(A) Salinity changes regime for FW and SW tanks. The FW tanks were kept in constant freshwater (0.1 ppt, gray triangles), while in SW tanks, salinity was increased in a stepwise manner every day (black squares), by addition of increasing amounts of sea salt. Salinity increase steps were of 7 ppt/day, except for the first day when the increase was of 3 ppt. (B) Opercular ventilation rate (OVR) in experimental tanks, measured as opercular beats/minute and expressed as the daily mean per tank (n = 7). The dotted line represent the 25% increment selected as experimental end point. Asterisks denote significant differences relative to the FW0 per day (P<0.05). Top: FW0, fresh water; FW0.5, FW plus 0.5 mg·L−1 CuO; FW5, FW plus 5 mg·L−1 CuO; Bottom: SW0, increasing salinity; SW0.5, SW plus 0.5 mg·L−1 CuO; SW5, SW plus 5 mg·L−1 CuO. n×tank = 7.