Christiansø AI-pedia

Christiansø is the largest island of Ertholmene (formerly spelled Ærtholmene), a small archipelago in the Baltic Sea that belongs to Denmark. The name Christiansø is often used for the whole archipelago. The islands lie about 18 kilometres (11 miles) northeast of the town of Gudhjem on the island of Bornholm. They include Denmark's easternmost point. The name Ertholmene comes from Danish and means "pea islands" or "pea islets".

Two of the islands, Christiansø and Frederiksø, are inhabited. In 2024 they had a combined permanent population of 90. The archipelago covers a total area of 39 hectares (96 acres), which is about 0.39 square kilometres. The islands are made of bare granite with small areas of grass, gardens and trees. The historic buildings, fortifications and wildlife are protected by law.

From 1684 until 1855 the islands were an important Danish naval fortress. The fortress has had no working military role since then. The islands are still formally administered by the Danish Ministry of Defence, and they do not belong to any municipality or region. This makes them unusual within Denmark. Today the local economy depends almost entirely on tourism. Tens of thousands of people visit each year, most of them on day trips from Bornholm in summer.

Name

The Danish name Ertholmene combines ert or ært ("pea") with holm ("islet", a small island), and ends with the Danish definite plural suffix -ene ("the"). The name therefore means roughly "the pea islets". The older spelling Ærtholmene is still found in historical texts. The name probably refers to how small the islands look, but the exact origin is not certain.

The two inhabited islands are named after Danish royalty. Christiansø ("Christian's island") is named after King Christian V, who reigned when the fortress was founded in 1684. Frederiksø ("Frederik's island") is generally understood to be named after a prince of the same royal house. The name of the uninhabited island Græsholm means "grass islet".

In everyday Danish, people often say "Christiansø" for the whole group of islands, especially when they talk about travelling there or about the community. Official sources use "Ertholmene" for the archipelago as a whole.

Geography

Location

Ertholmene lies in the southern Baltic Sea, northeast of Bornholm. Bornholm itself is a Danish island that lies far east of the rest of Denmark, closer to Sweden and Poland than to Copenhagen. Ertholmene lies even further east, and is therefore the most remote inhabited part of Denmark in that direction. The approximate coordinates of the archipelago are 55°19′12″N 15°11′16″E.

The closest town on Bornholm is Gudhjem, about 18 kilometres away. Boats to the islands usually leave from there.

The islands

The archipelago consists of three main islands and a number of small skerries. A skerry is a small, rocky islet, often too small or too exposed for vegetation or habitation.

Island	Status	Notes
Christiansø	Inhabited	Largest island; main settlement, harbour, church, Store Tårn (Great Tower) with lighthouse
Frederiksø	Inhabited	Smaller island across the harbour from Christiansø; Lille Tårn (Little Tower), which houses a museum
Græsholm	Uninhabited	Bird sanctuary; public access is not allowed
Østerskær	Uninhabited skerry	Denmark's easternmost point
Other skerries	Uninhabited	Several small rocks and islets, mostly important for birds and seals

Christiansø and Frederiksø lie very close together. Between them is a narrow, sheltered stretch of water that forms a natural harbour. This harbour was the main reason the islands were useful to the navy. A pedestrian footbridge crosses the harbour and links the two islands. It is a swing bridge, which means a section can be turned aside to let boats pass.

Landscape and geology

The islands are formed of granite bedrock, like much of Bornholm. Much of the surface is bare rock, polished and shaped by ice during past glaciations. There is little natural soil. Over the centuries, residents and the garrison created small gardens, often protected by stone walls, and planted trees and shrubs. As a result, parts of Christiansø and Frederiksø look green and sheltered, while other areas are open rock exposed to wind and sea.

The islands are low, and their small size means that no point on them is far from the sea. The whole archipelago could fit many times over inside a typical Danish town.

Climate

Ertholmene has a maritime climate, which means it is strongly influenced by the surrounding sea. Winters tend to be milder, and summers cooler, than in inland areas at the same latitude. The Bornholm area is known in Denmark for having relatively many hours of sunshine and relatively low rainfall, and the islands share in this. Wind and storms can be strong, especially in autumn and winter, and can sometimes interrupt boat services.

Fresh water

Because the islands have no rivers or lakes, fresh water has always been a practical problem. Historically, rainwater was collected and stored in cisterns, which are tanks or basins for storing water. Some of these structures are still visible. How water is supplied today, and what role the old cisterns still play, is not covered in detail here.

History

Before the fortress

Before the late 17th century, Ertholmene was used mainly by fishermen from Bornholm. The islands were a convenient base for fishing in the surrounding waters, especially for herring. There was no permanent military presence and probably little or no year-round settlement.

Strategic background

In the 17th century, Denmark and Sweden were rivals for control of the Baltic Sea and the straits leading into it. In 1658, under the Treaty of Roskilde, Denmark lost its provinces in what is now southern Sweden (Scania and neighbouring areas) to Sweden. Bornholm was also handed over at that time, but the island rebelled against Swedish rule and returned to Danish control soon afterwards.

After losing the Scanian provinces, Denmark no longer controlled harbours on both sides of the Øresund strait and had fewer naval bases in the eastern Baltic. During the Scanian War (1675–1679), Denmark tried and failed to win back the lost provinces. After that war, the Danish navy wanted a protected forward base in the eastern Baltic. A base there could be used to watch Swedish naval movements, especially around the Swedish naval base at Karlskrona, which Sweden established in the 1680s.

Founding of the fortress (1684)

In 1684, King Christian V ordered the fortification of the islands. The sheltered harbour between Christiansø and Frederiksø could hold ships, and the granite rock made a solid foundation for walls and towers. Construction began that year. The fortress was built with granite walls, bastions (projecting parts of a fortification designed so defenders can fire along the walls), gun positions, barracks, storehouses and workshops.

The two most visible structures are the round towers:

• Store Tårn ("Great Tower") on Christiansø. It later also became the base of the islands' lighthouse.
• Lille Tårn ("Little Tower") on Frederiksø. It now houses the local museum.

Both towers and much of the surrounding fortification still dominate the skyline of the islands.

Life in the garrison

For most of its existence as a fortress, Christiansø was a small military community. It had soldiers, naval personnel, officers and their families, craftsmen and other workers. The population was considerably larger than it is today. Almost everything, including food, building materials and often water, had to be brought by boat, so the community depended heavily on supplies from Bornholm and the rest of Denmark.

A garrison church was established for the soldiers and their families. This building survives as Christiansø Church and is still used as a church today (see the section on buildings below).

Christiansø was also used as a place of imprisonment. Like some other Danish fortresses of the period, it held prisoners, including people sentenced to forced labour, who could be used for building and maintenance work. Some prisoners were held for political reasons. The isolated location made escape very difficult.

The Gunboat War (1807–1814)

The Gunboat War was a naval conflict between Denmark–Norway and the United Kingdom that took place during the Napoleonic Wars. It began after the British attack on Copenhagen in 1807, in which the British seized much of the Danish fleet. Without large warships, Denmark–Norway relied on small, oared gunboats and on privateers (privately owned ships given official permission to attack enemy merchant ships).

Christiansø was one of the Danish positions involved in this war. Its harbour and guns gave Danish vessels a protected base in the eastern Baltic, from which they could operate against British shipping and merchant convoys passing through the area. The fortress came under British pressure during the war, but it remained in Danish hands.

Decline and closure (1855–1863)

During the first half of the 19th century, changes in naval technology and in the political situation in the Baltic reduced the military value of the small fortress. In 1855, the fortress was formally closed. The infobox gives its period of use as 1684–1855, with 1863 in brackets. This reflects the fact that the military presence was wound down over several years after the formal closure, rather than ending on a single day.

Since then there has been no working military infrastructure on the islands. However, the Ministry of Defence kept responsibility for them, and this arrangement continues today.

From fortress to civilian community

After the garrison left, a civilian population remained, made up mainly of fishermen and their families, along with people employed to maintain the islands. Fishing, especially for herring, was the main occupation for many decades. The fortress buildings were adapted for housing, workshops and other civilian purposes.

In the 20th century, the islands became known as a place of special natural and cultural value. The buildings and landscape were placed under protection, and visitor numbers grew as boat connections from Bornholm improved and tourism in the region increased.

Administration

A special status

Ertholmene has an unusual administrative position. Almost all of Denmark is divided into municipalities (kommuner), which provide local services and collect local taxes. Ertholmene does not belong to any municipality or to any of Denmark's regions. Instead, the islands remain under the formal administration of the Ministry of Defence, as a continuation of their history as a fortress.

An administrator appointed by the ministry acts as the local authority. This person is responsible for many of the tasks that a municipality would handle elsewhere in Denmark.

Staff and services

The Ministry of Defence employs about 25 to 30 people on the islands. They maintain the civilian infrastructure (such as the harbour, paths and utilities), the museum and the historic buildings. These employees make up a large part of the working population.

Taxes and residence

Because the islands are not part of a municipality, residents do not pay municipal tax. Municipal tax is a significant part of the income tax paid by most people in Denmark, so this is a notable difference. Residents remain subject to national Danish law.

The buildings on the islands are protected, and residence is closely controlled. People cannot simply move to the islands. Living there generally depends on having work or another role connected with the community, and on approval from the administration. These rules help to keep the population small and to protect the historic environment.

Buildings and cultural heritage

The islands form one of the best-preserved fortress environments in Denmark. Much of the layout dates from the 17th and 18th centuries. Key structures include:

• Store Tårn, the Great Tower on Christiansø, which carries the lighthouse.
• Lille Tårn, the Little Tower on Frederiksø, which houses the museum about the islands' history and nature.
• Granite walls and bastions surrounding parts of both islands.
• Former barracks, storehouses and officers' houses, many now used as homes, workshops or visitor facilities.
• Christiansø Church, originally built for the garrison stationed in the fortifications.

Christiansø Church

Christiansø Church stands on Christiansø. It was first the church for the soldiers and their families in the fortress. After the garrison left, it continued to serve the civilian population and it remains in use. It is one of the few public buildings on the islands and is a regular stop for visitors.

Lighthouse

The lighthouse on Store Tårn guides ships in the waters around Bornholm and the islands. Combining a lighthouse with a fortress tower made practical use of the existing tall structure.

Protection

The buildings, fortifications and landscape are protected by Danish law. Changes to buildings are strictly regulated. The aim is to keep the islands close to their historic appearance, while still allowing a small community to live and work there.

Nature and wildlife

International protection

Ertholmene was designated a wetland of international importance under the Ramsar Convention on 9 February 1977. The Ramsar Convention is an international treaty for the conservation and sustainable use of wetlands, which include coastal waters and the islands and rocks within them that are used by water birds. The site's Ramsar reference number is 165. The islands are also covered by Danish nature protection rules.

Birds

The islands are best known among naturalists for their seabirds. Important species include:

• Common eider (Somateria mollissima), a large sea duck. Many eiders breed on the islands, including in gardens and among the buildings on the inhabited islands.
• Common guillemot (Uria aalge) and razorbill (Alca torda), members of the auk family. Græsholm holds Denmark's only breeding colonies of these two species.
• Gulls, including herring gulls and lesser black-backed gulls, which nest in large numbers.

The islands are also a resting place for migrating birds, especially small land birds that cross the Baltic Sea in spring and autumn. For this reason they have long been used for bird observation, bird ringing and scientific study. Bird ringing is a research method in which birds are caught, marked with a small numbered ring on the leg and released, so that their movements and lifespan can be studied. A scientific field station on Christiansø carries out research and monitoring of the islands' birds and other wildlife.

Græsholm bird sanctuary

Græsholm, the second-largest island, is a strict bird sanctuary. The public is not allowed to land there. Access to the other uninhabited islands and skerries is also restricted. These measures protect breeding birds from disturbance.

Seals and marine life

Grey seals can be seen around the skerries. The surrounding sea has historically supported fisheries, especially for herring, and the waters are part of the protected area.

Plants

The natural vegetation on the bare granite is sparse. Over the centuries, residents brought in soil and plants and built sheltered gardens. Some garden plants have spread into the wild. The combination of rock vegetation and introduced plants gives the inhabited islands an unusual, garden-like appearance in places.

Rules on animals

To protect the birds, there are strict rules about keeping and bringing animals to the islands. Cats are not allowed, because they are a serious threat to ground-nesting birds such as eiders. Some sources say that dogs are also forbidden. The exact current rules for visitors' pets should be checked with the island administration or the boat operator, as this article cannot confirm every detail.

Community and daily life

The permanent population was 90 in 2024. This makes Ertholmene one of the smallest permanently inhabited communities in Denmark. The population is much smaller than in the fortress period.

There are no cars or other road vehicles for general use on the islands. People get around on foot, and goods are moved by hand or with small carts. The short distances make this practical.

Life on the islands depends closely on the boat connection to Bornholm. Supplies, mail, visitors and many services arrive by boat. In bad weather, crossings can be cancelled, and the islands can be cut off for a time.

The islands have a long tradition of preparing herring. Herring pickled or marinated in a local style, often called Christiansø-sild, is a well-known local product and is sold to visitors.

Tourism and access

Visitor numbers

Tourism is now the basis of almost the entire economy. The islands receive tens of thousands of visitors each year. Most come on day trips from Bornholm during the summer months. A smaller number stay overnight in local accommodation or at a small campsite.

Getting there

The islands are reached by passenger boat. The main route runs from Gudhjem on Bornholm, and the crossing takes roughly one hour. Services run year-round but are more frequent in summer. Timetables and departure harbours can change with the season, so travellers usually check current schedules.

What visitors see

• The fortress walls, bastions and the two towers.
• The museum in Lille Tårn on Frederiksø.
• Christiansø Church.
• The harbour and the footbridge between Christiansø and Frederiksø.
• Seabirds, including nesting eiders close to paths and buildings in spring and early summer.
• Views towards Græsholm and the skerries, including Østerskær, Denmark's easternmost point.

Rules for visitors

Because of the protected status of the islands, visitors must follow rules intended to protect birds and historic structures. These include staying away from the restricted islands, not disturbing nesting birds, and following the rules on animals. Landing on Græsholm is not allowed.

Debates and open questions

The islands' special situation raises several continuing questions:

• Administration. The arrangement under which the Ministry of Defence manages a civilian community without military functions is a historical legacy. People have sometimes discussed whether this arrangement is still the most suitable one, or whether the islands should be placed under another authority. The arrangement has remained in place.
• Balancing tourism and conservation. The islands depend on visitors for their economy, but large numbers of visitors in the breeding season can put pressure on birds and on fragile historic structures. Managing this balance is an ongoing task.
• Sustaining a small community. With fewer than 100 residents, the community faces questions about how to keep enough people, especially families and younger people, living on the islands year-round.
• Seabird populations. Seabird numbers in the Baltic change over time because of food supply, pollution, disease and other factors. Ongoing monitoring on the islands contributes to understanding these changes.

Name confusion: Christiansø Fortress in Norway

A different fortress, also called Christiansø Fortress, exists in Norway. It is one of the historic Norwegian fortresses and commands the western approaches to Flekkerøy harbour, just outside the city of Kristiansand in Agder county. It stood on the small island of Slottsholmen (now called Gammeløen, "the old island"), just north of the larger island of Flekkerøya. This Norwegian fortress dates from the period when Denmark and Norway were united under the same crown. It has no connection with the Danish islands of Ertholmene other than the shared royal name.

See also

• Bornholm
• Gudhjem
• Christiansø Church
• Christiansø Fortress (Norway)
• Gunboat War
• Christian V of Denmark
• Treaty of Roskilde
• Scanian War
• Ramsar Convention
• Extreme points of Denmark
• List of islands of Denmark
• Common eider
• Common guillemot
• Razorbill