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US Fish & Wildlife Service FieldNotes
REGION 8: Giant Garter Snake Population is First Confirmed Find in 15 Years
Region 8, September 9, 2009
Giant garter snake
Photo by Eric Hansen
Summer 2009
Giant garter snake Photo by Eric Hansen Summer 2009 - Photo Credit: n/a

by Caroline Prose
The July 2009 discovery of  giant garter snakes in California's San Joaquin County is the first confirmed population in the county in 15 years.  The disovery, by Eric Hansen, a private consultant and researcher, was part of a study funded by the Central Valley Project Improvement Act Habitat Restoration Program (HRP), a grant program conducted jointly by the U.S. Fish and Wildlife Service and Bureau of Reclamation.  The HRP grants annually fund a variety of habitat acquisition, restoration, research, and other projects such as outreach and planning.  As part of its Fiscal Year 2009 solicitation, the HRP requested proposals that would target status surveys for federally listed species, particularly those that will be the subject of 5-year reviews in 2010.  One of these species is the federally threatened giant garter snake. 

Subsequent to the proposal solicitation, the HRP received a proposal that involved surveying for the giant garter snake in an area that had not been surveyed since 1994.  The proposal was funded following a rigorous review and ranking process. The study commenced in July 2009,  conducted by Mr. Hansen.  Various data are being gathered which include numbers of individuals, size of occupied area, and reproductive status.  Tissue samples are also being collected for genetic analysis.  Mr. Hansen has so far trapped 11 large adult giant garter snakes in San Joaquin County.  Of the 11 snakes trapped, seven were male and four were female, including one pregnant snake. The pregnant female was trapped a second time and had already given birth, which shows that the snakes are reproducing.  

 The population is one of 11 extant populations in the Central Valley.  The specific location of where the 11 snakes were found is not being released  out of concern that the traps could be tampered with and/or the giant garter snakes harmed or killed.

The giant garter snake inhabits agricultural wetlands and other waterways such as irrigation and drainage canals, sloughs, ponds, small lakes, low gradient streams, and adjacent uplands in the Central Valley.  Because of the direct loss of natural habitat, the giant garter snake relies heavily on rice fields in the Sacramento Valley, but also uses managed marsh areas in national wildlife refuges and state wildlife areas.  Giant garter snakes are typically absent from larger rivers because of lack of suitable habitat and emergent vegetative cover, and from wetlands and streams with sand, gravel, or rock substrates.  Riparian woodlands typically do not provide suitable habitat because of excessive shade, lack of basking sites, and absence of prey populations, however  some riparian woodlands do provide good habitat.  Continuing threats are the main causes for the decline of the giant garter snake, including habitat loss and fragmentation, flood control activities, changes in agricultural and land management practices, predation from introduced species, parasites, and water pollution.    

For more information, please contact Caroline Prose, Program Manager for the CVPIA Habitat Restoration Program, 916-414-6575 or caroline_prose@fws.gov

Contact Info: Caroline Prose, 916-414-6575, Caroline_Prose@fws.gov