Tracking the expansion of the American mink (Neovison vison) range in NW Portugal

Invasive in many European countries, the American mink (Neovison vison) was introduced in Portugal in the late 1980’s, presumably escaping from Spanish fur farms close to the border. In spite of the biological richness of the invaded area, no study ever addressed the evolution of the invasion process. We aimed to investigate the current distribution and status of the mink in NW Portugal and discuss some contributing factors to explain the rate of invasion. We detected mink presence using floating rafts as footprint tracking devices, and scats as a molecular tool aiding in species identification. Results demonstrate a clear range expansion southwards, with mink already occupying most of the region’s hydrographic basins. After a first phase of slow expansion (55 km in 20 years), mink seems to have expanded its range quite rapidly in only 2 years (45 km). The initial delay could be due to local thriving otter populations, whereas the recent establishment of red swamp crayfish (Procambarus clarkii) in the area could be a plausible explanation for the acceleration in the mink’s expansion. Being a key food resource, crayfish may be playing an important role as an expansion facilitator. Mink eradication is probably no longer feasible since well established populations near the border continue to function as sources for the Portuguese population. Therefore, a control program should start immediately in the NW region, preferably in conjunction with Spanish authorities.