Battling an invasion of watersnakes

UNWELCOME IN THE WEST: Bob Reed, Ph.D, from the USGS Fort Collins Science Center in Fort Collins, Colo., displays one of the first live captured southern waternake (Nerodia fasciata) in the Colorado River Basin, near Yuma, Arizona. While the southern watersnake is non-venomous, it is aggressive and when threatened, will often strike and bite. Herpetologists do not recommend it as a pet. Credit: Jon Myatt/USFWS

The Service & our invasive species partners are collaborating in the fight to prevent the spread of southern and common watersnakes in the West

By Jon Myatt
October 31, 2016

“We found one,” said Bob Reed, as he emerged from an eight-foot tall wall of cattails and marsh grass into the chest-deep water and muck of an irrigation ditch. His expression reflected both good news and bad news.

A Ph. D., research biologist and invasive species branch chief for the U.S. Geological Survey (USGS) based at the Fort Collins (Colo.) Science Center, Reed had come to the Colorado River Basin north of Yuma, Arizona, in May to investigate the presence of the southern watersnake – Nerodia fasciata – a non-venomous snake native to the southeast U.S.

With support from the U.S. Fish and Wildlife Service, Bureau of Land Management, Bureau of Reclamation, the California and Arizona departments of fish and wildlife, and other organizations, Reed is leading a 10-week investigation. He is capturing the snakes and documenting their presence, in order to estimate their population and current range.

Bob Reed, describes watersnake behavior to Ashley Hall, the Southwest Region's invasive species strike team coordinator and Ryan Munes, a wildlife biologist at nearby Cibola NWR. Credit: Jon Myatt/USFWS

This species of watersnake is invasive to the western states and is a serious threat to native species, he said, and through its predation, “could contribute to continued population declines of endangered or threatened fish and amphibians throughout the West.”

The good news, of course, was that his traps were successful. The bad news: over the first few days of trapping, his team captured seven live southern watersnakes. His suspicion was confirmed: a thriving population of Nerodia fasciata exists in the Colorado River basin.

Mittry Lake is a man-made waterway a few miles north of Yuma, Arizona, formed in the 1940s to provide water for agriculture by divertng water from the Colorado River and damming the southern end of the basin.  It is bordered on the north and east by the U.S. Army's Yuma Proving Ground. Full of previously introduced species like large mouth bass and mosquito fish, and bullfrogs, the confirmation of a southern watersnake population is just the latest challenge facing native species along the California-Arizona border. Credit: Jon Myatt/USFWS

Though he has not investigated the California side of the Colorado River, Reed believes the snakes are present there too. 

Once introduced, they can spread rapidly. Each adult female watersnake gives birth to an average of 20-to-50 offspring every two years.

Very few aquatic species can move independently from native to foreign waters. Most often, humans are responsible for the importation of aquatic and semi-aquatic species in water bodies outside of their native ranges.

Such is the case with southern watersnakes.

DEFENSIVE POSTURE: The southern watersnake (Nerodia fasciata) can spread its jaw bones and flatten its head to appear like its venomous southeastern neighbor, Agkistrodon piscivorus, more commonly known as a water mocassin or cottonmouth. Photo: Jon Myatt/USFWS

Though the southern watersnake, and its close relative the common -- or northern -- watersnake, were initially found in three California locations in 1992, those populations were thought to be somewhat under control and contained.

After those initial discoveries, the “Nerodia Working Group," a collaboration of herpetologists, wildlife managers, university research biologists and invasive species coordinators from state and federal agencies based in the western states, formed to provide expertise and science funding to study the snakes, and also assist the states in planning strategies to confront an invasion.

U.S. Fish and Wildlife Service aquatic invasive species biologist Louanne McMartin, in the Lodi Fish and Wildlife Office, has been involved with the Nerodia issue in the state for more than 10 years.

Aquatic invasive species biologist, Louanne McMartin,
with captured watersnakes near Roseville, California.
Credit: Jon Myatt/USFWS

McMartin helped re-establish “the Nerodia Working Group” in 2007 to coordinate monitoring efforts and to advise the California Department of Fish and Wildlife and other agencies on early detection and eradication strategies.

She and the working group initiated a 2009 investigation of Lake Machado, a 31-acre lake that straddles the communities of Harbor City and Wilmington, near Los Angeles, where more than 300 southern watersnakes were caught.

The Service also formed a national "Invasive Species Strike Team" program during that time to focus on understanding and fending off invasive species before they become permanently established.

While the scientists do not know exactly how many watersnakes are in California today, more than 350 of two different species -- the common watersnake (Nerodia sipedon) and the southern watersnake (Nerodia fasciata) -- have been found near Sacramento (Roseville and Folsom).

In 2008, the Nerodia genus was added to California’s restricted species list. As a result, it is a violation of California law to capture, possess, import or distribute all snakes within the Nerodia family without a permit.

McMartin also helped Jonathan Rose, a Ph.D. in the UC Davis Graduate Group in Ecology, receive funding from the Service for a study of early detection of the snake using environmental DNA -- or eDNA.

Jonathan Rose, Ph.D., UC Davis, with a southern
watersnake captured near Folson, Calif.
Credit: Brian Todd/UC Davis

The eDNA technique “looks at water samples from a range of specific grid locations to determine the presence of animals in wetlands,” she said.

Since trapping watersnakes is labor intensive, Rose and Brian Todd, Ph.D., an associate professor with the UC Davis Department of Wildlife, Fish and Conservation Biology, thought a water sampling technique, if it met a sufficient standard of accuracy, would be more efficient and also provide a better picture of where the snakes were located.

Using the data from Rose’s work with eDNA, and models they created to estimate the population and range of the snakes, researchers were able to create an accurate graphical representation of the invasion in two northern California sites.

“The knowledge gained from Rose and Todd’s studies and work from participating state and federal agencies, and other environmental organizations, led the working group to recommend a Nerodia eradication program at the site identified in Roseville,” McMartin said.

Their procedures will be used at other locations where the snakes are thought to be, McMartin added.

“The Nerodia are not yet out of control,” said Rose, who also authored a recent study on suitable habitat for the invasive snakes. He remains concerned about their potential to spread to other areas.

In 2014, Rose recommended action be taken to control the emergent populations of these snakes while their populations remained low in California. “Waiting until they become entrenched could cost more ecologically and economically,” he said.

Retired Coast Guardsman, Clay Sharp is credited with
finding the first banded watersnake in the Yuma Basin
in October 2015. Photo courtesy of Clay Sharp

Then, in October 2015, Clap Sharp, a retired Coast Guardsman fishing at Mittry Lake on the Colorado River a few miles south of the U.S. Army’s Yuma Proving Ground ran across a snake he’d not seen in the area before.

He collected the snake and gave it to a game warden from the Arizona Department Game and Fish, thus providing the herpetology community the first sign of another possible population, this time along the California-Arizona border.

The following April, Sharp captured another watersnake in the same area. Sharp eventually captured five more snakes that week, turning them over to representatives of Arizona Department of Game and Fish in Yuma, Ariz.

Researchers, like Todd, who has studied the Nerodia populations in California extensively, suspect the presence of the nonvenomous snakes is most likely a result of the “pet trade” – introduction by people “setting free” their pet snakes.

Ron Smith, the Pacific Southwest Region's aquatic invasive species program coordinator, feels that the close proximity to the US Army base should also be considered.

"The movement of equipment in and out of the base could be a vector for the introduction on non-native species as well," he said.  " Bringing in equipment containing concealed Nerodia from a base in the southeastern U.S. is a real possibility."

Valerie Cook-Fletcher, Ron Smith and Louanne McMartin check snake traps along a creek in Roseville, Calif. Credit: Jon Myatt/USFWS

Regardless of the source, these snakes can thrive just about anywhere.

Todd and others are concerned about the danger of both Nerodia fasciata and Nerodia sipedon expanding their range where they would compete with and prey upon sensitive native species in vulnerable situations.

“Watersnakes are not picky eaters and can adapt to varied ecosystem types,” Todd said.

Andrew Holycross, Ph.D., herpetologist, professor at Mesa Community College and expert on reptiles in Arizona, assists Reed setting snake traps at along a cattail bank at Mittry Lake. Credit: Jon Myatt/USFWS

Nearly half of California’s amphibians are considered Species of Conservation Concern or are listed under state or federal Endangered Species Acts. he said.  Also, more than 80 percent of the state’s inland fish are listed as species of conservation concern.

“The giant garter snake, listed as 'threatened' by the state and federal governments, is an example,” Rose said. “The watersnake is a generalist and can adapt to the specific habitat of the garter snake and be a primary competitor for food.”

While the southern watersnake has a more restricted climatic niche, he added that “without action, it could spread through the Central Valley, where native fish and amphibians have already suffered significant declines.”

Calif. Department of Fish and Wildllife invasive species coordinator Valerie Cook-Fletcher (right), and Service aquatic invasive species biologist Louanne McMartin, bag a northern watersnake near Roseville, Calif., June 1, 2016. (Credit: Ron Smith/USFWS)

And the watersnakes not only have the potential to spread through Central California, but also farther north to Oregon’s Willamette Valley and to central Washington.

Valerie Cook-Fletcher, a senior environmental scientist with the California Department of Fish and Wildlife’s Invasive Species Program, has coordinated the state’s monitoring efforts and supervised the project for northern, or common watersnakes (Nerodia sipedon) in Roseville beginning in 2015.

California DFW is planning an eradication project in Folsom focused on the southern watersnake (Nerodia fasciata) scheduled to begin in 2017, she said.

Valerie Cook-Fletcher, California DFW, displays a common,
or northern, watersnake (Nerodia sipedon) caught near
Roseville, Calif.  Credit: Jon Myatt/USFWS

Eradication is labor intensive and costly. Funding for invasive species eradication is available only through grants, says Cook-Fletcher, and there is little funding available for the future.

“But this is where the public can help,” Cook-Fletcher said. “First, if you see one of these snakes in the wild, try to document it and report it. Early identification will assist our efforts to contain their presence.”

“Second, don’t release pet snakes into the wild,” she said. “If you find that you can’t keep your pet, try to find a new home for it with another pet owner, or try to return it to the pet store where you purchased it.”

Down in Yuma at Mittry Lake, a rotating team of biologists, graduate students and environmental technicians cycle in every 10 days to check and reposition snake traps daily, document captures and fill in pieces of the puzzle.

Bob Reed (center) sets  a snake trap along a drift fence, with the help of Ashley Hall and Ryan Munes. Reed perfected the snake trapping technique during the initial trapping operation at Lake Machado, near Los Angeles, in 2010. A paper detailing the Lake Machado investigation and co-authored by Reed, Louanne McMartin and  Robert Fisher  was published recently. Credit: Jon Myatt/USFWS

As the fourth week of the investigation approaches, Dr. Reed is hoping that the capture data shows a limited presence of Nerodia.

In this location in the Colorado River Basin, “We're still trying to find out exactly where these snakes are present in this ecosystem,” he said. “If they're limited to a small area, then we may have the opportunity to do local control, we may be able to set up barriers of traps or other detection tools to prevent spread and we may be able to slow their spread into more sensitive areas where there are more endangered species.”

“But the first step is to find out where they are, and how many there are, and that will help us decide where to allocate resources for control.”

“And so we shouldn't just throw the baby out with the bath water and say ‘all is lost, there is nothing we can do,’” he said. “Because, if we have the resources -- and we are early enough -- we may just be able to do something about it,”

Nerodia invading the west

Jon Myatt is the digital communications manager for the Pacific Southwest Region headquartered in Sacramento, Calif.

For more information about Nerodia watersnakes, visit its species profile pages or join California Nerodia Watch. For more on these watersnakes, see common watersnake or southern watersnake at For questions about the project or volunteering, please contact

All Nerodia watersnakes are restricted in California (14 CCR 671) and may not be imported, transported, or possessed without a permit from CDFW.