Optimizing small mustelid monitoring: Enclosed camera traps increase detection of the smallest carnivores

Abstract Land use changes in Europe contribute to the decline of once‐abundant species. While these declines are well documented for some species, other, more elusive species could quietly disappear. As a result, small mustelids are believed to be declining across their historical range. Their small size and elusive ecology make small mustelids challenging to monitor and thus remain understudied. In this study, we tested the effectiveness of three camera trap‐based methods to monitor common weasel Mustela nivalis , stoat Mustela erminea , and European polecat Mustela putorius . We deployed unenclosed, semi‐enclosed, and fully enclosed camera traps in a clustered design incorporating all methods during the fall of 2023 in two extensive agricultural areas in the Netherlands. Using a multi‐scale occupancy approach, we assessed (1) how detection probabilities differ among the three camera trap methods for each small mustelid species and (2) how scent‐based lures and placement near passages influenced detection probabilities. We found that weasels had the highest detection probability in fully enclosed camera traps placed within clusters containing a scent‐based lure. The detection probability of stoats was highest in fully enclosed camera traps, regardless of the presence or absence of lure, as well as in unenclosed camera traps with no lure nearby. Polecats had the highest detection probability in unenclosed camera traps, regardless of lure presence, and in semi‐enclosed camera traps without lure nearby. Placing camera traps near passages increased detection probability for all three species. Practical implication : This study advances monitoring protocols for small mustelids, a group facing suspected population declines despite limited data. We highlight different detection probabilities among three mustelid species using various camera‐trap methods. Camera trap placement and species‐specific use of scent‐based lures, beneficial for weasels but not for stoats or polecats, should be considered by researchers and wildlife managers. Combining fully enclosed and unenclosed camera traps enhances species detection and offers broader ecological insights by monitoring other prey and predator mammals as bycatch. Our findings provide practical guidance for large‐scale monitoring efforts of small mustelids across Europe.