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US Fish & Wildlife Service FieldNotes

SACRAMENTO FWO: A Day with the Giant Garter Snake

Region 8, July 1, 2014
Lily Douglas, a biologist with the Sacramento FWO, recently got out from behind the desk and into the fields in search of the endangered giant garter snake.
Lily Douglas, a biologist with the Sacramento FWO, recently got out from behind the desk and into the fields in search of the endangered giant garter snake. - Photo Credit: n/a
The giant garter snake can vary in coloration, with the most common a dark brown with yellow stripes. Snakes in the Natomas Basin can also tend more towards orange.
The giant garter snake can vary in coloration, with the most common a dark brown with yellow stripes. Snakes in the Natomas Basin can also tend more towards orange. - Photo Credit: n/a
USGS biologist Shannon Skalos measures the snout-to-vent length of a captured giant garter snake.
USGS biologist Shannon Skalos measures the snout-to-vent length of a captured giant garter snake. - Photo Credit: n/a
Biologists Lily Douglas (SFWO), Angela Calderaro (CDFW), and Shannon Skalos (USGS) kayak along the edge of the wetland, checking traps.
Biologists Lily Douglas (SFWO), Angela Calderaro (CDFW), and Shannon Skalos (USGS) kayak along the edge of the wetland, checking traps. - Photo Credit: n/a

By Lily Douglas

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.

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.

Staff from 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).

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.

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.

For more information on the giant garter snake, check out the SFWO fact page (http://www.fws.gov/sacramento/es_species/Accounts/Amphibians-Reptiles/Documents/giant_garter_snake.pdf) or CDFW’s life history account (https://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=3457&inline=1).

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

Lily Douglas is a fish and wildlife biologist at the Sacramento Fish and Wildlife Office in Sacramento, California. Additional thanks to Margaret Mantor (CDFW), Brian Halstead (USGS) and Heather McPherron (USFWS).

Contact Info: Sarah Swenty, 916-414-6571, sarah_swenty@fws.gov