Endangered Species Recovery Program

Survivorship of translocated and repatriated riparian brush rabbits.

Presented to:

American Society of Mammalogists, Annual Meeting, Humboldt State University, Arcata, CA, June, 2004;

2^nd World Lagomorph Conference, Vairão, Portugal, July, 2004.

Daniel F. Williams, Matthew R. Lloyd, Laurissa P. Hamilton, and Patrick A. Kelly

Endangered Species Recovery Program, Department of Biological Sciences, California State University, Stanislaus, Turlock CA 95382, USA

Abstract

In 2002-2003 we propagated endangered riparian brush rabbits (Sylvilagus bachmani riparius) in outdoor pens, each of approximately 0.50-0.57 ha. Only one pen was available in 2002. In November 2001, six (3f, 3m) wild-caught rabbits were released into the pen. Three surviving adults were removed in June-July 2002 and repatriated to their original capture points. Additionally, three animals born in confinement were released at the capture sites of the dead, adult brood-stock. A predator killed one of the repatriated adults about 1 month after release. The other two survived into February 2003 when monitoring ceased. The three captive-born young did not survive long enough to breed after being released at the capture sites of adult breeders that died in captivity. Forty-nine captive-born rabbits were released in unoccupied, historical habitat at the San Joaquin River National Wildlife Refuge (SJRNWR) between July and October 2002. Between July 2002 and February 2003 18 of the translocated rabbits died—mostly of predation (63% survival). In December 2002, six wild-caught rabbits (3f, 3m) were released into each of the three propagation pens. In February 2003, two additional wild-caught rabbits were released into one pen where two breeders had died. Of the 20 wild-caught rabbits, 13 died in captivity. Between July 2003 and March 2004, 214 captive-born rabbit were released at SJRNWR, and by March 2004 116 of the translocated rabbits died, mostly of predation (46% survival). We believe the lower survival of rabbits released in 2003 was due to more predators being attracted by the population of rabbits established in 2002, and because in 2003 much of the suitable habitat was already occupied by the rabbits released in 2002 and their progeny.