Eurasian otters are well-adapted to water as semi-aquatic carnivores. Water provides their main source of food – fish – and it also serves as protection from predators and competitors. Otters are able to hunt for fish year-round and when diving, otters are also protected against things like bullets, spears, or hunting dogs. Living close to a water source means for the otters that there is enough food availability, which is needed so that their energy budget says balanced. When in the water, an otter expends over 25 times more energy than when on land. This means that to keep the otter’s body temperature at a normal range while active, between 39° - 40° Celsius, an otter weighing 6-12kg needs to eat around 1kg of food per day. Thus, calorie-rich fish such as fatty eel and carp quickly become an otter’s favourite food.
The Eurasian otter is primarily nocturnal and therefore mainly hunt for fish during the night, and are seldom observed being active during the day. Although considered predominately nocturnal, these otters have 4-6 active phases per day and, therefore, several resting periods also occur during the night.
There are only fragmented research findings available on the space-time behaviours of otters. It is very difficult to observe otters in the wild or to equip them with data transmitters, such as tracking devices. It is assumed that Eurasian otters live within large territories, which can cover around 10 – 20 km along a river and depending on food availability, number of hiding opportunities, and population density. Otters of the same sex do not tolerate each other in one territory, but otters of the opposite sexes can live in territories that overlap completely. Male otters have larger territories, which can then overlap with 1 – 3 female territories.
As semi-aquatic carnivores, Eurasian otters stay close to water. Here, they can find food and protection. But otters can also run long distances over dry land, especially to travel between pond areas or to reach other waterbodies.