Widespread endocrine disruption and reproductive impairment in an estuarine fish population exposed to seasonal hypoxia.

The long-term effects on marine fish populations of the recent increase worldwide in the incidence of coastal hypoxia are unknown. Here we show that chronic environmental exposure of Atlantic croaker (Micropogonias undulatus) to hypoxia in a Florida estuary caused marked suppression of ovarian and testicular growth which was accompanied by endocrine disruption. Laboratory hypoxia studies showed that the endocrine disruption was associated with impairment of reproductive neuroendocrine function and decreases in hypothalamic serotonin (5-HT) content and the activity of the 5-HT biosynthetic enzyme, tryptophan hydroxylase. Pharmacological restoration of hypothalamic 5-HT levels also restored neuroendocrine function, indicating that the stimulatory serotonergic neuroendocrine pathway is a major site of hypoxia-induced inhibition. Inhibition of tryptophan hydroxylase activity to downregulate reproductive activity could have evolved as an adaptive mechanism to survive periodic hypoxia, but in view of the recent increased incidence of coastal hypoxia could become maladaptive and potentially affect fish population abundance and threaten valuable fishery resources.

Reproductive endocrine dysfunction in Atlantic croaker exposed to hypoxia.

Although there is extensive evidence for impaired endocrine function in fishes exposed to environmental chemicals, information is currently lacking on reproductive endocrine effects of other environmental stressors such as hypoxia. The effects of ten weeks exposure to low dissolved oxygen (DO: 2.7 ppm and 1.7 ppm) on reproductive morphometric and endocrine responses in female Atlantic croaker (Micropogonias undulatus) were investigated in controlled laboratory studies, and compared to the effects observed in fish collected from hypoxic sites in Mobile Bay, Alabama. Exposure of croaker to moderate hypoxia during ovarian recrudescence, both in the laboratory and at the field sites, caused significant impairment of ovarian growth as well as decreased production of fully grown oocytes, resulting in dramatic reductions in the number of viable gametes (fecundity). Ovarian dysfunction was associated with significant decreases in endocrine indicators of the estrogen signaling pathway regulating production of vitellogenin, the yolk protein precursor sequestered by the growing oocytes. The results indicate that reproductive morphometric and endocrine biomarkers in croaker are sensitive to moderate hypoxia, and are potentially useful as early warning indicators of reproductive failure.