The best swimmer of the terrestrial animals – the otter – has found a safe and stable home at Krka National Park

Due to the broad research conducted to date, and thanks to video surveillance and the detection of physical tracks, the presence of otter in the Park has been confirmed from Bilušića buk to Skradin.

From 13 to 15 September 2021, non-invasive data collection methods were implemented to monitor the presence of otter in the territory of Krka National Park. The current knowledge of its presence throughout the Krka River canyon, from Skradin to Bilušić buk, and at the confluence of the Krka and Čikola Rivers was confirmed. In Croatia, otter is a strictly protected species, protected under the Convention on the protection of European wild taxa and natural habitats (Bern Convention) and the Directive on the protection of natural habitats and wild plant and animal species. It is threatened primarily due to the destruction and fragmentation of suitable habitats and water pollution.

There is no precise knowledge on the abundance of otter in Croatia. It is a target species of the Natura 2000 ecological network, HR2000918 Broader area of Krka National Park, as a significant conservation area for species and habitat types. The goal has been set to conserve 1320 ha of suitable habitat needed to retain an otter population of at least 21 to 23 individuals in that area. Along with regular monitoring of otter presence, conducted once every six years, the Public Institute of Krka National Park is also planning to conduct genetic analysis as the primary and most precise method to determine the population size. Analysing DNA isolated from hair or freshly collected faeces can be used to determine the population size and structure (male/female), the inhabitation range, distribution and the genetic diversity within this otter population.

Otter (Lutra lutra) is a carnivore in the weasel family. It is adapted for an aquatic life and its habitats are primarily freshwater. It is an agile swimming since its body structure is adapted for swimming and diving. The body is slender, elongated, with short arms and legs, and a uniform cylindrical tail. The adult male weighs about 10 kilograms and is from 100 to 135 cm long. Females are smaller, weighing about 7 kg and 90 to 125 cm long. The webbed feet and power tail make it a strong swimmer, the “whiskers” are sensitive to touch, and its strong sense of sight enables it to hunt underwater. The thick fur, brown to dark brown with a lighter belly that is grey to white, maintains its body temperature. The otter call is a clear but quiet whistle, and it also squeaks and growls. Communication between otters is by smell.

Otters can remain in water for a long time, and they use their sharp, strong teeth to catch prey. They feed primarily on fish, crustaceans, bivalves, amphibians, birds, small mammals and aquatic invertebrates. They spend several hours a day hunting, and can cover great distances in search of food. Adults can swim 1.5 to 2 km per hour, and can spend seven to eight hours hunting at night.

The otter is mostly nocturnal, spending the day in its den underground or in another aboveground shelter. In its territory, it uses up to 30 safe and secure resting areas that it uses for various purposes, and the most secure and least accessible is used as a den for raising its young.

It is a highly territorial animal that lives alone, except during mating season and the first year of life. This solitary life is due to its strong need for a large living space. Males become sexually mature at 18 months, females at 2 years. The gestation period is 2 months, and the female usually gives birth to two to three young. The mortality rate among young is high, and the female usually raises just one young per year, and has three litters at most during her life.  During the first six weeks of life, the young are completely helpless and live just off of mother’s milk. Males do not participate in raising the young. The young otters learn to swim at an age of 12 weeks, once their thick water-resistant fur has grown. After 8 to 9 months, the young otters first start to separate from their mother, and are completely independent after one year.

Otters are predators at the very top of the food web, and they contribute to establishing ecological balance in aquatic ecosystems. Often, the available of prey is even more important than the size of the habitats for otter colonisation. A moderate density of otter is when there is one individual on 15 km of river. All predators larger than the otter are its natural enemies. Its lifespan in the wild is short, from three to four years, and rarely it can live up to 10 years, while it can live up to 15 years in captivity.