Reducing effort while improving inference: Estimating Dall’s sheep abundance and composition in small areas

Schmidt, J; Rattenbury, K

Rcent work has demonstrated that aerial distance sampling surveys are more efficient and effective than unadjusted minimum count surveys for estimating Dall’s sheep (Ois dalli) abunda, although larg requirements (e.g., 150–200 detections) may discourage implementation of these methods in small (<2,500 km2 ) or low density areas. However, using informed priors and borrtection information across survencrease precision and decrease onducted distance sampling surves a majority of the Dall’s sheeervice units in Alaska during 2010–2011. We compared 4 analyticon information to demonstrate the increases in efficiency that approach. Based on our analysis using all available survey info detection function, we estimated that 2,252 (1,871– 2,765), 2,0) sheep occurred in Denali National Park and Preserve (DENA), rve subarea of Gates of the Arctic National Park and Preserve, ctively. These estimates were achieved with relatively small num00), suggesting that sample size requirements for Dall’s sheep over previously recommended levels when adequate prior information is available. In addition, we describe a formal approach fical framework as part of the abundance estimation process. This approach to the estimation of population composition could repd years. We found that ratios of full curl rams:100 ewe-like sheep in WEAR and Itkillik preserve subarea populations were lesstent productivity. We recommend that aerial distance sampling survey methods using prior information, combined with direct estiroughout their range. 2013 The Wildlife Society.evncee sample sizea Bayesian analytical approachowing deys can irequired sample sizes. Using these methods, we cys acrosp habitat within National Park Sal scenarios using increasing amounts of detectican be gained over time through the use of this rmation in the estimation of the809 (2,361–3,379), 1,669 (1,339–2,120), and 12,428 (10,780–14,47the Western Arctic National Parklands (WEAR), the Itkillik preseand Wrangell-St. Elias National Park and Preserve (WRST), respebers of group detections in DENA (n ¼ 57), the Itkillik preserve area (n ¼ 48), and WEAR (n ¼ 1distance sampling surveys can be reduced by an additional 50–75%or estimating the size of individual composition classes (i.e., lambs, ewe-like sheep, <fullcurl rams, full-curl rams) and sex and age ratios, corrected for incomplete detection. We implemented the composition analysis within the distance sampling analytlace commonly used indices and provides more detailed and rigorous estimates that are directly comparable among survey areas an than in the DENA population, whereas lamb:ewe-like ratios were similar across all 4 survey areas, suggesting relatively consismation of population composition, be used to increase the effectiveness of Dall’s sheep population monitoring and management th

Reducing effort while improving inference: Estimating Dall’s sheep abundance and composition in small areas

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