Developing an empirical approach to optimal camera-trap deployment at mammal resting sites: evidence from a longitudinal study of an otter Lutra lutra holt

Abstract

The study of nocturnal mammals relies on indirect evidence or invasive methods involving capture and tagging of individuals. Indirect methods are prone to error, while capture and tagging mammals have logistical and ethical considerations. Off-the-shelf camera traps are perceived as an accessible, non-intrusive method for direct data gathering, having many benefits but also potential biases. Here, using a 6-year camera-trap study of a Eurasian otter holt (den), we evaluate key parameters of study design. First, we analyse patterns of holt use in relation to researcher visits to maintain the camera traps. Then, using a dual camera-trap deployment, we compare the success of data capture from each camera-trap position in relation to the dual setup. Finally, we provide analyses to optimise minimum survey effort and camera-trap programming. Our findings indicate that otter presence and resting patterns were unaffected by the researcher visits. Results were significantly better using a close camera-trap emplacement than a distant. There was a higher frequency of otter activity at the holt during the natal and early rearing period which has implications for determining the minimum survey duration. Reducing video clip duration from 30 to 19 s would have included 95% of instances where sex could be identified, and saved 35–40% of memory storage. Peaks of otter activity were related to sunrise and sunset; exclusion of diurnal hours would have missed 11% of registrations. Camera-trap studies would benefit by adopting a similar framework of analyses in the preliminary stages or during a trial period to inform subsequent methodological refinements.