We compared different methods of monitoring an American mink, Mustela vison, population in an European mink, Mustela lutreola, area. The study was carried out in the Butron basin, northern Spain. We compared sign surveys and trapping in the same non-continuous 500 m sections of river. We also radio-tagged 10 minks to evaluate the efficiency of the two methods. There was no significant association between the presence of tracks and trapping success. We noticed the presence of minks in sections where none was captured and vice versa. Furthermore, by monitoring the radio-collared minks, we found inter-sexual differences in spatial use that produced differences in the success of the trapping and sign surveys, with males using the main streams and females selecting little streams where tracks were difficult to survey. Moreover, we detected tracks in the sand of 17 minks that actively avoided the traps. Hence, 1) neither sign surveys nor trapping are reliable methods for estimating relative abundances because both are subjected to strong biases. 2) the spatial use of habitat by minks could severely affect management projects that consider only main streams and 3) projects aimed at extracting American minks need to consider those individuals that never trigger traps, females living in very small streams and the floating population living in other habitats.