first identify the frequency characteristics for the fish's frequencies of 38 and 120 kHz and differences between them. In this case, the dB-difference represents the difference in the mean volume backscattering strength (MVBS) at multiple frequencies. To set \(\Delta\)MVBS as positive values, target strength (TS) for each frequency of the target species for separation is compared, and the frequency with the smaller TS value is subtracted from the frequency with the large TS value.