Effects of carbon dioxide on Atlantic salmon ( Salmo salar L.) smolts at constant pH in bicarbonate rich freshwater

Abstract

Atlantic salmon ( Salmo salar L.) smolts (mean weight 53 g) in bicarbonate rich freshwater were exposed to three levels of carbon dioxide partial pressure for 62 days in an open flow system: 2 mm Hg (control), 5 mm Hg (medium) and 9 mm Hg (high). Water temperature was 3–7°C and oxygen levels above 9 mg l −1. All groups were transferred to normal seawater (34‰ salinity) on day 63. The seawater period lasted for 123 days. After 3 days, a transient but significant increase in the mean plasma cortisol concentration of the high carbon dioxide group (32 mg l −1; P CO 2 =9 mm Hg) was observed. The respiration frequency of the high CO 2 group was significantly increased during the freshwater period from days 0 to 58. The respiration frequency of the medium group (19 mg l −1; P CO 2 =5 mm Hg) was intermediate between the control (7 mg l −1; P CO 2 =2 mm Hg) and the high group. There were no significant differences in plasma glucose or haemoglobin levels between groups. Haematocrit levels of smolts in the high CO 2 group were slightly increased after 3 days and significantly increased after 25 days, compared to the control group. After 61 days, however, no significant differences between groups were found. The mean plasma chloride concentrations of smolts in the medium and high CO 2 groups were significantly reduced after both 3 and 61 days of exposure. After 62 days, there were no significant differences in mean fish length between the groups. The mean weight was reduced, although not significantly in the high group. The mean condition factors in the medium group and high groups (5 and 9 mm Hg, respectively) were though significantly reduced. Nephrocalcinosis was observed in smolts in all groups, including the control group, after 62 days in freshwater. The prevalence of fish showing nephrocalcinosis was significantly higher in the high CO 2 group compared to the control and medium CO 2 groups. Cumulative mortality in the control group during the freshwater period (62 days) was 1.5%. The mortalities in the medium and high carbon dioxide groups were 4.6 and 7.7%, respectively. After 123 days in seawater, the mean weight and length of fish in the high CO 2 group were significantly greater and the condition factor was significantly reduced, compared to the control group. Nephrocalcinosis was only observed in one fish. Mortality during the seawater period was low for all groups.