Oxygen starved fish more likely to be male

While a host of gender-bending chemicals in the environment may be affecting fertility and reducing the ratio of male to female fish, new research from China suggests that in water clogged with pollutants the swing has been reversed because of a lack of oxygen and more males are being spawned.

With up to three quarters of some species now hatching as male, there could be a serious threat of extinction, say researchers from the City University of Hong Kong.

Professor Rudolf Wu, director of the university’s Centre for Coastal Pollution and Conservation, said a recent discovery showed that oxygen depletion, or hypoxia, could lead to more male fish and an imbalance in the sex ratio.

Such a discrepancy would have a dramatic impact on the reproductive success of certain fish, and could mean population decline and possible extinction.

This is the first report to have linked hypoxia with sexual development.

Hypoxia in fish occurs when there are less than two parts per million of oxygen in water.

This can happen naturally in areas where fresh and salt water meet but more commonly is down to pollution from agriculture and sewerage.

In his study Prof Wu found that under normal conditions 61% of zebra fish spawned into males, but under hypoxic conditions that figure shot up to 75%.

“This discovery is very relevant to Hong Kong and the region because waters in Tolo Harbour, Deep Bay and Victoria Harbour as well as waters to the south of Hong Kong are often hypoxic, especially in the summer,” he said.

In order to counter this it would be necessary to reduce by-products of human development such as human waste, industrial pollution, and fertilizers from farming, said the professor.

Sam Bond