Is it possible to fish under an area?
Male loss among fish due to human hormones
Exeter / Vienna - The year is 1960. In the USA, a drug called Enovid comes onto the market for the first time. It is the birth of the "pill" - the contraceptive that will change the world. The product contains artificial hormones and thereby inhibits ovulation. Human fertility becomes easily controllable.
Most birth control pills consist mainly of EE2, a synthetic estrogen derivative. Millions of women take it. Every day. However, after the active ingredient has fulfilled its function, it is excreted in the urine and discharged via the sewer system. In conventional sewage treatment plants, however, it is not possible to effectively remove EE2 from the wastewater. The substance finds its way into rivers, lakes and sometimes even into the groundwater. There, too, it is only slowly being degraded. In the environment, EE2 has a half-life of around 14 days. And that has serious consequences.
In European and North American waters, EE2 often occurs in concentrations of one nanogram per liter or more. Unfortunately, the molecules develop their effect even in such minimal quantities. As early as the 1990s, some British researchers sounded the alarm. They observed a strange feminization of male fish that lived downstream from sewage treatment plants. Egg cells increasingly formed in their gonads. Several species were affected.
The phenomenon is now known from various regions. "There are an enormous number of levels of feminization," reports the biologist Patrick Hamilton from the University of Exeter in an interview with the Standard. Some male fish even seem to have undergone a complete sex change.
However, such specimens cannot be precisely proven, explains Hamilton, because they can no longer be distinguished from normal females. The animals do not have sex chromosomes. "So we don't have a genetic test for masculinity." The scientists can only determine that there is a significant excess of females in some fish stocks.
When it comes to gender issues, however, there is a lot of confusion among fin bearers. Some species, such as the sea bream (Sparus aurata), a popular delicacy, are so-called protandric hermaphrodites - they spend their young years as males and transform into fish women as they mature. Environmental factors can also have a formative effect. At higher water temperatures, female sockeye salmon larvae (Oncorhynchus nerka) originally grow into males - a remarkable flexibility.
The advancing feminization of whole fish populations, however, is regarded by experts as a kind of ecological time bomb. In the absence of males, the affected populations could eventually collapse. But such a collapse has not yet been documented anywhere - although the pollution of the water with EE2 has persisted for decades. Apart from that, other hormonally active substances also find their way into lakes and rivers, including the notorious nonylphenols, which are found in textile dyes. Why aren't the consequences much more severe?
Hamilton is investigating this question with a few colleagues from Exeter and Brunel University. The team focuses on the feminization of Rutilus rutilus, in English roach, a species of fish that occurs frequently in large parts of Europe. In order to track down a possible population decline, the researchers took almost 1,800 roach from 39 different areas in six English river systems and subjected their genome to a genetic analysis.
The highlight: If reproduction in one or more of these populations is seriously impaired, genetic impoverishment should become visible there. It would also be conceivable that fish would migrate from more distant river sections, but such an influx could also be recorded in genetic comparisons. The examined waters show very different EE2 loads. The mean concentrations vary from undetectable amounts to 11.6 nanograms per liter.
The study, details of which in the specialist journal BMC Biology published shows a surprising result. Despite the widespread feminization of male roaches, there is virtually no evidence of decreased reproduction. Immigration does not seem to play an essential role in this species either - at least not in the sampled English rivers, which are often interrupted by weirs. Scientists are puzzled. "Maybe it doesn't take that many males to maintain a population," says Hamilton. And many partially feminized males still appear to be able to reproduce.
There are probably other factors at play as well. In some fish species, there is sometimes a strong reproductive dominance of larger, older specimens. This could possibly be compensated for in the case of hormonally impaired roaches. Feminization is an ongoing process, explains Hamilton. If a few dominant males turn into females as a result, the younger generations would have a better chance of offspring. This would stabilize both genetic diversity and the reproductive success of the entire population.
The British experts have other points in their sights. The environmental concentrations of artificial hormones are subject to considerable periodic fluctuations, especially depending on precipitation and the resulting flow rates in the rivers. The more rain, the greater the dilution. The duration and intensity with which the fish are actually exposed to EE2 could therefore play a decisive role, as could a different, hereditary susceptibility. "We're taking a closer look at that at the moment," says Hamilton.
Poison for embryos
However, there can be no all-clear regarding EE2. The substance is apparently also toxic to fish embryos. Swiss researchers have tested the effect of the artificial hormone on the fertilized eggs of whitefish species of the Coregonus genus. "There are developmental disorders and increased mortality from just one nanogram per liter," reports Claus Wedekind from the University of Lausanne.
However, there are also clear individual differences here. The offspring of some fish show a certain tolerance towards EE2. This is genetically determined, as the scientists found in genetic analyzes that they found in Evolutionary Applications (Online pre-release) published.
The whitefish thus have the evolutionary potential to adapt to the hormone in the environment, says Wedekind. How strongly this ability can develop in natural populations in the future remains to be seen for the time being. (Kurt de Swaaf, DER STANDARD, November 12, 2014)
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