Illegal hunting threatens the establishment of a lynx population and highlights the need of a centralized judiciary approach

O_LIIllegal hunting (poaching) represents a major threat to the conservation of large predators. Yet, its impact remains difficult to quantify as there are strong incentives to conceal this criminal activity. Attributing changes in the population status of large carnivores in part to poaching is therefore an important conservation challenge. C_LIO_LIWe present a case study of lynx (Lynx lynx) in southwestern Switzerland (canton of Valais) where the current distribution range is much smaller than it was in the recent past and population density is now >80% lower than in other lynx populations in the Swiss Alps, particularly in the adjacent Pre-Alps. We tested four hypotheses to explain this far lower density: 1) a too low density of trail camera-traps deployed for lynx surveys in Valais compared to the Pre-Alps (i.e. a methodological artefact); 2) less favourable environmental conditions around the camera-trap sites; 3) lower densities of the main prey; and 4) poaching. C_LIO_LIWe estimated lynx and ungulate densities and environmental conditions at trail camera sites, and were able to clearly reject the first three hypotheses: 1) the monitoring protocol was similarly effective; 2) environmental conditions around the trapping sites in Valais were even more favourable to lynx detection than in the Pre-Alps; and 3) prey supply was even larger. Concerning hypothesis 4, we discovered a local, but dense network of 17 illegal lynx traps in the narrow main immigration corridor into Valais from the thriving adjacent lynx population in the Pre-Alps, suggesting intense local poaching. C_LIO_LIOur findings substantiate the suspicions of long-lasting lynx poaching as a threat to the establishment and survival of the Valais population. The fact that instances of poaching were publicly known since 1995 but remained unabated for at least two decades, until a first conviction occurred, questions the commitment of local authorities to address this case of wildlife crime. Our study shows that inquiries about wildlife crime such as top predator poaching may need to be carried out at the highest levels of jurisdiction to avoid any risk of collusion between law enforcement agents and poachers. C_LI