*2.1. Study Sites and Sample Collection*

The sampling sites selected for this study are located in Bohai Bay, which is situated in the west of the Bohai Sea, in northeast China (Figure 1). The four sampling sites were as follows: P0 (117.935◦ E, 39.0415◦ N), P1 (117.626544◦ E, 38.852927◦ N), P2 (117.912634◦ E, 39.304451◦ N) and P3 (117.964958◦ E, 39.264878◦ N). There were three types of samples, including intestinal tracts of *C. semilaevis* (A), sea water (B) and soil (C), collected from the natural sea area or fish farms. Fish samples A0–A3 (*C. semilaevis*) were intestinal tracts collected using sterile plastic bags, followed by separation with sterile forceps and homogenization in a sterile mortar with liquid nitrogen. A volume of 5 L of water samples so-called B0–B3 were collected from 30 cm below the surface with a stainless steel sampler, and then transferred to sterile polyethylene bottles. For the extraction of DNA, 200 mL of each sample B was filtered with vacuum filtration apparatus with a sterile membrane filter (0.22 μm pore diameter), which was aseptically stored at −80 ◦C. About 100 g of dry soil samples C0–C3 was collected from a depth of 0–10 cm with the help of a grab sampler and gently mixed using sterile polyethylene bottles. For the fish farm area, soil at the drain outlet was sampled. Soil samples were then freeze-dried, ground with a mortar and sieved through a 100-mesh screen. All samples were collected in three replicates, rapidly stored in a portable ice box, which were then transported to the laboratory and the pretreatment was performed within 1 day (Table 1).

**Figure 1.** Map of the sampling location and sampling sites: The four sampling sites were P0 (117.935◦ E, 39.0415◦ N), P1 (117.626544◦ E, 38.852927◦ N), P2 (117.912634◦ E, 39.304451◦ N) and P3 (117.964958◦ E, 39.264878◦ N) in the west of the Bohai Sea, in northeast China.


**Table 1.** Samples from different sites.
