Metabolic capacities of common carp (Cyprinus carpio) following combined exposures to copper and environmental hypoxia

• Acute and chronic hypoxia increased aerobic scope but all Cu2+ treatments showed adverse effects.
• Hypoxia can act as a limiting stressor while Cu2+ can account as a loading stressor.
• Cu2+ exposures increased the number of mucus cell in gills, but hypoxia did not show the same.

In aquatic ecosystems, a decline in water O2 level is the main factor that can release heavy metal ions from top sediment layer. Therefore, hypoxia in turn, and in association with heavy metals might provide undesirable environment and impairs physiological functions of aquatic animals. To address this, metabolic capacities, including standard metabolic rate (SMR), maximum metabolic rate (MMR), aerobic scope (AS) and factorial aerobic scope (FAS) of common carp were determined following exposures to different levels of water-borne Cu2+ as well as hypoxia. Treatments for Cu2+ were included: 100% (acute), 50% (sub-lethal) and 10% (chronic) of LC50-96 h for immediately, 24 h and 7 days exposures respectively. Hypoxia treatments were assigned as acute for immediately, sub-lethal for 24 h and chronic for 7 days. Combined effects of treatments were also considered as acute Cu2++hypoxia, sub-lethal Cu2++hypoxia and chronic Cu2++hypoxia. While SMR of carp was reduced by chronic hypoxia, significant (P<0.05) increase was observed during acute hypoxia, as compared with control. The MMR and AS were significantly reduced (P<0.05) following all hypoxia treatments. The acute and chronic Cu2+ treatments showed significant (P<0.05) increases in SMR and MMR values. All acute and sub-lethal combined treatments showed significant (P<0.05) reductions in SMR, MMR and AS values, whilst chronic combined treatments showed generally increasing trends for MMR and AS. PCrit was relatively reduced following all treatments except for acute and sub-lethal Cu2+-treated fish that showed higher value (P<0.05) and no change respectively. Although all Cu2+ treatments increased the number of mucus cell, hypoxia treatments did not show any remarkable differences when compared with control group. In general, the results of present study reveal that hypoxia acts as limiting stressor whilst Cu2+ do act as loading stressors in the case of common carp metabolism. The interactive exposures mostly showing a synergist effect in all metabolic capacities with an exception for chronic treatments.