Bioenergetics and RNA/DNA ratios in the common carp (Cyprinus carpio) under hypoxia

Hypoxia caused by eutrophication occurs over large areas in aquatic systems worldwide. Common carp (Cyprinus carpio) exposed to hypoxia (1 mg · O₂ · 1^-1 and 2 mg · O₂ · 1^-1) for 1 week showed a significant reduction in feeding rate, respiration rate, faecal production and nitrogenous excretion compared to those maintained at normoxia (7 mg · O₂ · 1^-1). Fish exposed to hypoxia showed negative scope for growth (SfG), but no significant difference in the specific growth rate was revealed after 1 week in both hypoxic groups. A significant reduction in RNA/DNA ratio was, however, clearly evident in the white muscle of the 1 mg · O₂ · 1^-1 treatment group, but not in the 2 mg · O₂ · 1^-1 treatment group. Both specific growth rate and RNA/DNA ratio were significantly reduced when fish were exposed to severe hypoxia (0.5 mg · O₂ · 1^-1) for 4 weeks. At all levels of hypoxia, growth reduction was accompanied by a significant decrease in RNA/DNA ratio in white muscle. Covariance analysis showed no significant difference between the slope of RNA/DNA ratio and growth rate under normoxic conditions and 0.5 mg · O₂ · 1^-1 for 4 weeks (F = 1.036, P > 0.326), as well as 1.0 mg · O₂ · 1^-1 and 2.0 mg · O₂ · 1^-1 for 1 week (F = 0.457, P > 0.5), indicating that the RNA/DNA ratio serves as a biomarker of growth under all oxygen levels, at least under controlled experimental conditions. SfG also appears to be more sensitive than the RNA/DNA ratio in responding to hypoxia in fish.