*3.1. The Concentration of PCDD/Fs and DL-PCBs in Farm Crabs*

A total of 17 PCDD/Fs and 12 DL-PCBs were detected in the edible parts of the sampled crabs, and the PCDD/F and DL-PCB concentrations in the crab samples from the different farms varied greatly. Detailed results are shown in Table S5. The mean PCDD/F and DL-PCB concentrations of the crab samples were 264.20 ± 260.14 and 506.25 ± 226.80 pg/g ww, respectively. The PCDD/F concentrations ranged from 20.45 to 686.64 pg/g ww, with OCDD having the greatest concentration (612.75 pg/g ww), accounting for 38.7%, followed by 1,2,3,4,6,7,8-HpCDD at 549.05 pg/g ww, accounting for 34.6% (Figures 2A and 3A). The DL-PCB concentrations ranged from 246.51 to 894.47 pg/g ww, with the highest concentration of 2,3 ,4,4 ,5-PeCB (1573.97 pg/g ww) accounting for 51.8%, followed by 2,3,3 ,4,4 - PeCB (86.63 pg/g ww), which accounted for 19.8% (Figures 2B and 3B). The remaining PCDD/F and DL-PCB congeners in the crab samples accounted for less than 10%.

**Figure 2.** Concentrations of PCDD/Fs (**A**) and DL-PCBs (**B**) in farmed Chinese mitten crabs (*n* = 90).

**Figure 3.** Ratios of PCDD/Fs (**A**) and DL-PCBs (**B**) in farmed Chinese mitten crabs.

The TEQs of the total PCDD/F and DL-PCB levels in the crab samples ranged from 1.20 to 29.04 pg TEQ/g ww, and the mean was 9.37 pg TEQ/g ww. Similarly, the TEQ range for the PCDD/Fs was 0.54–27.86 pg TEQ/g, and the mean was 8.50 pg TEQ/g ww; the TEQ range for the DL-PCBs was 0.60–1.18 pg TEQ/g ww, and the mean was 0.87 pg TEQ/g ww. The TEQs of the PCDD/Fs and DL-PCBs in the crabs from three farms (MH, RJ, and NXC) were higher than the standard limits (3.5 pg/g for PCDD/Fs and 6.5 pg/g for the total PCDD/Fs and dl-PCBs) [27]. The TEQs of the PCDD/Fs and DL-PCBs in the farm crab samples are shown in Supplementary Table S7.
