Prior posts have described the scope and potential causes for the "dead zone" (low oxygen) in the Gulf of Mexico; the zone exists for part of the year, and its scope has been expanding over time. Nutrients (nitrogen, phosphorus) from various agricultural operations and waste treatment effluent have been tied into triggering the growth of organism that consume the oxygen in the waters. Prior posts have also noted the multistate program that is seeking to address this issue.
Many fish species in the Gulf are male by default; exposure to estrogen is required to transform the testes into ovaries. What has been found is that some of these fish show evidence of the ovaries reverting to testes, though the testes are often deformed and deficient. Laboratory experiments have demonstrated that an exposure to just ten weeks of hypoxic conditions can trigger these deformities. Laboratory analysis of the fish revealed that the masculinized female croakers, for example, had decreased levels of a key chemical found in the brain and ovaries called aromatase (see http://en.wikipedia.org/wiki/Aromatase). This enzyme regulates the production of the female sex hormone estrogen, which is critical for proper development of the ovaries. As such, it would appear that as the oxygen levels in the water decrease, this affects the brain and the neurohormones and neuropeptides that it produces, including aromatase.
Thus, the fish may be impacted by two different mechanisms. One is the presence of estrogenic chemicals in the treated sewage effluent, and the other is the impact on brain chemistry that arises from the low-oxygen environment in which the fish find themselves.
The report can be found at http://rspb.royalsocietypublishing.org/content/early/2011/05/25/rspb.2011.0529.short.