576 N. van Doormaal et al.


search, the distance walked and the area covered, with the R package sf (Pebesma, 2018). Wecompared the number of reported imitation snares to


the number of imitation snares available for detection. We used non-parametric bootstrapping to estimate the variance in detection probabilities, because the number of reported snares was low, which precludes maximum likelihood esti- mation techniques. Non-parametric bootstrapping repeat- edly draws new samples of equal size with replacement from the collected data, and each original observation has an equal probability of being drawn into the new sample. Detection probabilities were then calculated using the newly drawn sample. By repeating these steps 10,000 times for the baseline sample and every patrol strategy, we calculated the average detection probability and a 95% confidence interval.We used the base functions in R for the bootstrap resampling and the ggplot2 package for visuali- zation (Wickham, 2016). Fisher’s exact test (Fay, 2010)was used to compare the detection probabilities of every strategy to the baseline detection probability.


Results


In total, 69 snare searches were conducted, six of which had no recorded GPS tracks because rangers forgot to record them or because there were technical problems with the equipment. Because of logistical and technical issues, only two snare searches were conducted in April 2019, and these were not recorded. The final dataset included 63 re- corded searches (39 searches by rangers and 24 systematic searches), covering c. 263 km over 132 patrol hours (Table 1). We combined all 63 recorded snare searches and their observations to assess how many imitation snares remained undetected during the study period. A total of 39 (23%) of the 166 imitation snares were reported. In addition, a total of 11 real, but old snares (i.e. probably placed some time be- fore the study) and one bundle of snares (several snares tied together with a piece of string; not set up to catch animals) were reported by the rangers and volunteers during the study period. The probability that an imitation snare re- mained undetected at the end of the study (after 200 days) was c. 0.77 (Fig. 2). The first imitation snares were reported after 77 days (c. 2.5 months) and seven recorded patrolswere conducted during this period. The median time until detec- tion could not be estimated because more than half of all imitation snares remained undetected during the study period. We estimated the baseline detection probability to be


19%(Table 1, Fig. 3). Circa 15% of imitation snares within the 10 m buffer of the spatially focused patrols were re- ported, and detection was 27% for patrols with independent observers (Table 1, Fig. 3). The largest number of imitation snares were located in the systematic searches: searches


Oryx, 2022, 56(4), 572–580 © The Author(s), 2021. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605320001301


TABLE 1 Patrol effort and detections for different search strategies for locating snares. Patrol strategy


Patrol effort


No. of recorded searches


Standard patrols


Systematic searches Parallel lines


Quadrant pattern Total


Spatially focused patrols 12 Independent observers


20 7


12 12 63


Total distance walked (km)


101.7 58.2 24.8


45.5 50.9


263.6 1Tested against the baseline detection rate of standard patrols.


Total duration (hours & minutes)


34 h 1 min 24 h 40 min 10 h 33 min


26 h 45 min 36 h 30 min 132 h 29 min


Total coverage (km2)


0.92 0.80 0.38


0.67 0.55 2.27


% of study area covered


22 19 9


16 13 54


Detections


No. of imitation snares in range


26 27 11


31 31


126


No. of imitation snares detected


5 4 3


14 13 39


Estimated detection rate


0.19 0.15 0.27


0.45 0.42


Bootstrap 95% CI


0.08–0.31 0.04–0.26 0.09–0.54


0.32–0.61 0.29–0.58


Fisher’s exact test1


0.79 0.44


0.04 0.06


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