[Pharmwaste] Pollution can change sex ratio among fish

Tenace, Laurie Laurie.Tenace at dep.state.fl.us
Wed Jun 7 11:54:13 EDT 2006


So maybe it's NOT always (or just) the pharmaceuticals...as if this issue
isn't already confusing enough! - Laurie

Pollution can change sex ratio among fish
Wed Jun 7, 2006 11:05 AM BST
By Tan Ee Lyn

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

In a three-and-a-half 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 water that was depleted of oxygen.

The other half was 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 in places where
salt and fresh waters meet, though it is now also caused by pollution in many
parts of the world.

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

This is the first study ever to suggest that hypoxia can affect sex
development, differentiation and ratio in any animal species. It was
published in the May issue of the journal Environmental Science & Technology.

>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, Wu said, four genes that are responsible for
regulating sex hormones in fish become suppressed. 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," Wu said.

Such a development is understandable because oxygen deprivation threatens
survival, and functions not directly related to survival, such as those
regulating sex hormones, may be brought down to a minimum to conserve energy.

"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," Wu

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

Laurie J. Tenace
Environmental Specialist
Florida Department of Environmental Protection
2600 Blair Stone Road, MS 4555
Tallahassee, Florida 32399-2400
PH: (850) 245-8759
FAX: (850) 245-8811
Laurie.Tenace at dep.state.fl.us  
view our mercury web pages at: 

Please Note:  Florida has a very broad public records law.  Most written
communications to or from state officials regarding state business are public
records available to the public and media upon request.  Your e-mail is
communications and may therefore be subject to public disclosure.


More information about the Pharmwaste mailing list