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Stop the Spread of Non-Native Water Snakes in California

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California Nerodia Watch at iNaturalist


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RECENT NEWS

May 2016 Trapping for Water Snakes in Yuma, AZ

This past May, Dr. Robert Reed led a team of biologists in a trapping program for exotic water snakes along the Colorado River, near the Arizona-California border. As reported here, a population of water snakes was discovered at the Laguna Dam and connecting riparian habitats late last year. To assess the distribution and density of the population, Dr. Reed (USGS) organized a trapping survey, with help from state and federal biologists from California and Arizona. Read about the trapping study and what they found.

installing traps southern water snake caught in Yuma
Photo credits: Jon Myatt (USFWS)

For general information on invasive water snakes in California, or to report on your observations, please contact: Mike Fuller.


Purpose of this Site

The purpose of this web site is to report on the distribution of non-native water snake species in California. Here, you will find information on the location of established populations and new sightings, as well as updates on control measures taken to reduce threats to native fish and wildlife. Natural resource agencies involved include the US Fish and Wildlife Service, US Geological Service, and California Department of Fish and Game.

Two species of exotic water snake (genus Nerodia) have become established in California:

Confirmed Sightings and Populations (two species):

  1. Southern water snake ( N. fasciata): self-sustaining population within Machado Lake, at the Kenneth Malloy Memorial Park, in Harbor City, Los Angeles County1
  2. Southern water snake: self-sustaining population in the Folsom area of Sacramento County2,3,4
  3. Northern water snakes (N. sipedon): captured in Roseville, CA.
  4. Southern water snakes (N. fasciata): captured at Mittry Lake, near Yuma, AZ.

A third species, the diamond-backed water snake (N. rhombifer), was recorded at the Lafayette Reservoir, east of Berkeley, in the 1990s. That population declined to undetectable levels and may have died out. However, recent reports of water snakes at the Lafayette Reservoir suggest that the diamond-backed water snakes may be recovering at that location.

Map of Water Snake Sightings

map of water snakes populations

References Cited

  1. Fuller, M.M. and B.W. Trevett. 2006. Geographic distribution: Nerodia fasciata pictiventris (Florida water snake). Herpetological Review 37:363.
  2. Balfour, P. S., and E. W. Stitt. 2002. Geographic distribution: Nerodia fasciata fasciata (banded watersnake). USA: California: Sacramento Co. Herpetological Review 33:150
  3. Stitt, E.W., P.S. Balfour, T. Luckau, and T.E. Edwards. 2005. The southern watersnake (Nerodia fasciata) in Folsom, California: history, population attributes, and relation to other introduced watersnake in North America. Final report to US Fish and Wildlife Service. ECORP Consulting Inc
  4. Balfour, P.S., E.W. Stitt, and M.M. Fuller. 2007. Geographic distribution: Nerodia fasciata pictiventris (Florida water snake). Herpetological Review 38:363

How Did Water Snakes Invade California?

For many years, water snakes and other non-native reptiles were routinely imported to California for the pet trade. We believe the source of most water snakes in California is the intended or accidental release of snakes purchased at pet shops.

The fact is, water snakes make poor pets. They are prone to biting and spray-defecating with a foul-smelling musk secretion when handled. Water snakes rarely become tame and do not prosper in captivity. Reptile enthusiasts accustomed to more easily handled species can become discouraged with pet water snakes. Not wanting to kill the snakes, owners may simply release them into the closest aquatic habitat. We believe this is a likely reason that water snakes can now be found outside their native range.

Photos of Non-Native Water Snakes

Threat Posed by Non-Native Water Snakes

Within North America, snakes in the genus Nerodia are native to the eastern and southeastern US, eastern Canada, and Mexico. Unlike the cottonmouth water moccasin (Agkistrodon piscivorus), which has not become established in California, water snakes are not poisonous and do not present a threat to human safety.

Although water snakes are not dangerous to people, as introduced (exotic) predators they pose a threat to natural systems and native species. Water snakes feed primarily on small fish, frogs, tadpoles, and possibly crayfish. Exotic predators can damage native ecosystems by decimating populations of their prey. Exotics also compete with native species for food, and can carry new parasites or diseases which may spread to native species.

Many Californian fish, amphibians, and aquatic snakes have become rare and in danger of extinction in California, largely as a result of the loss of natural freshwater habitats (over 90 percent of the original extent of freshwater habitat in California has been lost; US Geological Survey). Already suffering from loss of habitat, native aquaitc organisms now face the added stress of introduced water snakes.

Similar Native Snake Species

Several aquatic snake species that are native to California superficially resemble non-native water snakes, and care should be taken not to harm unidentified snakes that appear to be water snakes. Garter snakes (genus Thamnophis) are the native snake species most commonly found in or near fresh water in California. Many garter snakes can be distinguished from water snakes by the central stripe that runs down the back of garter snakes. The stripe may be yellow, orange, red, tan, grey, or white. Water snakes lack the central stripe, and have crossbands or no obvious markings instead. However, two species of California garter snake, the Sierra garter snake and two-striped garter snake, lack the central stripe, and one other species, the Oregon garter snake, sometimes has a stripe, and sometimes does not. Therefore, the latter three species may be more easily confused with non-native water snakes.

Native garter snakes found in California include the following:
  • California red-sided garter snake (Thamnophis sirtalis infernalis)
  • Checkered garter snake (Thamnophis marcianus)
  • Giant garter snake (Thamnophis gigas)
  • San Francisco garter snake (Thamnophis sirtalis tetrataenia)
  • Santa Cruz garter snake (Thamnophis atratus atratus)
  • Oregon garter snake (Thamnophis atratus hydrophilus)
  • Sierra garter snake (Thamnophis couchii)
  • Two-striped garter snake (Thamnophis hammondi)
  • Valley garter snake (Thamnophis sirtalis fitchi)
Photos of the above native garter snakes can be found at the California Herps Website

Legal Status of Water Snakes in California

As of January 2008, all non-native water snakes (genus Nerodia) are regulated as restricted animals by the California Department of Fish and Game (Sec. 671, Title 14, Calif. Code of Regulations). It is now unlawful to import, transport, or possess water snakes of the genus Nerodia in California without a permit.

If you are currently in possession of one or more water snakes of the genus Nerodia, PLEASE DO NOT RELEASE THEM. The snakes should be disposed of humanely, such as with the assistance of a veterinarian.

More Photos of Water Snakes (with captions)
Click the above text, or photo bar below, to view photos of water snakes captured in California. photo bar

Where to Report Sightings of Water Snakes

If you have photographs of aquatic snakes in California that you believe to be non-native water snake species, you can send them to Mike Fuller for confirmation. Please include the location and date of the photos, as well as the name and contact information of the photographer.

LINKS

General Documents About Invasive Species


More Information

For more information, contact Mike Fuller

Return to Mike Fuller's homepage

References Cited, with Notes
Balfour, P. S., and E. W. Stitt. 2002. Geographic distribution: Nerodia fasciata fasciata (banded watersnake). USA: California: Sacramento Co. Herpetological Review 33:150.

Balfour, P.S., E.W. Stitt, and M.M. Fuller. 2007. Geographic distribution: Nerodia fasciata pictiventris (Florida water snake). Herpetological Review 38:363. (Provides updated information on populations found in Northern and Southern California).

Fuller, M.M. and B.W. Trevett. 2006. Geographic distribution: Nerodia fasciata pictiventris (Florida water snake). Herpetological Review 37:363. (First published record for the Florida water snake population in California).

Stitt, E.W., P.S. Balfour, T. Luckau, and T.E. Edwards. 2005. The southern watersnake (Nerodia fasciata) in Folsom, California: history, population attributes, and relation to other introduced watersnake in North America. Final report to US Fish and Wildlife Service. ECORP Consulting Inc.