Pollution Found to Harm Fish in Lake Mead

November 19, 1996

 

Disturbing new evidence that chemical pollution can deform reproductive systems has turned up in fish in Lake Mead, one of the nation's most popular recreation spots and a major source of drinking water for Las Vegas and Southern California.

The results of federal research at the lake behind Hoover Dam and more than 20 sites around the country reinforce a growing body of science that indicates common chemical contaminants have interfered with hormonal systems in wildlife, feminizing males and generally wreaking havoc with sexual development in several types of animals.

Unlike most places where evidence of endocrine disruption in animals has shown up, Lake Mead is the centerpiece of a national recreation area that attracts 7 million to 8 million people a year. It is one of the nation's five most popular destinations under the jurisdiction of the National Park Service.

Although there is no known direct link between hormone disruption and risk to humans, the Park Service is expected to ask for more research after the study on so-called endocrine disrupters is officially released today in Washington at the annual meeting of the Society of Environmental Toxicology and Chemistry.

In the Lake Mead study, evidence of endocrine disruption showed up in "the presence of female egg protein in blood plasma samples of male carp," the report said.

The fish were taken from areas of the lake, Las Vegas Wash and Las Vegas Bay, that receive much of the treated and untreated waste, including pesticides and industrial chemicals, from the Las Vegas area.

Testing the same parts of the lake for organic compounds associated with endocrine problems, researchers found higher levels of the suspect material than they did elsewhere in the lake.

The chemicals are considered dangerous because in the earliest stages of animal development they are able to imitate natural hormones, scrambling genetic instructions and permanently distorting sexual identities and reproductive systems.

"Endocrine disrupters have become a popular concern, and these findings suggest the potential for a significant problem," said Dennis Fenn, chief biologist for the U.S. Geological Survey, the agency that conducted the research.

The phenomenon has shown up in a broad variety of wildlife exposed to chemical contamination, including river otters and alligators with abnormally small sex organs, beluga whales with fertility problems and male panthers with high estrogen levels and low sperm counts.

Researchers rarely have been able to link similar disorders in humans to environmental pollutants. However, scientists are exploring possible connections between chemical contaminants, low sperm counts and high rates of prostate and testicular cancers.

Moreover, scientists point out that the developing embryos of humans and animals are similar, and that in the earliest stages of development, all creatures may be vulnerable to the same influences.

Lake Mead, on the Colorado River, was created by the construction of Hoover Dam, which was finished in 1935, and lies just downstream from the Grand Canyon.

"Fishing has always been an important recreational activity," said Karen Whitney, a Park Service spokeswoman. "The lake is famous for the striped bass caught here. We have tournaments every year."

Only carp were tested as part of the government study, and carp is not a game fish.
Nevertheless, the Park Service is not ruling out the possibility that pollution has had similar effects on striped bass and catfish, the two favorite game fish.

"We want to do a similar analysis on other fish, looking at the parts of the fish that people tend to eat," Whitney said.

Theo Colborn, a scientist with the World Wildlife Fund who has written a book about endocrine disrupters, said Monday that people may have more to fear from drinking polluted water than from eating fish that have ingested the pollution.

"It doesn't tend to be in the parts of the fish that people eat," Colborn said.

Routine measurements of water pollution by the Las Vegas Water District indicate almost undetectable levels of organic compounds linked to endocrine system disorders, said Linda Blish, the agency's water quality manager.

However, scientists have determined that the compounds in question can be harmful to hormonal systems even at very low levels.

"The message is not the amount but the unpredictability of the mixture," Colborn said.

Pesticides and the long-lived residue of numerous household and industrial chemicals are all suspects. But scientists have had difficulty isolating possible chemical culprits because the affected species often dwell in places where they are exposed to a stew of pollutants.

According to Colborn, recent research in Britain has been turning attention away from synthetic compounds and looking at the possibility that naturally occurring estrogen, the female sex hormone, may be at the center of the problem.

"The theory is that excreted female hormones, though inactive when released from our bodies, can become reactivated by some process in sewage treatment plants."