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

Abstract

Hypoxia caused by eutrophication occurs over large areas in aquatic systems worldwide. Common carp (Cyprinus carpio) exposed to hypoxia (1 mg · O₂ · l⁻¹ and 2 mg · O₂ · l⁻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 · l⁻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 · l⁻¹ treatment group, but not in the 2 mg · O₂ · l⁻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 · l⁻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 · l⁻¹ for 4 weeks (F=1.036, P > 0.326), as well as 1.0 mg · O₂ · l⁻¹ and 2.0 mg · O₂ · l⁻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.