622 A. Balestrieri et al.
FIG. 2 Neighbour joining tree showing the phylogenetic relationship between otters from the River Ticino (both non-invasive samples and one individual found dead on the road) and European, Chinese and Korean lineages.
FIG.3 Vortex simulations (scenario 1) for the reintroduced otter population, with initial population size N0 = 2 and the supplementation of four individuals in the following years (two per year): (a) population trend, (b) probability of survival, (c) proportion of iterations (F) that lead to extinction, and (d) proportion of iterations that lead to population size =N, 21 years after the reintroduction.
Sensitivity analyses indicated inbreeding depression as
the main cause of variation in the probability of population survival (Fig. 4). For numbers of lethal equivalents below the default value proposed by Vortex it is an index of inbreeding depression used by Vortex, as added in theMethods (6.29), the probability of survival increased from 12 to 45%.
Discussion
Despite the small number of founder individuals and lack of post-release monitoring, an otter population has survived on the River Ticino since 1997. Signs of otter presence were recorded on a 7 km stretch of the river in 2010 (Prigioni & Balestrieri, 2011), and along 60 and 32 km in
2017 and 2018, respectively. Throughout the study period, the population comprised few individuals. Although the sample size and number of re-captures were insufficient to assess population size effectively, the number of individuals was considerably smaller than the predicted carrying cap- acity. The minimum number of individuals was consistent with that assessed by the most likely scenario in the popula- tion viability analysis, based on the information available: an initial population consisting of only one pair of otters, supplemented with a further pair per year in the following 2 years. Faecal samples collected in 2016–2017 were stored for 1
year before extraction, whereas those found in 2018 were all analysed irrespective of how fresh they were. Despite
Oryx, 2022, 56(4), 617–626 © The Author(s), 2021. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605321000107
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