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Helping a Remnant Bighorn Sheep Population Survive
Bighorn sheepBackground:
During the last 50 years, nearly all of the endemic bighorn sheep populations in the interior of Arizona have been extirpated. The Silver Bell Mountains near Tucson maintain the last endemic bighorn sheep population from what was no doubt a population complex that included the Santa Rita, Catalina, and Rincon mountains. As a result, it is important to ensure that this unique population is maintained and to evaluate the potential management options to sustain long-term viability in this remnant population. Our research objectives are to: (1) evaluate habitat conditions in the Silver Bell complex to assess potential for natural expansion or supplemental transplants; (2) identify travel corridors between mountainous habitats; (3) use frequent GPS-equipped collar locations to evaluate adequacy of previous habitat evaluation models currently applied to sheep management; (4) monitor a recent disease outbreak in this population, treat infected individuals, and identify potential implications; and (5) seek funding to examine genetic variability of this population relative to other desert bighorn sheep populations and/or evaluate survey techniques that provide statistically valid population estimates to enhance monitoring of bighorn sheep populations in the future.

Currently occupied sheep habitat in the Silver Bell Complex includes the Silver Bell Mountains, West Silver Bell Mountains, Ragged Top Mountain, and the Waterman Mountains, which are part of a series of low desert mountain ranges in south-central Arizona about 20 miles west of Marana. This area supports about 75-100 bighorn sheep.

The recent advent of Global Positioning System (GPS) technology incorporated into radiocollars provides opportunities to evaluate detailed habitat use patterns, where multiple locations are collected throughout each 24-hour period over several seasons. GPS locations will be attempted every 5 hours to assess habitat use and movement patterns from at least 18 bighorn sheep of various sex and age classes. Habitat use patterns from collared bighorn sheep will be used to evaluate habitat conditions of adjacent areas to assess potential for natural expansion or supplemental transplants if warranted. In addition, habitat use patterns of bighorn sheep from fine-scaled analyses in the Silver Bell Mountains will be used to evaluate previously developed habitat use models applied at larger scales.

Disease has been widely documented as a potential factor significantly limiting bighorn populations. Recently, blindness has been observed in several bighorn sheep in the Silver Bell Mountains. This condition has been diagnosed as a bacterial caused form of pinkeye, which may have been contracted from a domestic goat herd that escaped a nearby allotment. A similar disease resulted in a 60% mortality rate in a Yellowstone bighorn population during the early 1980s. It is unlikely the Silver Bell population could withstand a similar mortality rate due to its small size and degree of isolation. Identifying and treating infected sheep is underway, and survival and reproduction will be monitored to assess potential implications to the population.

Once populations become isolated at relatively low densities, extinction risk increases due to reduced resiliency of that population to other factors such as predation, disease, annual forage and moisture fluctuations, and reduced genetic variability. Ability to cope with limiting factors is likely reduced as populations become bottlenecked and individuals in the population become more closely related. If adequate funding is obtained, genetic variability of the Silver Bell bighorn population will be compared to other, more robust populations in southern Arizona to determine if this population is becoming genetically inhibited.

Ability to estimate the size of wildlife populations is a desirable and often difficult aspect of wildlife management. Statistically valid population estimates become increasingly important when managing isolated populations at low density, typical of desert bighorn sheep populations. The Silver Bell bighorn project will result in about 25 to 35% of the population being radiocollared to accomplish other objectives and will provide an excellent opportunity to evaluate survey techniques for estimating population size. Funding will be solicited to examine multiple methods to estimate size of bighorn sheep populations.

The primary benefit from this project will be a set of recommendations that resource managers can use to ensure continued existence of this endemic bighorn population including: (1) identification of important use areas such as movement corridors, lambing sites, watering areas, and foraging sites; (2) development of a detailed habitat use model for evaluating habitat quality of adjacent, unoccupied ranges and to evaluate adequacy of previous habitat evaluation methods; and (3) identify prevalence of a recent disease outbreak and implement measures to reduce impacts. In addition, funding will be solicited to evaluate genetic variation of the Silver Bell bighorn sheep population and to investigate survey methods that provide statistically valid population estimates for bighorn sheep management.

For more information contact:
Richard Ockenfels, Arizona Game and Fish Department, 5000 W. Carefree Highway Phoenix, AZ 85086-5000 .
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