Estimating Sizes of Wild Pig Populations in the North and Central Coast Regions of California

Abstract

We developed a mark-sighting approach with automatic camera systems and assessed the size and density of wild pig (Sus scrofa) populations in the North and Central Coast regions of California in 1994 and 1995. Eighty and 149 wild pigs were captured and tagged at 4 and 6 study sites in 1994 and 1995, respectively. Of those animals, 202 (88%) were subsequently sighted among over 3,000 photographs of wild pigs obtained from 54 camera stations. Sighting rates for both tagged and nontagged wild pigs were similar between years and averaged 2.8 ± 0.5 and 2.8 ± 0.4 wild pigs/camera-station night, respectively. Sighting rates for all wild pigs during the study averaged 5.7 ± 0.8 wild pigs/camera-station night. Mark-sighting data analyzed with program NOREMARK provided estimates of population size, and narrow 95% confidence intervals (95% CI) for most research sites suggested we obtained sufficient sightings for reliable estimates of size of populations. It also proved possible to identify nontagged wild pigs in photographs for determining minimum population sizes, and the 95% CI from mark-sighting estimates encompassed the minimum population sizes for 8 of 10 study sites. An important factor influencing the reliability of population estimates was the proportion of tagged animals resighted. At one site where few tagged animals were sighted, 95% CIs were large and did not encompass the minimum population estimate. Mean population densities ranged from 0.7 to 3.8 wild pigs/ km 2 , comparable to previous research on wild pigs in the regions we studied. Densities increased from 1994 to 1995, potentially related to higher rainfall and increased forage availability in 1995. Also, densities were lower in areas with relatively high hunting pressure, suggesting that sport hunting may be effective at reducing wild pigs numbers in some areas.