Evaluating likelihood-based photogrammetry for individual recognition of four species of northern ungulates

Estimating abundance is a key component of wildlife management and capture–mark–recapture (CMR) methods are commonly used. Photography has been widely explored as a method of ‘marking’ individuals for CMR. However, it is most often used on species with unique markings, and application to species without obvious marks remains challenging. We tested a likelihood-based photogrammetric method to identify individuals from four species of ungulates: muskox (Ovibos moschatus; 31 photos of 16 individuals), thinhorn sheep (Ovis dalli; 110 photos of 32 individuals), mountain goats (Oreamnos americanus; 47 photos of 19 individuals) and mule deer (Odocoileus hemionus; 60 photos of 30 individuals). Misidentification rates (false rejection [FRR] and false acceptance [FAR] rates) varied widely between species (FRR =  < 1–13%, FAR = 2–22%), as did matching success rates (48–96%). Muskox and sheep had the highest matching success rates (96% and 88%, respectively), while those for goats (80%) and deer (58%) were lower. Automated matching success rate for deer and sheep, calculated based on the top-ranked photograph, was compared to a user-generated matching success rate. The latter was significantly higher in all cases, indicating that the final subjective step was important. Significant observer bias was found for deer, but not sheep. The main findings of our study were that a photogrammetric approach to individual recognition of ungulates was species specific and sensitive to the inclusion of horn measurements, season, photo quality, and final selection from top choices by the user. While matching success rates of photogrammetry was reasonably good (> 90%) for muskox, it was only moderately accurate 80–90% for sheep and goats, and poor (< 80%) for deer. Future work should address the temporal stability of horn measurements, seasonal effects, feasibility of capturing useable photographs from remote cameras, and application to other species.