In some sections of European rivers, up to 100% (!) of the male fish population has begun to show expression of enzymes normally found only in females, and even begun generating eggs and female sexual tissue. Studies of this effect, which is present in North America as well, began in about 1995, and remain ongoing. While the cause is more-or-less understood, the long term effects on the fish population and the animals which feed on it are not.

The effect was first discovered accidentally, when researchers were looking at fish different distances downstream from industrial and waste treatment sites. They noticed that within range of both of them, many males were carrying eggs along with their allotment of sperm. To standardize the testing, British researchers John Sumpter and Susan Jobling decided to examine bloodstream levels of a protein named vitellogenin, which is responsible for making the yolk material for eggs, and is ordinarily only expressed in the presence of estrogen. They found that male trout, when caged near sewage treatment plants, showed highly elevated vitellogenin levels within two or three weeks of being placed.

Next, the researchers tested industrial chemicals with trout in a laboratory, to see which, if any, would stimulate production of the protein. They found quite a few industrial chemicals (octylphenol and nonylphenol, bisphenol-A, the banned pesticide DDT, and a variety of PCBs) which did just that, and did so in a dose-dependent manner. In analysis, they also found that at the levels and combinations found in English rivers could easily cause the observed effects. Also, because the chemicals are fat soluble, they tend to build up in the flesh of the fish, so not only will the fish tend to feminize in the long term at low exposure, they will pass the chemicals on up through the food chain.

Meanwhile, it was discovered in the United States that pollutants weren't solely responsible for the changes, but birth control products were as well. Medical research on the various kinds of oral contraceptives showed that when the body metabolized these estrogen-like chemicals, their molecular structure was modified to become biologically inert before excretion. However, in 2000, laboratory work showed that in conditions common to waste treatment plants could strip the modification from the molecules, making them true estrogens again. All that remained was for the British researchers mentioned above to show that these un-modified chemicals could affect fish sexuality, which they did, even at the unbelievably small level of one part-per-trillion.

Besides the worry of these various chemicals causing illness, being carcinogenic, and so forth, the researchers indicated that since they could imitate estrogen in one kind of animal, they could probably do so in others. The ramifications of, essentially, a global (due to movement through the food chain) raise in estrogen levels are completely unknown.

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