Experimental design and water sampling
We conducted tank experiments and field sampling at the Maizuru Fisheries Research Station (MFRS) of Kyoto University, Japan, which is located in front of Maizuru Bay, in October 2020 (Fig. 1; Table S1). Two 60-L aliquots of rearing water were simultaneously transferred to two other tanks (sampling tanks) from a 200-L tank (stock tank), in which five Japanese jack mackerel individuals were kept (total length: approximately 20 cm). We then added 60 mL of BAC solution (Osvan S, Nihon Pharmaceutical Co., Ltd, Japan; 0.01% final concentration of BAC in the sampling tank) to one of the sampling tanks and thoroughly mixed the water. Subsequently, we collected four replicates of 1000 mL water samples using plastic bottles from both sampling tanks (defined as time 0). Subsequently, water sampling was performed 6, 12, 24, 48, 72, and 96 hours after time 0 (i.e., at time 6, 12, 24, 48, 72, and 96, respectively), during which the water temperature was also measured in the sampling tanks (Table S1). The sampling tanks were aerated by a pump and placed in a water bath to minimize fluctuations in water temperature throughout the experiment. After water collection, water samples were immediately filtered with a 47‐mm‐diameter GF/F glass microfiber filter (nominal pore size, 0.7 μm; GE Healthcare Life Science, UK). At each time point, 1000 mL distilled water was filtered as a negative filtration control. All filtered samples were kept at -20 °C until eDNA extraction.
In addition, we collected 18 1-L seawater samples using plastic bottles from a floating pier in the MFRS. Surface seawater temperature and salinity were 20.9 °C and 30.9‰, respectively. We added 1 mL BAC solution to nine of the collected seawater samples and thoroughly mixed them. Subsequently, we randomly collected three seawater samples with and without BAC addition and filtered them in the same manner as described above (defined as time 0). The remaining seawater samples were placed in a water bath at a constant temperature (18 ± 1°C). Further, after 6 and 24 hours (i.e., at time 6 and 24), we randomly sampled three seawater samples with and without BAC addition, respectively, and filtered them in the same manner as above. For each time point, 1000 mL distilled water was filtered as a negative filtration control. Throughout the experiments, we wore disposable gloves to collect and filter water samples and bleached the filtering devices (i.e., filter funnels [Magnetic Filter Funnel, 500 mL capacity; Pall Corporation, USA], 1 L beakers, tweezers, and sampling bottles) before every use in 0.1% sodium hypochlorite solution for at least 5 min (Yamanaka et al., 2017).