U.S. Fish & Wildlife Service

Sacramento Fish & Wildlife OfficeServing the people, conserving the fish, wildlife, and plants of California

A Unit Of The Pacific Southwest Region

A Day with the Giant Garter Snake

July 7, 2014

By Lily Douglas, Sacramento Fish & Wildlife Office

Photo of Biologist Lily Douglas with Sacramento Fish and Wildlife Office

Biologist Lily Douglas, in search of the endangered snake. Photo Credit: Angela Calderaro/CDFW

Most people are familiar with our more common species of garter snakes, but finding a giant garter snake is a rare sight! The giant garter snake (Thamnophis gigas) is listed as a threatened species under both the federal and California Endangered Species Acts. The species is also notoriously wary, silently sliding into the water or tule rushes before you can even catch a glimpse.

Lily Douglas, a biologist with the Sacramento Fish and Wildlife Office, spent a day with biologists from the California Department of Fish and Wildlife (CDFW) and the U.S. Geological Survey (USGS) on a research site in the Natomas Basin, north of Sacramento. Preserves in the Basin are managed for listed species as part of the Natomas Basin Habitat Conservation Plan (HCP).

Photo of giant garter snake

Snakes in Natomas Basin tend to be more orange. Photo credit: Lily Douglas/USFWS

Monitoring for giant garter snakes is both a requirement under the HCP and allows the USGS to collect valuable data on population demographics and prey use of the snake. Each snake is marked with a microbrand and/or a PIT tag, similar to the microchip on your pet, for identification.

Historically, the giant garter snake swam throughout the wetlands and marshes of the floodplains of California’s Central Valley. Loss of this historic habitat has been the greatest contribution to the snake’s decline. However, giant garter snakes in the Sacramento Valley have been able to use agricultural canals and rice fields as an alternative to their natural habitats.  

Photo of biologist Shannon Skalos measuring the snout-to-vent length of a giant garter snake

Biologist Shannon Skalos measures the length of a captured giant garter snake. Photo Credit: Lily Douglas/USFWS

So how do the biologists capture these secretive, largely aquatic snakes? Modified minnow traps are floated along the edge of wetlands, where giant garter snakes follow the tules in search of their fish or tadpole prey. The snakes are able to enter the funnels on the ends of the trap, but are unable to escape. Checking traps that are floating in the water can be challenging, requiring boots, waders, or even kayaks.

Studies such as this one provide important information on the life history requirements of endangered species. Learning about the needs of the giant garter snake allows biologists to work towards the survival and recovery of this unique Central Valley species.

Photo of biologist kayaking along the edge of the wetland

Biologists kayak along the edge of the wetland, checking traps. Photo credit: Andrew Hughan/CDFW

For more information on the giant garter snake, check out our SFWO fact document (giant_garter_snake.pdf) or California Department of Fish and Wildlife (CDFW) life history account (https://nrm.dfg.ca.gov).

For more information on USGS studies on the giant garter snake, see http://www.werc.usgs.gov/Project.aspx?ProjectID=89.

Additional thanks to Margaret Mantor (CDFW), Brian Halstead (USGS) and Heather McPherron (USFWS).

Last updated: December 6, 2017