Ocean dead zones trigger sex changes in fish,
posing extinction threat
WASHINGTON, March 29 Oxygen depletion in the worlds 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 Web site of the American Chemical Society
journal, Environmental Science & Technology. The study is
scheduled to appear in the May 1 print issue of the journal.
The finding, by Rudolf Wu, Ph.D., and colleagues at the City
University of Hong Kong, raises new concerns about vast areas of
the worlds 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.