Low signs of territorial behavior in the Eurasian otter during low-water conditions in a Mediterranean river

Abstract

The Eurasian otter Lutra lutra is a territorial semi-aquatic carnivore usually found at low densities in rivers, coastal areas, and wetlands. Its diet is based on prey associated with aquatic environments. Mediterranean rivers are highly seasonal, and suffer reduced flow during the summer, resulting in isolated river sections (pools) that sometimes can be left with a minimal amount of water, leading to concentrations of food for otters. To our knowledge, this process, which was known to field naturalists, has not been accurately described, nor have otter densities been estimated under these conditions. In this study, we describe the population size and movements of an aggregation of otters in an isolated pool in the Guadiana River in the Tablas de Daimiel National Park (central Spain), which progressively dried out during the spring-summer of 2022, in a context of low connectivity due to the absence of circulating water in the Guadiana and Gigüela rivers. Using non-invasive genetic sampling of 120 spraints collected along 79.4 km of sampling transects and spatial capture-recapture methods, we estimated the otter density at 1.71 individuals/km of river channel length (4.21 individuals/km²) in a progressively drying river pool, up to five times higher than previously described in the Iberian Peninsula. The movement patterns obtained with the spatial capture-recapture model are not quite different from those described in low density, which seems to indicate a wide home range overlap, with low signs of territoriality.

Keywords: Lutra lutra; Aggregation; Density estimate; Mediterranean rivers; Pools; Spatial capture-recapture.

Figure 1

Map showing the location of the Tablas de Daimiel National Park (top left), the study area (red rectangle), and changes in the flooded area between January and August 2022. The hexagonal grid was used to divide the sampling area into sampling cells, the samples in each cell being assigned to its centroid, to generate 'detectors' for use in the spatial capture-recapture model. Created using ArcGIS.

Figure 2

Local maxima for each individual otter Lutra lutra activity center, calculated from contour maps of the posterior distribution of activity centers in the Tablas de Daimiel National Park (Spain) in 2022. Table show direct familial relationships inferred among the 13 individuals. The six offspring are listed in the first column, along with their sex, father (when observed), full-siblings, and half-siblings. The probability of the relationship is given in parentheses for parent–offspring and full-siblings (see also Supplementary Information S2–S6, and Figure S7). Created using R.

Figure 3

Histogram plots of observed values (red vertical lines) and posterior predictive values for the random thinning-spatial capture-recapture model. In each plot the Bayesian p value is shown (top right). Created using R.