Executive Summary of Contingency Captive Breeding Plan for Sierra Nevada Bighorn Sheep
Report for Interagency Agreement # P9980059 between California Department of Fish and Game and Wildlife Health Center, School of Veterinary Medicine, University of California, Davis CA.
Holly Ernest DVM PhD 2001
The Captive Breeding Contingency Plan (Ernest 2001), contracted by the California Department of Fish and Game's (CDFG) Sierra Nevada Bighorn Sheep Population Recovery Program, includes several tools to facilitate decisions relating to the captive breeding of bighorn sheep. This analysis was also provided to the multi-agency Sierra Nevada Bighorn Sheep Recovery Team (SNBSRT) to assist recovery planning. The concept of captive breeding in general, along with past Sierra Nevada bighorn sheep (SNBS) captive breeding attempts were reviewed in the Introduction. A model for decision tree analysis was presented in a dichotomous format: a series of questions requiring yes or no answers to lead to specific recommendations for captive breeding.
Next, to assess the impact that captive breeding-associated sheep removal and augmentation would have on extinction probabilities in populations, population modeling was conducted. Preliminary models for the populations at Wheeler Ridge, Mt. Baxter region, and a theoretical captive herd were run under three different scenarios representing a range of mortality and survival values. Since this pilot set of models detailed is very preliminary and simplistic, they should be used only for initial guidance decision-making and construction of future models. Perhaps most importantly, these models demonstrate the conspicuous need for age- and cause-specific mortality, survivorship, and census data. Although the models were run with limited available data, they revealed that the potential for Wheeler population to serve as a reliable source of transplantation stock may be limited and tenuous due to small population size. Using data available at the time of writing from the Sierra Nevada and existing captive bighorn sheep facilities, models indicated that a captive herd would produce a more reliable source of translocation stock than Wheeler Crest alone. Depending on factors specific to the contemporary populations, well planned and conducted captive breeding and translocation of animals may facilitate recovery goals by increasing the rate of population growth and achieving population numbers to reduce likelihood of extinction.
Captive breeding site selection guidelines were presented, along with a detailed assessment of a site (Paoha Island in Mono Lake) that had been under consideration by the California Department of Fish and Game. Preliminary assessments were made for potential sites west of Big Pine between Baker and Fuller Creeks. The Plan includes information (including strengths and weaknesses) on existing captive breeding facilities in southern California and other states collected by site visits and communications with facility managers, veterinarians, and biologists. Most of the problems experienced in the past would be eliminated or at least greatly reduced with proper facility planning and management. Also included are guidelines and recommendations for constructing and maintaining a facility for captive breeding, selection of founder breeding stock, husbandry, and veterinary care, along with a summary of diseases that may impact a captive herd. A preliminary cost estimate worksheet for start-up and first year is provided. Start-up and first year costs range from $600,000-1,000,000 (roughly estimated, since there are many unknowns).
My general conclusion at the time of writing, from literature review, consultation with captive breeding and bighorn sheep experts, and preliminary population modeling was that establishment of a well-managed captive herd would reduce the risk of extinction of Sierra Nevada bighorn sheep, given the year 2000 population estimates. The captive herd should consist of a minimum of 30-40 founder animals, collected over at least fifteen years, from the Wheeler population (and other populations, as available) to preserve a minimum level of genetic diversity (at least 90-95% of original heterozygosity). Well-planned breeding, and pedigree and genetic analyses should be conducted under consultation of a geneticist experienced in ungulate captive breeding. The captive herd would provide a new population (estimated to be 50-100 animals) as a safeguard against wild extinctions. Based on models, within 4-7 years, reliable translocation stock should be available for translocation and reintroduction to Sierra Nevada populations. Simulation models specific to the Sierra Nevada metapopulation should be constructed and that further modeling with updated population estimates be conducted. One potential problem that needs further research is the translocation success of captive raised vs. wild caught sheep (see Thompson et al. 2000; Clark et al. 1988). Other issues for further research include detailed examinations of the risks of pathogen exposure and infection in a captive herd and transmission to wild sheep.
The final products of a captive breeding herd should be healthy, behaviorally normal individuals capable of surviving and reproducing in the wild. A large enclosure with an abundance of natural forage, escape terrain and protection from predation is a must. Disease may be an unavoidable occurrence in a captive herd, especially an intensively managed herd in a small enclosure. Prevention will be the key to minimizing and delaying this event. In the case of a disease event in the captive herd, the eventual release of captive animals into the wild must be managed very conservatively. It is within the realm of possibility that disease could totally prevent the release of any captive animals into the wild. A long-term commitment (i.e. greater than 10 years) by CDFG, USFWS, and the Recovery Team for high-quality facility planning, construction, and management will be critical to the success of a captive breeding program.
Reduced adult survival and high environmental variation in reproduction and lamb survival are likely to be important factors driving Sierra Nevada populations toward extinction. A captive population can be managed to have optimum reproduction and survival without the high environmental variation that is present in wild populations. Without the potential stability of captive herd, the Wheeler population, as currently modeled, may have a limited potential to supply translocation stock for augmentation of existing Sierra Nevada populations and for reintroduction of new populations, and therefore, population recovery goals may not be achieved in the desired time frame.
Finally, as the Sierra Nevada metapopulation of bighorn sheep and their ecosystems are dynamic, so should be captive breeding contingency planning. This document is meant as a starting point, and as a living document it should be revised and supplemented as new science becomes available.
Very special thanks to Vern Bleich, Walter Boyce, Ben Gonzales, Esther Rubin, Tom Stephenson, Steve Torres, and John Wehausen who helped with several aspects of this document.
Many other people were very helpful in gathering and supplying information and providing ideas for models and decision trees for this document: Dan Beck, Dori Borjesson, Karl Chang, Roger Cook, Paul Crosbie, Jim DeForge, Mark Drew, Bill Dunn, Kris Ernest, Amy Fisher, John Glenn, Phil Hedrick, Sharon Hietala, Bruce Hoar, Doug Humphreys, Bob Jellison, Paul Krausman, Troy Kelly, Terry Kreeger, Dave Jessup, Ray Lee, John Maas, Karla Michelson, Mike Miller, Phil Miller, Stacey Ostermann, Nancy Ottum, Jim Richards, Eric Rominger, Becky Pierce, Margaret Wild, Mara Weisenberger, Margaret Wild, Dave Zeiler, Dave Zezulak, staff of the UC Davis Wildlife Health Center, California Department of Fish and Game, California Department of Food and Agriculture veterinarians, and the Sierra Nevada Bighorn Sheep Recovery Team.
Ernest HB 2001. Captive breeding Contingency plan: A Guide for Captive Breeding of Sierra Nevada Bighorn Sheep. Report for Interagency Agreement # P9980059 between California Department of Fish and Game and Wildlife Health Center, School of Veterinary Medicine, University of California, Davis CA. 147 pages.