Population Estimates with Age and Genetic Structure of a Harvested Bear Population in Eastern North Carolina

Abstract

Noninvasive genetic sampling (NGS) is appealing because it facilitates the use of more robust, capture-recapture models to estimate population size. NGS is expensive, however, and current sub-sampling approaches, though made a priori, are made with incomplete knowledge of the ramifications. I compared model selection and population estimates from all hair samples to those from subsets of samples chosen by simulating 4 published sub-sampling approaches. I used 4 weeks of samples collected from black bears (Ursus americanus) at scented DNA hair traps during Spring 2003 and again during Spring 2004 in Hyde County, North Carolina. I found that follicle filters deleted individuals from the data set without altering sex ratio, but random sub-sampling both deleted capture histories and altered the sex ratio. Collectively, these decisions biased population estimates low and produced inconsistent model selection among 10 replications.

I also conducted a 13-week study in Spring⁄Summer 2004 to investigate effects of using food and scent to lure bears to DNA hair traps. Food and scent collected twice as many hair samples as just scent, but produced similar estimates. I do not recommend using follicle filters or sub-sampling; my data suggest they may reduce NGS to an expensive population index. Instead, I recommend using only scent to lure bears, identifying all samples for gender, and genotyping just female samples. This approach estimates the female population size and, combined with ages from trapped bears and ages with fecundity data from hunter harvested bears, allows estimation of reproductive rate, which are especially valuable for population monitoring.

Model Mo fit females best and model Mb fit males best for both 2003 and 2004 and produced population estimates of 223 females and 160 males. Using reproductive tract data from hunter harvested bears and Spring estimates of breeding-age females, I estimated yearly cub production as 97 cubs of each sex for a total population estimate of 577 bears in our 404.3 mi2 (1,047.2 km2) study area. My study area has averaged about 120 hunter harvested bears the past 15 years. Because I estimated the net reproductive rate was 1.0, the maximum sustainable yield appeared to be 20.7 %.