Use of small rich patches by Eurasian otter (Lutra lutra L.) females and cubs during the pre-dispersal period

Direct observation of Eurasian otters was used to demonstrate the characteristics and interannual use of small patches of habitats by females and litters in stretches of two rivers in north-east Spain. During censuses and other vigils, 215 otter sightings (involving 265 otters) were obtained, 14–18% of which were observations on females and litters. Thirty-five additional sightings were obtained in stretches used by litters of large cubs (47 individuals). Six small cub (2- to 6-month-old) and 15 large cub (>7-month-old) different family units (litters) were detected in both rivers as a whole. Average sizes of litters of small and large cubs were significantly similar, being respectively 1.17 (SD=0.41, range 1–2, n=6) and 1.27 (SD=0.46, range 1–2, n=15), so mortality of cubs was low at the period before dispersal. Litters were detected in significantly different patches from other otters, and litters of small and large cubs were also found in significantly different river sections. Litters of large cubs used small home ranges and core areas, and selected sections at reservoir tails, reservoirs, wider stretches, rough waters and refuges. Small cubs were found in narrow river stretches with calm waters. Selected patches had sheltered zones with abundant fish. This reduced use of space may result in the low rate of mortality.