We conducted several seasonal aquarium experiments to determine survival and body chemistry changes of Atlantic salmon (Salmo salar) after 24‐h exposures to salinities of 0, 16.5, 33, and 40‰ at 10°C. Fish transferred directly to test aquaria from an ozonated, 100‰ closed water‐reuse system at 18°C were compared with fish acclimated to single‐use water at 10°C for 1–3 weeks before salinity challenges in solutions of sea salt. In the initial (December and January) challenges, all Atlantic salmon from the closed system with a fork length of at least 200 mm survived exposure to 16.5 and 33‰ salinity, but not 40‰; fish 135 mm long or shorter did not survive exposure to 33‰ salinity. In later experiments, prior acclimation of fish to single‐use water at 10°C helped protect them against exposure to high salinity (i.e., 33 and 40‰) at 10°C. Few fish that were moved directly from the water‐reuse system into test aquaria lived for 24 h in 33 or 40‰ salinity. Atlantic salmon contained less water at 33‰ than at 16.5‰ or in fresh water, but carcass water content did not differ within salinity treatments between fish previously held at 18°C and those acclimated to 10°C and single‐use water. At each salinity, serum sodium concentration tended to rise less in those fish acclimated to the singleuse system than in those transferred directly from the reuse system to the aquaria. The exposure of Atlantic salmon to an elevated acclimation temperature (18°C) in the water‐reuse system exacerbated the disruption of their ionic exchange at exposures to 33 and 40‰ sea salt. A trace of copper (27 μg/L) in the reused water also may have reduced the salinity tolerance of these fish.
