The riparian brush rabbit (Sylvilagus bachmani riparius) is an endangered species found in dense, brushy habitat in the California's Central Valley. We implemented a reintroduction program to bolster populations at a Federal Wildlife Refuge and to assess factors influencing mortality and subsequent survival of released individuals. Between July 2002 and July 2005, we reintroduced 325 captive-bred individuals to unoccupied habitat within their historic range using a soft-release strategy and monitored their subsequent survival with radiotelemetry. Longer time in soft-release pens resulted in increased monthly survival. Rabbits were most susceptible to post-release mortality during the first 4 weeks following reintroduction and both body mass and length of time in the soft-release enclosure influenced this relationship. When we controlled for release mortality during this acclimation period, subsequent monthly survival probabilities were most strongly influenced by release year (year 1 vs. years 2 and 3) and by a catastrophic flooding event; length of time in the soft-release enclosure remained an important variable in longer-term survival. Cause of mortality was unknown for the majority of deaths (61.9%), but predation (including presumptive predation) was the greatest known cause of death in translocated rabbits (26.4%). Reintroduction programs should employ an adaptive management approach with ongoing monitoring of target animals and concurrent analysis to allow managers to adjust methods as conditions dictate.

The objective of this study was to establish reference hematologic and biochemistry values for adult riparian brush rabbits (Sylvilagus bachmani riparius). Between February 2002 and December 2003, complete blood cell counts and serum biochemistry panels were done on blood samples collected for 252 healthy, adult riparian brush rabbits that were bred in captivity and released to the wild in the California, USA, San Joaquin Valley. Significant differences in mean values of some analytes were found among males, nonpregnant females, and pregnant females and among seasons. However, when reference values for each subgroup (sex, pregnancy status, and season) were compared with reference values for all riparian rabbits combined, differences were not sufficiently relevant clinically to warrant the adoption of separate subgroup-specific reference values. The reference ranges reported herein will be of value to veterinarians and wildlife biologists assessing the health of riparian brush rabbits and other wild Sylvilagus species.

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