The entire male fish population of some European rivers show feminising effects from so-called "gender-bending" chemicals, according to new research.

Freshwater fish in five out of seven northern European countries surveyed so far showed some signs of exposure to the chemicals, which mimic female hormones and are present in sewage effluents.

"We are finding this problem right across northern Europe, it is clearly widespread," Professor Alan Pickering of the Natural Environment Research Council's Centre for Ecology and Hydrology, UK, said.

He was addressing the British Association's Festival of Science in London.

Male and female

"It seems to relate to a mixture of chemicals both industrial and also some of the natural excretory products from the human body."

Symptoms of exposure to the chemicals, known as endocrine-disrupters, ranged from relatively minor effects to, in the worst cases, fish developing both male and female reproductive organs.

In some places, such as the River Aire, Yorkshire, UK, 100% of male fish were found to show evidence of feminisation.

Professor Pickering said it was still not clear whether the chemicals were having a direct impact on the ability of fish to reproduce normally. But in one UK river at least, he said, detrimental effects had been established.

Food chains

This has raised concern that predators that eat the fish, such as otters and kingfishers, could also be at risk.

"We don't know how serious an environmental issue this is, " Professor Pickering said. "If there is a serious impact on fish populations, we should be worried about it."

In a separate presentation at the festival, it was revealed that fish might face a threat from global warming.

Climate research predicts that rivers may become too warm for the brown trout, while habitat for cold-water lake fish, such as the charr, may be reduced.

Oxygen declines

The evidence comes from scenarios for future climate change commissioned by the UK Climate Impacts Programme.

These forecast an increase in temperature of between 0.6 and 1.4C by the 2020s.

If these predictions are realised, increases in water temperature, and the subsequent decline in water oxygen, threatens some valued fish populations.

Professor Nigel Arnell of Southampton University, UK, carried out the research.