Ocean 'dead zones' trigger sex changes in fish, posing extinction threat

Mar 29, 2006
Ocean water

Oxygen depletion in the world’s oceans, primarily caused by agricultural run-off and pollution, could spark the development of far more male fish than female, thereby threatening some species with extinction, according to a study published today on the website of the American Chemical Society journal, Environmental Science & Technology.

The finding, by Rudolf Wu, Ph.D., and colleagues at the City University of Hong Kong, raises new concerns about vast areas of the world’s oceans, known as "dead zones," that lack sufficient oxygen to sustain most sea life. Fish and other creatures trapped in these zones often die. Those that escape may be more vulnerable to predators and other stresses. This new study, Wu says, suggests these zones potentially pose a third threat to these species — an inability of their offspring to find mates and reproduce.

The researchers found that low levels of dissolved oxygen, also known as hypoxia, can induce sex changes in embryonic fish, leading to an overabundance of males. As these predominately male fish mature, it is unlikely they will be able to reproduce in sufficient numbers to maintain sustainable populations, Wu says. Low oxygen levels also might reduce the quantity and quality of the eggs produced by female fish, diminishing their fertility, he adds.

In their experiments, Wu and his colleagues found low levels of dissolved oxygen — less than 2 parts per million — down-regulated the activity of certain genes that control the production of sex hormones and sexual differentiation in embryonic zebra fish. As a result, 75 percent of the fish developed male characteristics. In contrast, 61 percent of the zebra fish spawn raised under normal oxygen conditions — more than 5 parts per million — developed into males. The normal sex ratio of zebra fish is about 60 percent male and 40 percent female, Wu says.

"Reproductive success is the single most important factor in the sustainability of species," Dr. Wu says. "In many places, the areas affected by hypoxia are usually larger than the spawning and nursery grounds of fish. Even though some tolerant species can survive in hypoxic zones, they may not be able to migrate out of the zone and their reproduction will be impaired."

Hypoxia is considered one of the most serious threats to marine life and genetic diversity, Wu says. It occurs when excessive amounts of plant nutrients, particularly nitrogen, accumulate in oceans, freshwater lakes and other waterways. These nutrients trigger the growth of huge algae and phytoplankton blooms. As these blooms die, they sink to the ocean floor where they are decomposed by bacteria and other microorganisms. Decomposition depletes most of the oxygen in the surrounding water, making it difficult for marine life to survive.

Although some hypoxic areas — dead zones — develop naturally, scientific evidence suggests in many coastal areas and inland waters, hypoxia is primarily caused by agricultural run-off (particularly fertilizers) and discharge of domestic and industrial wastewaters.

Dead zones are developing along the coasts of the major continents, and they are spreading over larger areas of the sea floor, Wu says. The United Nations Environmental Programme estimates nearly 150 permanent and recurring dead zones exist worldwide, including 43 in U.S. coastal waters.

In the Gulf of Mexico, for instance, a dead zone the size of New Jersey, some 7,000 square miles, develops each summer. Other affected areas of the United States include coastal Florida and California, the Chesapeake Bay and Long Island Sound.

Source: American Chemical Society

Explore further: SpaceX making Easter delivery of station supplies (Update 2)

add to favorites email to friend print save as pdf

Related Stories

Efficient operation of wastewater treatment plants

Apr 08, 2014

The German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt) announces the launch of a groundbreaking project for which the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) is developing sophisticated ...

Recommended for you

NASA's space station Robonaut finally getting legs

8 hours ago

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Quest for extraterrestrial life not over, experts say

Apr 18, 2014

The discovery of an Earth-sized planet in the "habitable" zone of a distant star, though exciting, is still a long way from pointing to the existence of extraterrestrial life, experts said Friday. ...

Sun emits a mid-level solar flare

Apr 18, 2014

The sun emitted a mid-level solar flare, peaking at 9:03 a.m. EDT on April 18, 2014, and NASA's Solar Dynamics Observatory captured images of the event. Solar flares are powerful bursts of radiation. Harmful ...

Impact glass stores biodata for millions of years

Apr 18, 2014

(Phys.org) —Bits of plant life encapsulated in molten glass by asteroid and comet impacts millions of years ago give geologists information about climate and life forms on the ancient Earth. Scientists ...

The importance of plumes

Apr 18, 2014

The Hubble Space Telescope is famous for finding black holes. It can pick out thousands of galaxies in a patch of sky the size of a thumbprint. The most powerful space telescope ever built, the Hubble provided ...

User comments : 0

More news stories

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Cosmologists weigh cosmic filaments and voids

(Phys.org) —Cosmologists have established that much of the stuff of the universe is made of dark matter, a mysterious, invisible substance that can't be directly detected but which exerts a gravitational ...

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

Filipino tests negative for Middle East virus

A Filipino nurse who tested positive for the Middle East virus has been found free of infection in a subsequent examination after he returned home, Philippine health officials said Saturday.

Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...

Airbnb rental site raises $450 mn

Online lodging listings website Airbnb inked a $450 million funding deal with investors led by TPG, a source close to the matter said Friday.