HONG KONG - A lack of oxygen in highly polluted waters can sharply alter the sex ratio among fish, resulting in far more males than females, which could result in extinction, a study has found.

In a 3 1/2-year study, researchers at the City University of Hong Kong used more than 10,000 embryos of zebra fish - a small, hardy freshwater species - and raised half of them in oxygen-depleted water. The other half were raised in normal, oxygenated water.

Hypoxia, or oxygen depletion, occurs when there are less than two parts of oxygen for every million parts of water. It occurs naturally where salt and fresh waters meet, though it is now also caused by pollution.

"We found that whereas 61 per cent of zebra fish spawned into males under regular oxygen conditions, under hypoxic conditions, the number of males increased to 75 per cent," said chair of biology Rudolf Wu.

This is the first study to suggest that hypoxia can affect sex development, differentiation and ratio in any animal species.

From the start, fertilised zebra fish embryos have gonads that look like ovaries, whether they are genetically male or female. Sex differentiation only begins 23 to 25 days later and the ovaries will continue to develop in about half of the embryos, while testicles replace ovaries in the remaining.

That process is completed only by day 42.

But when faced with hypoxia, four genes that are responsible for regulating sex hormones in fish become suppressed, Professor Wu said. These genes control the enzyme aromatase, which is normally responsible for converting the male hormone testosterone into the female hormone estradiol.

"When the genes are suppressed, you've got less enzyme converting testosterone into estradiol. So you get an accumulation of testosterone, the male hormone. That directs the fish to turn into a male," Professor Wu said.

"When a fish faces hypoxia, it tries to downregulate [suppress] its metabolism altogether in order to conserve its energy for survival ... building body muscle will be downregulated, and then they reduce feeding."

While it is not known if this phenomenon has depleted fish stocks, Professor Wu stressed the need to counter pollution and hypoxia by reducing the dumping of human and industrial waste in the sea.