2.4. Frequency of air–surface respiration (ASR)
After a two-day pre-acclimation in the normoxic condition, fish
were transferred to acidic water and fresh water (control group),
respectively. This experiment was continued for 7 days, and a 45-min
video recording (DCR-HC 46, Sony, Japan) was conducted on days 0
(before transfer), 1, 2, 4, and 7 (n = 8). The video recording was
always taken between 10:00 and 16:00 h. The air–surface respiration,
ASR, was recorded when fish directly swallowed air on the water's
surface. The frequency of ASR was calculated by determining the
numbers of ASR events divided by the recording periods.