Individual identification and distribution assessment of otters (Lutra lutra) through non-invasive genetic sampling: Recovery of an endangered species in the Basque Country (Northern Spain)

Non-invasive genetic techniques have proven to be cost-effective for monitoring and studying otter populations, largely due to the linearity of otter territories and the marking behavior of the species. After a severe decline, in the 60, the Eurasian otter is recovering in the Iberian Peninsula. However, the recovery pattern is not homogeneous and the species is still considered “Threatened” in many regions. During 2007–2010 a systematic non-invasive genetic sampling effort was carried out to determine the spatial distribution and to estimate the population size of an endangered otter population in northern Iberian Peninsula (Basque Country). Samples were identified to species level by sequencing a 226 bp mtDNA fragment prior to genotyping. Among the 132 obtained samples, 127 (98.4%) belonged to the study species, one sample was genetically identified as European mink (Mustela lutreola) and one as American mink (Neovison vison) while genetic species confirmation was not possible in the three remaining samples. These results provided novel and accurate data on species distribution, highlighting an overall increase of 25% in 10 × 10 UTM grids occupied by otter and a clear pattern of re-colonization upstream of the main rivers. All samples corresponding to otter were subsequently individually genotyped using a novel multiplex panel of 11 microsatellite markers and sexed by typing the sex-chromosome-related gene ZFX/ZFY. We obtained a complete individual genetic profile for 55 samples (genotyping success 43%), corresponding to 20 different individuals (11 females, 6 males, and 3 individuals of unknown gender). The mean otter density in occupied areas estimated to be 0.09 (0.06–0.12) individuals per river kilometer. The present study enabled us to obtain updated and relevant information about this elusive species’ distribution and population size, essential to define population status and to design successful and effective management and conservation programs.