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

Atlantic salmon (Salmo salar L.) smelts (mean weight 53 g) in bicarbonate r ich freshwater were exposed to three levels of carbon dioxide partial press ure for 62 days in an open flow system: 2 mm Hg (control), 5 mm Hg (medium) and 9 mm Hg thigh). Water temperature was 3-7 degrees C and oxygen levels above 9 mg l(-1). All groups were transferred to normal seawater (34 parts per thousand salinity) on day 63. The seawater period lasted for 123 days. After 3 days, a transient but significant increase in the mean plasma corti sol concentration of the high carbon dioxide group (32 mg l(-1); P-CO2 = 9 mm Hg) was observed. The respiration frequency of the high CO2 group was si gnificantly increased during the freshwater period from days 0 to 58. The r espiration frequency of the medium group (19 mg l(-1); P-CO2 = 5 mm Hg) was intermediate between the control (7 mg l(-1); P-CO2 = 2 mm Hg) and the hig h group. There were no significant differences in plasma glucose or haemogl obin levels between groups. Haematocrit levels of smelts in the high CO2 gr oup were slightly increased after 3 days and significantly increased after 25 days, compared to the control group, After 61 days, however, no signific ant differences between groups were found. The mean plasma chloride concent rations of smelts in the medium and high CO2 groups were significantly redu ced after both 3 and 61 days of exposure. After 62 days, there were no sign ificant differences in mean fish length between the groups. The mean weight was reduced, although not significantly in the high group. The mean condit ion factors in the medium group and high groups (5 and 9 mm Hg, respectivel y) were though significantly reduced. Nephrocalcinosis was observed in smel ts in all groups, including the control group, after 62 days in freshwater. The prevalence of fish showing nephrocalcinosis was significantly higher i n the high CO2 group compared to the control and medium CO2 groups. Cumulat ive mortality in the control group during the freshwater period (62 days) w as 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 CO2 group were significantly greater and the co ndition factor was significantly reduced, compared to the control group. Ne phrocalcinosis was only observed in one fish. Mortality during the seawater period was low for all groups. (C) 1999 Elsevier Science B.V. All rights r eserved.