Invasive water snakes at Lake Machado could be here to stay, study says

Lake Machado in the South Bay’s Ken Mallow Harbor Regional Park is an extreme example of what can happen when invasive species gain a foothold in an urban habitat.

The 31-acre lake straddling the communities of Harbor City and Wilmington captures runoff from Los Angeles County storm drains, and along with it a thriving menagerie of creatures including bullfrogs, apple snails as big as baseballs and snapping turtles. (The lake’s most famous resident was Reggie, the abandoned alligator who eluded animal control officers for two years before finally being relocated to the Los Angeles Zoo.)

Now scientists are studying Lake Machado to see whether it’s feasible to eradicate thousands of 3-foot-long southern water snakes that are not native to the area but have made themselves at home among the lake’s floating mounds of trash.

“Our goal is to get a handle on the biology of these snakes as part of an effort to keep them from spreading,” said U.S. Geological Survey biologist Robert Fisher, who is working with colleagues from his agency, the U.S. Fish and Wildlife Service and UC Davis. “There’s concern about them eventually entering the Los Angeles, San Gabriel and Santa Ana River systems, and beyond.”

Southern water snakes hail from the southeastern United States. In their natural habitat, they eat frogs and fish and can give birth to up to 57 young in one breeding season.

Lake Machado’s water snakes are believed to be the offspring of pets that were turned loose in the area about a decade ago.

Fisher and his colleagues conducted necropsies on 306 water snakes that were captured in 2010. They found no direct evidence that the serpents are affecting native species in Lake Machado  -- but only because there were no remains of native species in their digestive tracts.

Instead, the snakes are preying on other invasive species in the lake, a state-designated “impaired body of water” fouled by trash, algae, coliform bacteria and hazardous substances.

“We found that young water snakes prey mostly on nonnative mosquito fish,” said Robert Reed, a USGS snake ecologist. “Older snakes hunt bullfrogs and bullfrog tadpoles.”

The results of the investigation were published this week in the print edition of the journal Herpetologica.

Field work for the study concluded two years before the Los Angeles Department of Public Works launched an ecological rehabilitation project that includes mechanical dredging along the shore and removal of sediment laced with pesticides and other hazardous substances from the bottom of the lake.

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When completed next year, the effort is expected to improve water quality and make it easier for native plants and fish to thrive. However, it almost certainly won’t reduce the number of invasive water snakes.

“Improvements in water quality will help the snake by increasing the biomass of fish and frogs they  prey on,” Reed said. “The only sure way to get rid of the water snakes is to eradicate their food sources by draining the lake for at least a year.”

The problem isn’t unique to Lake Machado. Experts are also trying to control southern water snake populations discovered in the vicinity of Folsom, near Sacramento, and along the Colorado River near Blythe, Calif.

Nor is the problem limited to water snakes, Reed said.

“The number of invasive snake species throughout the nation is increasing exponentially,” he said. “The situation will only get worse without a concerted effort to regulate the live animal trade.”

louis.sahagun@latimes.com

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