Western gray squirrel ecology and augmentation of the population in the South Puget Trough: 2010 Annual Progress Report

Western gray squirrel (Sciurus griseus) populations in Washington have declined over the last century and their range has diminished. Of the three disjunct populations of western gray squirrel remaining in Washington, the Puget Trough population faces the greatest risk of extinction. The majority of the Puget Trough population occurs on Joint Base Lewis-McChord (JBLM), an area of over 35,000 ha that contains some of the last patches of oak-pine forest in western Washington. The Recovery Plan for this State-threatened species lists augmentation of the population in the Puget Trough as a high priority. In 2007, Washington Department of Fish and Wildlife and the Department of Defense.Fort Lewis, engaged in a cooperative study of the ecology of western gray squirrels on the Fort and implemented a plan to augment this endangered population. Research on the resident population has focused on quantifying population parameters including survival, causes of mortality, and productivity, as well as habitat use and use of space. This information will be critical for assessing why the Puget Trough population has contracted over the last few decades and for focusing management efforts to allow the population to recover. Augmentation has focused on introducing animals from other populations in Washington and northern Oregon in order to increase the genetic diversity of the resident Puget Trough population and expand the occupied area.

From January through December 2010 we captured a total of 30 new resident western gray squirrels during routine and grid trapping. This brings to 121 the total number of resident western gray squirrels captured on the study area since October 2006. Of the squirrels captured in 2010, 15 were fitted with radio-collars and 15 were ear-tagged only and released. We collected 7152 telemetry locations of 80 radio-collared western gray squirrels in 2010 and we documented 179 new nests.

Annual survival of resident western gray squirrels was estimated at 0.55 for 2010; survival of translocated squirrels was similar at 0.54. Fifteen squirrels (residents and translocations) were depredated, 6 succumbed to disease, and 2 died of other causes. Nineteen of 25 resident females tracked through the breeding period attempted to rear young (5 of 6 that did not breed were yearlings); litter size varied from 1 to 4. Two females had a second litter after successfully rearing young; this finding represents the first documentation of 2 litters in a year for this species. All 10 translocated females tracked through the breeding period attempted to rear young; at least 5 were successful producing a total of >12 young.

We translocated 23 western gray squirrels from Klickitat County, WA and from Hood River and Wasco Counties, OR to JBLM in autumn 2010. As of 31 December, 8 animals from the 2010 release were known to be alive (fate of 4 was unknown because they were not radio-collared; fate of an additional 7 was unknown due to loss of their radio signal). Eight western gray squirrels (6 females, 2 male) from the previous augmentations retained working radio-collars and were known to survive through December 2010. Plans for 2011 include continued monitoring of radio-collared animals and intensified monitoring of reproductive success. Additional translocations will take place in fall 2011 as part of the species recovery effort.