Gulf of Mexico Dead Zone Severely Impairs Reproduction in Atlantic Croaker, Researchers Find

June 14, 2011

AUSTIN, Texas — Atlantic croaker living in the large Gulf of Mexico "Dead Zone" exhibit severe reproductive impairment with potential long-term impacts on the fish's population abundance, researchers from The University of Texas at Austin's Marine Science Institute have found.

Males and females were found to produce dramatically fewer sperm and eggs. In addition, females in the hypoxic Dead Zone waters were masculinized — some 20 percent actually produced sperm in their ovaries. The sex ratio was also heavily skewed toward males in the hypoxic area.

Hypoxic Dead Zone leads to smaller ovaries

Top: Ovaries in Atlantic croaker are smaller if the fish are found in the Dead Zone's hypoxic waters. Bottom: Female croaker were found to produce sperm in hypoxic water.

"This is the first evidence of impairment of reproductive output across a wide region in a fish population inhabiting a hypoxic coastal zone," says Peter Thomas, professor of marine science.

Thomas and his coauthor, marine scientist Saydur Rahman, published their research in Proceedings of the Royal Society B.

The occurrence of low oxygen (hypoxic) waters in coastal regions worldwide has increased dramatically over the past 25 years, but its impact on marine animals is just beginning to be understood.

The northern Gulf of Mexico "Dead Zone," off the coast of Louisiana, is the second largest seasonal hypoxic zone in the world, averaging 6,447 square miles annually. Coastal areas affected by hypoxia are often economically and environmentally important for maintaining fish populations and fisheries.

Thomas and Rahman collected and studied fishes from sites throughout the northern Gulf of Mexico hypoxic zone and compared them with fishes collected from areas with normal oxygen levels.

Masculinization of the female croaker and other reproductive problems were associated with declines in neuroendocrine function and ovarian and brain expression of an enzyme called aromatase that converts androgens (male sex hormones) to estrogens (female sex hormones). This was found to be the case in field and laboratory experiments.

"Our results show that ovarian masculinization is a specific response to hypoxia and is due to decreased aromatase activity," Thomas says.

For more information, contact: Peter Thomas,, 361-749-6768.

4 Comments to "Gulf of Mexico Dead Zone Severely Impairs Reproduction in Atlantic Croaker, Researchers Find"

1.  Sandy Carter said on June 23, 2011

If the female ovary has both egg and sperm can it self-fertilize?

2.  Jerry Salaiz said on June 23, 2011

What exactly is causing the decreased aromatization. How is the hypoxia connected and what causes these hypoxic areas to form? I believe that our society right now is suffering from the opposite overall effects, that is, increased aromatization. Toxins in our water, food supply, and atmosphere are causing early development of females and female characteristics in males. I believe some studies have already shown that many substances in our society currently mimic the hormone estrogen and their proliferation is affecting us all. In men, one of the effects is gynecomastia or breast development. Further research by you in this area could lead to answers for reversing effects we have caused in our overall environment. Perhaps the simple answer is oxygen?

3.  Laura Shaw said on June 23, 2011

Mr. Salaiz, it would be great if the problem could be solved simply by ensuring better oxygen supplies, but the problems are more complex than this. The dead zones are created by nitrogen and phosphorous, found in commercial fertilizers, run-off from the Mississipi River into the Gulf, primarily. People use fertilizers on the land, excess is washed into smaller water conveyances, these empty into rivers, the rivers empty into the Gulf. The fertilizer stimulates algae growth, the algae consumes all the oxygen in the area, then die off, leaving enormous areas with not enough oxygen to sustain marine life.

The problem of hormonal disruptions in land animals, including human beings, reptiles, amphibians, and other species, is caused by other chemicals in the food chain that get into the environment, and from there into the food chain. The addition of bovine growth hormone to commercially prepared milk is one example of a hormone disrupter consumed by people that is causing accelerated growth and sexual maturity in children, among other consequences.

There is now five times the biochemical toxins in our environment as were needed in laboratory conditions to cause some frog species to change from male to female, just to provide one example.

I would love to see some research into the question of whether some of these environmental hormonal disrupters, especially those related to the female hormone estrogen, may be contributing to the obesity epidemic, since female hormones promote fat (androgens, male hormones, promote a leaner body with less fat).

4.  Joanna Lowry said on June 23, 2011

Has damming up the rivers, Mississippi, etc. impacted the oxygen content in the waters of the Gulf. Is there a map available of the 'dead zone'?

jelowry '62