Hypoxia inhibits fish spawning via LH-dependent final oocyte maturation

To evaluate the effects of long term hypoxia exposure on fish spawning, mature common carp, Cyprinus carpio carpio (Linnaeus) were subjected to either normoxia (7.4 ± 0.2 mgO2mg O2 L− 1) or hypoxia (1.0 ± 0.2 mgO2O2 L− 1) for more than two months. Gonadosomatic index (GSI), and concentrations of serum luteinizing hormone (LH), testosterone (T), and estroldiol (E2) were measured and gonad histology examined. Hypoxia inhibits fish spawning even though the gonad and oocytes developed under hypoxia exposure. LH levels of female carp were significantly decreased upon chronic exposure to hypoxia, and the final oocyte maturation in hypoxic females was significantly retarded. The results indicated that hypoxia may inhibit fish spawning through LH-dependent final oocyte maturation. In addition, no courtship was observed in hypoxic males. In conclusion, hypoxia impairs fish ovulation and, therefore, spawning and reproduction. LH levels were reduced leading to a failure of oocyte maturation. This, along with a lack of courtship by males may be the major mechanisms involved in hypoxic inhibition of reproduction in carp.