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).