DISTRIBUTION AND ABUNDANCE OF BIGHORN SHEEP IN ROCKY MOUNTAIN NATIONAL PARK, COLORADO

McCLINTOCK, Brett

Anecdotal evidence of a pneumonia epizootic among bighorn sheep in Rocky Mountain National Park (RMNP), Colorado, during the mid-1990s prompted Park officials to examine the current condition of the herds. With an emphasis on the adult ewe population, we designed a mark-resight study to estimate abundance, recruitment, survival, minimum fecundity, rate of population change, distributions, and movements based on observations from December 2002 to May 2005. Fifty-nine adult ewes were captured, sampled, and radio-collared via helicopter net-gunning during winter 2002 – 2003. Mark-resight data for Bowden’s Estimator of abundance were collected in 2003 and 2004 from ground surveys conducted May – September. The 2003 adult ewe population was estimated at 175.7 (SE = 11.1) sheep. Overall observed ratios of yearlings:ewes, lambs:ewes, and rams:ewes produced abundance estimates of 42.1 (SE = 5.2) yearlings, 78.7 (SE = 9.4) lambs, and 93.3 (SE = 9.2) rams for a total 2003 bighorn population estimate of 389.9 (SE = 34.9, 95% CI = 327.2 – 464.6). In 2004, adult ewe abundance was estimated at 168.9 (SE = 11.9). Overall yearling, lamb, and ram ratios produced abundance estimates of 33.7 (SE = 4.8), 92.1 (SE = 10.0), and 71.7 (SE = 8.1), respectively, providing a total 2004 population estimate of 366.4 (SE = 34.7, 95% CI = 304.4 – 441.0). The majority of sheep continue to be found in the West-side herd. Marked ewes were observed to have 100% fidelity to 4 subranges. No evidence of movements between the East-side and West-side ranges was recorded. Minimum fecundity for 2003 and 2004 averaged 0.74 (SE = 0.05) lambs per ewe, but annual lamb recruitment per ewe and lamb survival from July 2003 to July 2004 were 0.15 (SE = 0.07) and 0.24 (SE = 0.09), respectively. Mean adult ewe survival for 2003 – 2004 was 0.89 (SE = 0.09) for the East-side herd and 0.91 (SE = 0.04) for the West-side herd. Mean adult ewe survival for 2004 – 2005 was 0.44 (SE = 0.20) and 0.91 (SE = 0.04) for the East-side and West-side herds, respectively. We found some evidence of declining adult ewe survival as age increases.