Abstract Hatchery reared Atlantic salmon ( Salmo salar L.) smolts (mean start weight 80 g) in soft freshwater were exposed to three levels of carbon dioxide partial pressure for 38 days (25 days for the high group) in an open flow system: 0.5 mm Hg (1.8 mg/l CO 2 ; pH 6.6; control group), 2.7 mm Hg (9.3 mg/l CO 2 ; pH 6.0; medium group) and 5.7 mm Hg (19.4 mg/l CO 2 ; pH 5.7; high group). The different pH levels were a consequence of the addition of carbon dioxide to the water. Water temperature was 6–8 °C and effluent oxygen levels above 9 mg/l. During the experiment, the total aluminium concentration in the water was relatively stable in the range 115–140 μg/l. Labile Al concentration was low in all groups (about 10 μg/l). Despite this, the mean gill aluminium deposition was five times and seven times higher in the medium and high carbon dioxide groups, respectively, compared with the control group ( p p 2 were focal to diffuse hypertrophy and hyperplasia of chloride cells and adhesion of lamellae. Even though no significant differences were found in mean plasma cortisol concentrations in the medium group compared to the control group at any sampling time, mean plasma chloride and gill Na + ,K + -ATPase activity in the medium group were lower than controls on day 19 ( p p Days 2 and 19 mean plasma cortisol and hematocrit were significantly increased in the high group, and mean plasma chloride concentration was significantly reduced compared to the control group. There was a significant depression in gill Na + ,K + -ATPase activity between days 2 and 19 ( p 2 group, to a level of approximately 30% of control levels. The specific growth rate for this group was negative (−0.25%/day) between days 3 and 38. Accumulated mortality in the high group was 42% after 25 days and the experiment was discontinued for ethical reasons. The present investigation has shown that the build-up of carbon dioxide, reduction in pH and the changes in Al chemistry may be detrimental to Atlantic salmon smolts even at low concentrations. Further investigations are, however, needed to study combined effects of aluminium, pH and carbon dioxide and effects of each parameter in isolation on Atlantic salmon smolts.

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