Because the islands are small and isolated, they are not home to any indigenous land mammals.
In the 1800s, seals were common on the islands, but in the early 1900s, they were hunted to near extinction, on both Christiansø and Bornholm. Happily, the grey seal has returned in recent years. With up to 400 resting grey seals in 2013, Ertholmene is now one of the largest resting sites for this big seal.
Some bats are found as migratory visitors.
The animal life on the islands is primarily concentrated around the waterholes and ponds. The noisiest animal is the marsh frog, but it is notoriously difficult to spot, because it is so quick to hop away. The rain brings out all the toads, so watch where you step. Other amphibians include little newts and the European green toad. There are not many mammals on Ertholmene, but there is still a small colony of hedgehogs, which were released on the islands in the 1950s.
Ertholmene has played a key role in the fishery industry in the Baltic Sea for centuries, and some of the most important fish spawning grounds are located in these waters. Herring, flounder, turbot, garfish, eel, cod and salmon can all be caught here. Because the Baltic Sea in this area is brackish water, it is not uncommon to find sculpin, herring, sprat, eel, eelpout and pike in the low waters around the islands.
Fishing has declined with the dwindling fish populations, especially as a result of the presence of the grey seals, but also due to fishing quotas. Consequently, fishery has lost its importance for the business community on the island.
Today, the herring processed at the island’s little fish processing plant, Ruths Kryddersild, is all imported.
HONEYBEES ON CHRISTIANSØ
Ertholmene is a reservation for the Nordic honeybee.
This is an old species of honeybee which was the only type of honeybee north of the Alps until around 170 years ago.
In Denmark, this old species is only found on the islands of Læsø, Endelave, Anholt and Ertholmene.
Other locations are home to other bee species or to hybrids of different species.
The Nordic honeybee thrives at lower temperatures than other species of bee. This means that they do not need as much winter feed and that they survive winter better.
Out of consideration for breeding, the bees on the island must be protected from other species of bee. In return, Christiansø can serve as a breeding station, where bee farmers can mate their unfertilised queens with Nordic drones.
The islanders love the bees. They say that they harvest ten times as many strawberries and apples thanks to pollination by the bees, compared to when there were no bees.
The bees also improve pollination of many of the wildflowers and flowering plants, so they produce more seeds and berries for the islands’ many migratory birds.