Capture–recapture methods 415 Cost comparison


We recognize that a direct comparison of costs associated with different methods of sampling is difficult given the dif- ferent tasks and information acquired with each method; however, we chose to evaluate the cost of obtaining an abun- dance estimate using aerial methods versus genetic capture– recapturemethods. The cost of aerial flights changes little, if at all, with an increase in population size. The cost of genetic capture–recapture methods, however, generally increases with an increase in population size and the need to collect and analysemore samples. Using our simulation results, we determined what level of sampling effort (i.e. sample size and number of sessions) would produce a CV equivalent to that of the aerial methods (c. 21%) at true abundance equal to the 2014 aerial survey estimate (202, 95%CI 171– 334).Wealso determined the cost for genetic capture–recap- ture to obtain a more precise CV of at least 10% with 200 individuals. For genetic capture–recapture, costs included supplies


for sample collection, DNA extraction and analysis, and as- sociated labour for field and laboratory work (i.e. time spent collecting samples, recording them in a database, extracting DNA, generating consensus genotypes and a capture his- tory; Supplementary Table 2). The time does not include conducting analysis for abundance and/or survival esti- mates. This is difficult to estimate as it will vary based on experience and could be conducted by in-house personnel or by a contractor. We also did not include travel time to the drinkers because management personnel visit drinkers for other management tasks with the same frequency (c. every 7 days) as our genetic capture–recapture sampling design. Travelling to distant locations will obviously result in increased fuel and time costs and must be accounted for in the study design. Although the time required for sam- ple collection will vary between studies, we estimated c. 27 hours annually (2 technicians at 13.5 hours each) for collec- tion of 1,000 samples. As pay rates vary between field and laboratory personnel, labour costs were based on an average rate (USD 25.00/hour). For comparison, we divided the cost of the biennial flight into an annual cost. This cost includes flight time and pilot salary, but does not include the salaries of personnel conducting the counts or personnel perform- ing analysis of sightability models.


Results Simulations


Weran 126 simulations (83 inMARK, 43 in capwire) varying sampling intensity and true abundance, with estimator per- formance dependent on sample size and true abundance (Fig. 1, Supplementary Table 1). As expected, increasing sample size (relative to number of sessions and number of


FIG. 1 Abundance estimates (y axis) from simulations for Sonoran pronghorn Antilocapra americana sonoriensis, with true abundance of 200 individuals in one session for single session models in capwire and two and three session closed capture (MARK) models. No shading indicates relative mean squared error (RMSE) .0.5, and hatching represents RMSE #0.5. Trends were the same in all simulations but see Supplementary Table 1 for complete results.


individuals) generally led to less bias and lower RMSE va- lues for both estimators. Abundance estimates were always positively biased in capwire (mean bias = 14.58%) and always


negatively biased inMARK(mean bias =−4.88%). Bias ran- ged from overestimating by 64 individuals to underestimat- ing by 25 individuals, and was larger with higher population sizes and with fewer samples per individual (Table 1). The


Oryx, 2020, 54(3), 412–420 © 2018 Fauna & Flora International doi:10.1017/S003060531800011X


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