• Can Male Fish Lay Eggs?


Can Male Fish Lay Eggs?

Jul 17 2017

Common sense should dictate that by definition, Arnold Schwarzenegger and male fish have at least one thing in common – they can’t reproduce. But just as Arnie gave birth in the film Junior, male fish in rivers all across England are defying logic and convention by laying eggs.

The bizarre discovery was made by scientists the University of Exeter, who carried out investigations at 50 different test sites throughout the country. Though the exact reasons behind the phenomenon are not certain, it’s thought that excessive levels of oestrogen and progesterone – probably caused by the use of contraceptive pills – in the water is to blame.

A gender-bending study

Though it’s not the first study to highlight the sex-changing effects of oestrogen on marine life, it is the first one to take place in the UK. The University of Exeter study took samples of freshwater fish from 50 sites across England, including species such as roach, and found that one in five of them were exhibiting “feminised” traits and even laying eggs.

Most commonly, this behaviour manifested itself in less aggressive attitudes and reduced competitivity, which is thought likely to affect the fish’s ability to find a mate and procreate. Even more concerning, it’s believed that the offspring of the affected fish will be even more susceptible to having their behaviour and physiology affected by the chemicals, potentially resulting in a snowballing effect.

“We are showing that some of these chemicals can have much wider health effects on fish that we expected. Using specially created transgenic fish that allow us to see responses to these chemicals in the bodies of fish in real time, for example, we have shown that oestrogens found in some plastics affect the valves in the heart,” explained Professor Charles Tyler, keynote speaker on the subject.

“Other research has shown that many other chemicals that are discharged through sewage treatment works can affect fish including antidepressant drugs, which reduce the natural shyness of some fish species, including the way they react to predators.”

Chemicals to blame

An influx of oestrogen into rivers, streams and other bodies of water is the most likely culprit behind the transgender fish. Where that oestrogen comes from, however, is a more complicated matter.

One school of thought believes that abundant use of contraceptives among the female population in the UK is affecting the urine composition of those who take them, which in turn is being flushed out into the sewage system and finding its way into these fish. Others are suggesting that pills are being flushed themselves, though it seems more likely that the urine is the principal cause.

However, a recent study concluded that our waters are even more polluted than we thought and there are over 200 household products including cosmetics, plastic items and cleaning agents which are known to contain oestrogen in some capacity. Therefore, there are a whole variety of possible sources from whence the offending chemical could have come.

Professor Tyler presented his work at the 50th Anniversary Symposium of the British Fisheries Society, which took place earlier this month. It was the opening lecture among dozens of others about the various threats which fish face from humans and climate change.

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