Abstract
Accurate and precise abundance estimation is vital for informed wildlife conservation and management decision-making. Line transect surveys are a common sampling approach for abundance estimation. Distance sampling is often used to estimate abundance from line transect survey data; however, search encounter spatial capture-recapture can also be used when individuals in the population of interest are identifiable. The search encounter spatial capture-recapture model has rarely been applied, and its performance has not been compared to that of distance sampling. We analyzed simulated datasets to compare the performance of distance sampling and spatial capture-recapture abundance estimators. Additionally, we estimated the abundance of North Atlantic right whales in the southeastern United States with two formulations of each model and compared the estimates. Spatial capture-recapture abundance estimates had lower root mean squared error than distance sampling estimates. Spatial capture-recapture 95% credible intervals for abundance had nominal coverage, i.e., contained the simulating value for abundance in 95% of simulations, whereas distance sampling credible intervals had below nominal coverage. Moreover, North Atlantic right whale abundance estimates from distance sampling models were more sensitive to model specification compared to spatial capture-recapture estimates. When estimating abundance from line transect data, researchers should consider using search encounter spatial capture-recapture when individuals in the population of interest are identifiable, when line transects are surveyed over multiple occasions, when there is imperfect detection of individuals located on the line transect, and when it is safe to assume the population of interest is closed demographically. When line transects are surveyed over multiple occasions, researchers should be aware that individual space use may induce spatial autocorrelation in counts across transects. This is not accounted for in common distance sampling estimators and leads to overly precise abundance estimates.
Highlights
The cornerstone of effective wildlife population management is the ability to estimate population abundance accurately and precisely
The spatial capture-recapture (SCR) abundance estimator had lower root mean squared error (RMSE) than all distance sampling (DS) estimators and had nominal credible interval coverage, whereas DS estimators had below nominal coverage
The search encounter SCR model has not been applied beyond its conception [22, 38], and we are unaware of any such comparison between DS and SCR models
Summary
The cornerstone of effective wildlife population management is the ability to estimate population abundance accurately and precisely. Two widely used methods for estimating abundance are distance sampling (DS) and spatial capture-recapture (SCR), a recent development of mark-recapture [1], and there are situations in which both methods could be suitable. We compare DS and SCR abundance estimation after providing a brief introduction for both approaches. Distances to detected individuals are used to estimate the scale of a detection function, which describes the probability that an individual is detected, given its distance from the observer. The detection function and abundance can be estimated simultaneously as in one-stage, hierarchical distance sampling [8]. A two-stage approach can be taken where the detection function is estimated first, and abundance second using a Horvitz-Thompson estimator based on the average detection probability over the surveyed region and the number of individuals detected [6]
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