## 2 Materials and Methods
### PCDD/Fs and DL-PCBs Analysis  
The analytical method followed the \"Analytical Guideline of Dioxins in Food\" with slight modifications. Briefly, after analyzing the total length and body mass of the freeze-dried specimens, six specimens were mixed and powdered. \({}^{13}\)C-labeled internal standards (17 congeners of \({}^{13}\)C-2,3,7,8-PCDD/Fs, 50 pg each); 12 congeners of \({}^{13}\)C-DL-PCB, 200 pg each) were spiked into the weighed specimen. Specimens were extracted using a high-speed solvent-extraction device (ASE-350; Thermo Fisher Scientific, Waltham, MA, USA) with extraction solvent acetone/hexane (1:2) at 150 \({}^{\circ}\)C. The extract was concentrated to near dryness by using a rotary evaporator (R-200; BUCHI, Sankt Gallen, Switzerland), and the concentrate was transferred into a weighed clean beaker. After additional drying at 105 \({}^{\circ}\)C for 1 h, the specimen was dissolved with \(\sim\)10 mL of hexane, transferred into a 50 mL glass tube that contained 5 mL of sulfuric acid, and left overnight. The hexane layer was purified by using a silver nitrate silica gel column and an activated carbon column. A separated layer was loaded onto two connected glass tubes that were filled with silver nitrate silica gel (0.5 g) and activated carbon (0.5 g) and eluted with 15 mL of hexane. The silver nitrate silica gel column was removed and 10% of dichloromethane in hexane (v/v) was added to the activated carbon column to elute most PCBs, except the non-_ortho_ PCBs (PCB fraction). Then, 25 mL of toluene was added to the activated carbon column to elute the PCDDs, PCDFs, and non-_ortho_ PCBs (DXN fraction). The PCB fraction was evaporated to near dryness and reconstituted with 1 mL of hexane. The reconstituted specimen was purified with a sulfoxide column. After concentration by N\({}_{2}\) gas and transferal into a 1.5 mL gas chromatograph (GC) vial, the specimen was spiked with a \begin{table}
\begin{tabular}{c c c} \hline \hline
**Site Number** & **Sampling Date** & **Sampling Site (City, Prefecture)** \\ \hline
1 & 21 January 2012 & Tateyama, Chiba (TAT) \\
2 & 20 October 2011 & Hachinobe, Aomori (HAC) \\
3 & 19 October 2011 & Usushiri, Hokkaido (USU) \\
4 & 23 October 2011 & Sado, Niigata (SAD) \\
5 & 14 October 2011 & Ishinomaki, Miyagi (ISH) \\
6 & 26 October 2011 & [PERSON], Aomori (AOM) \\
7 & 19 October 2011 & [PERSON], [PERSON] (NOR) \\
8 & 19 October 2011 & [PERSON], [PERSON] (NOT) \\
9 & 27 October 2011 & [PERSON], [PERSON] (GAM) \\
10 & 27 October 2011 & [PERSON], [PERSON] (SAS) \\
11 & 27 October 2011 & [PERSON], [PERSON] (ONA) \\
12 & 07 June 2012 & Matsukawa, Fukushima (MAT) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sampling date and location (city, prefecture). -syringe spike solution (13C-PCB #111, 200 pg) and concentrated to 20 \(\upmu\)L (mono-_ortho_ PCB sample). The DXN fraction was concentrated and the solvent was changed from toluene to hexane. After concentration by N\({}_{2}\) gas and transferal into a 1.5 mL GC vial, the specimen was spiked with a syringe spike solution (13C-1,2,3,4-tetrachloro-p-dioxin, 50 pg) and concentrated to 20 \(\upmu\)L (DXN sample). Prepared specimens were analyzed by high-resolution GC/MS (Agilent 6890 GC; Autospec-Premier MS; Waters, Milford, MA, USA) in selected ion monitoring mode. We analyzed 17 PCDD/Fs (4-8 chlorinated). The PCDDs analyzed were as follows: 2,3,7,8-Tetrachlororibenzo-p-dioxin (2,3,7,8-TeCDD), 1,2,3,7,8-pentachlororibenzo-p-dioxin (1,2,3,7,8-PeCDD), 1,2,3,4,7,8-hexachlororibenzo-p-dioxin (1,2,3,4,7,8-HxCDD), 1,2,3,6,7,8-hexachlororibenzo-p-dioxin (1,2,3,6,7,8-HxCDD), 1,2,3,7,8,9-hexachlororibenzo-p-dioxin (1,2,3,7,8,9-HxCDD), 1,2,3,4,6,7,8-hptachlororibenzo-p-dioxin (1,2,3,4,6,7,8-HpCDD), and octachlororibenzo-p-dioxin (OCDD).