WATER and environment chiefs have moved to allay fears that the discovery of "sex change fish" in a North Yorkshire river could pose a threat to human fertility.

Male fish with both male and female reproductive systems have been found in the River Ouse in the county.

But it is it is not thought that the high levels of oestrogen which have created this problem will affect humans.

The sexuality of fish in Britain's lowland rivers is changing because of hormones present in the urine of women taking the contraceptive pill.

The Environment Agency has confirmed that it has commissioned a report which reveals high levels of oestrogen in rivers throughout the country, including the Ouse in North Yorkshire and the Aire and Calder in West Yorkshire. This report will be published later this month.

The research, financed by the Department of Environment, Food and Rural Affairs and the Natural Environment Research Council studied roach from ten rivers, including the Ouse, over the past five years.

Some environmentalists fear that the hormone - which has a negative effect on the fertility of fish, causing some to be sterile or have damaged sperm - could make its way into the country's water supply. But Yorkshire Water was adamant that the chemical will not affect drinking water taken from rivers.

A spokesman from Yorkshire Water said: "The whole industry is looking at this along with the Environment Agency. Research done in the UK and abroad shows that existing water treatment processes are effective in removing these substances should they enter any source used as a water supply."

A spokesman from the Environment Agency was also keen to reassure people that while there is an impact on fish populations in the region, there is no evidence there will be an affect on humans.

He said: "There has been research done which has shown conclusively that these endocrine disrupting substances are not present in drinking water. According to this information, it shouldn't have any effect on human fertility."

He added: "While there is an impact on fish, it is not dramatic. Fish populations aren't going to be wiped out overnight."

Updated: 11:29 Monday, March 18, 2002