Aarhus University Seal / Aarhus Universitets segl

We focus on building and maintaining extensive knowledge on the basic biology of the animals and their interactions with human activities. In Denmark, we mostly work with harbour seal, grey seal and harbour porpoise, and monitor the population dynamics on an ongoing basis. This knowledge forms the basis for the management of marine mammals and enables assessments of future changes in the marine environment and of the efficiency of measures introduced to protect marine mammals.

We regularly have projects involving other species in Danish waters, including the minke whale and the white-beaked dolphin. In Greenland, we work with ringed seals, walruses, narwhals, and other species. Furthermore, we work with polar bears, which are also considered a sea mammal.

Walrus at Hellefiskebanke in Greenland
Walrus at Hellefiskebanke in Greenland. Photo: AU
Porpoise / Photo: Signe Sveegaard ©
Porpoise / Photo: Signe Sveegaard ©

Harbour porpoise

Denmark's only breeding whale species has had a major impact on our research into the conservation of whales worldwide. Both in connection with by-catch and noise pollution, and in connection with physiology and behaviour. The harbour porpoise is one of the world's smallest whales. It is difficult to see the difference between the size and gender of porpoises, but studies have shown that the mother and calf stay together for at least one year and that young and mature males often swim together, just as groups of young animals can be seen together.

Read more about porpoises

Harbour porpoise

Phocoena phocoena

Our researchers have specialized in research on porpoises, their biology, way of life, prevalence and migration, as well as the problems surrounding porpoises that drown in fishing nets.  In connection with this work, we have specialized in a number of different techniques, such as the tagging of animals with, among other things, satellite transmitters and acoustic data loggers, counts from aircraft and boat, genetic studies and registration of the porpoises’ echo-location sounds with acoustic instruments. Read more about our projects and find information about our technical equipment and capabilities under Research facilities.

Behind the name

The Danish name

The Danish name for the porpoise, marsvin, actually means sea swine.

The Latin name

The name phocoena comes from Phocaena, which was originally given because the porpoise looks like seals that are called Phoca in Latin. 

The English name

Harbour Porpoise.

Porpoise comes from the Latin name: Porcus Piscis, which means swine fish.

Appearance

Like all other whales, the porpoise is incredibly well adapted to life in water. They are torpedo-shaped and have smooth skin without hair growth, which minimizes resistance when swimming. Their nostrils are on top of their heads, which gives them a great advantage if they quickly need to come to the surface to breathe, for example, while hunting. The porpoise’s very muscular tail gives the animal a strong thrust, while the pectoral fins and the dorsal fin ensure stability and give the animal excellent ability to maneuver in the water. The porpoise can be submerged for up to 10 minutes and can reach the bottom throughout the Danish waters, but often it prefers shorter dives lasting 1-3 minutes.

Colour and shape

The head is short and rounded, the two pectoral fins point back, and the dorsal fin is small and triangular. The caudal fin is horizontal and, as with other whales, is used to produce thrust, while the pectoral fins are used to navigate. The back is grey-blue to grey-black, while the stomach is very light grey or completely white.

Age

Porpoises can grow to a maximum of 20 years in the wild. Due to their short lifespan, they become mature relatively fast. The males become sexually mature when they reach a length of approx. 130-140 cm, at the age of 2-3 years. Females become sexually mature when they reach a length of approx. 140-150 cm, at the age of 3-4 years.

Way of life

Sound

Like all toothed whales, the porpoise has a well-developed sonar system, which it uses to localize prey. The porpoise uses the sonar system by sending strong high-frequency sound waves through a fat lump in its forehead. The fat lump, called the melon, gathers the sound waves into a ray. When the ray e.g. hits a fish, the sound waves are thrown back. Through the lower jaw, which is connected to the ear, the porpoise can capture the reflections and thereby determine the direction and distance to the prey. The same principle is seen in bats, which are the only other group of animals to use sonar.

Food

Porpoises have a varied diet, depending on the type of prey present in their habitat. They eat a wide range of fish, including cod, herring, flatfish, etc., but also small squid. It was previously believed that porpoises caught most of their prey in the water, but new research has shown that they spend a lot of time digging through the seabed with their noses to catch fish hiding here.

Reproductive biology

Porpoises mate in late summer, and females are pregnant for about 11 months. She will then give birth to a young calf of 70-80 cm, weighing just over 10 kg. The calf feeds on the mother’s milk for up to 11 months, and most sexually mature females give birth to a pup every year.

Prevalence

In Denmark, porpoises prefer coastal areas or other shallow areas that are less than 200 meters deep.

The porpoise is the smallest and most abundant whale in Denmark, but it is extremely shy by nature. This makes it difficult to monitor the behaviour of the animal. The porpoise lives alone or in small flocks of normally 2-5 animals, although larger groups may occur. Researchers have estimated that approximately 200,000 porpoises live in the entire North Sea. There are approximately 20,000 animals in the inner Danish waters, and they probably only have limited contact with the animals in the North Sea and the Baltic Sea. In the Baltic Sea, the population is estimated to count 100-1000 individuals.

The harbour porpoise is found throughout the northern hemisphere from Morocco in the south to northern Norway in the north. In Denmark, the porpoise is common in the North Sea, Skagerrak, Kattegat and the belts, whereas it is very rare in the Øresund and in the waters around Bornholm.

There are several separate porpoise populations in the North Atlantic, e.g.  along the east coast of the US and Canada, in west Greenland, Iceland, the Faroe Islands, the west coast of Africa, the North Sea, the inner Danish waters and the Baltic Sea. In addition, there is also an isolated population in the Black Sea. The porpoise is only rarely seen in the Mediterranean.


Harbour seal / Photo: Anders Galatius ©
Harbour seal / Photo: Anders Galatius ©

Harbour seal

The harbour seal figures in projects and annual surveillance programmes, which help to provide an overview of the population with a focus on conservation of areas, fishing interactions and health. The harbour seal is Denmark's most common seal species. It is found in many places in Denmark where human disturbances are rare. It is found in Skagerrak, Kattegat, Limfjorden, Øresund, the Baltic Sea and the Wadden Sea. Counts from year 2015 show that approx. 17,000 harbour seals live in Denmark.

Read more about the harbour seal

Harbour seal

Phoca vitulina
Photo: Anders Galatius ©

Harbour seals can grow as big as 1.5 m and weigh up to 100 kg.

Our researchers have specialized in research on seals, their biology, way of life, prevalence and migration, as well as the problems surrounding seals and fishery. In connection with this work, we have become specialized in a number of different techniques, such as the tagging of animals with, among other things, satellite transmitters and acoustic data loggers, counts from aircraft and boat and genetic studies. Read more about our projects and find information about our technical equipment and capabilities under Research facilities.

Harbour seal / Photo: Anders Galatius ©
Harbour seal / Photo: Anders Galatius ©

illustration

Tagging of seals / Photo: Rune Dietz ©
The harbour seal being tagged with a satellite on Anholt/Photo: Rune Dietz ©

Behind the name

The Danish name

The name refers to the coat's speckled markings, and in Greenland this seal is also called the mottled seal. The word ' seal ' presumably comes from an old German word that means ' the one who drags himself along '.

The Latin name

Phoca means seal, while vitulina means calf. The harbour seal has also been known as the lake-calf or lake-dog.

The English name

Harbor seal, Harbour seal and common seal 

Appearance

The harbour seal belongs to the group of true seals. Specific for true seals, they have short paws, lack an outer ear, and move on land by crawling on their stomachs. They cannot curl their tails the way sea lions can.

Colour

The coat is light grey to grey-brown with white and grey-black spots on the surface, and the colour becomes brighter towards the stomach.

  • Pups: length 80 cm, weight 9 kg
  • Females: length 146 cm, weight 75 kg
  • Males: length 156 cm, weight 100 kg

Fun facts

During its embryonic state, the harbour seal has a long, white coat, so-called embryonal fur, which, unlike the ring seal and the grey seal, is felled before the pup is born. The new coat is similar to the adults’ and for this reason, the pups are able to follow the mother into the water immediately after they are born.

Age

The oldest harbour that we know of got to be 36 years old. They become sexually mature when they measure 130 cm at the age of 3-5. Most sexually mature females have a pub each year.

Way of life

Adaptation to water

Like all seals, the harbour seal lives on land and in water. The seal finds its food in the water, and it has no difficulty navigating the relatively shallow Danish waters, which rarely exceed 50 metres in depth. Their torpedo-shaped body makes it possible for them to gain high speed in the water, where the back part of the body is used for thrust, and they usually only stay submerged for 1-10 min. at a time.

On land, they are far more clumsy; they crawl on their stomachs like caterpillars. This is where they rest, breed and change coat. It is often possible to see large numbers of seals gathered on land. Because they are clumsy on land, they rarely move far up the beach, as they need to be ready to flee into their favourite element again. 

Food: All kinds of fish, squid and crustaceans

Mating: from July-August

Pregnancy: 10-11 months.

Breeding: June - July

Moulting: August

Prevalence

The harbour seal population in the Eastern Atlantic counts 100,000 - 140,000 animals. The largest populations are found around Iceland, the Shetland Islands, Orkney Islands, the Wash in East England, the Wadden Sea, Skagerrak and Kattegat, and smaller populations are found in Ireland, Norway, the Hebrides, Scotland, Svalbard, Limfjorden and the western part of the Baltic Sea. In the Danish waters alone, the harbour seal is divided into four populations. These are: The Wadden Sea, the Limfjord, Kattegat, and the western Baltic Sea, to which Øresund belongs. Their preferred habitats are characterised by sandy beaches and reefs around undisturbed small islands.

Media and articles

Articles from the Nature Park Wadden Sea


Gray seal / Anders Galatius ©
Gray seal / Anders Galatius ©

Gray seal

In recent years, research has documented an increase in the Danish population of grey seals, which has otherwise been in decline over a number of years. With tagging and monitoring, we have helped to make the survival conditions significantly better for the species. The grey seal is Denmark's largest seal and is also the most common seal in Denmark, following the harbour seal. The shape of the head is the best way to distinguish the two species. The grey seal has a straight cone-shaped profile, while the harbour seal has a more dog-like round head shape with a short muzzle.

Read more about the gray seal

Gray seal

Halichoerus grypus

Photo: Anders Galatius

Our researchers work on seal biology, their life history, prevalence and migration, as well as the problems surrounding seals and fishery. In connection with this work, we have become specialized in a number of different techniques, such as the tagging of animals with satellite transmitters and acoustic data loggers, counts from aircraft and boat and genetic studies. Read more about our projects and find information about our technical equipment and capabilities under Research facilities.

Gray seal / Photo: Morten Abilstrøm ©
On the left is a harbour seal and on the right a grey seal on Anholt. Photo: Morten Abilstrøm ©

Behind the name

The Danish name

The grey seal's name refers to the grey hue of the coat when it is dry. The word 'seal ' presumably comes from an old German word that means 'the one who drags himself along'.

The Latin name

Halichoerus originates from Greek and means lake (Halios) pig (choiros). Grypus also originates from Greek and means eagles' nose (grupos) and refers in particular to the curved nasal bridge of the male seal.

The English name

Grey seal

Appearance

The grey seal belongs to the group of true seals. Specific for true seals, they have short paws, lack an outer ear, and move on land by crawling on their stomachs. They cannot curl their tails the way sea lions can. The smaller limbs help the animal reach higher speeds in the water and reduces heat loss when animals are submerged in the cold water.

Colour

As the name suggests, the coat is usually greyish with scattered dark spots and the belly is often brighter than the back. The males often have a darker appearance, and in older males, the muzzle has a slightly upward-curved nasal bridge, unlike the females, who have a straighter nose. 

Size

The male is about 1½ times larger than the female and can measure more than 2 m and weigh up to 300 kg.

Facts

As a pup, the grey seal has an almost completely milky-white coat, which is replaced by the proper seal coat after 2-3 weeks.

Age

Grey seals becomes sexually mature when they are around 4-5 years old. Most sexually mature females give birth to a pup each year. The maximum life expectancy in nature is 46 years for females and 44 years for males, but it is likely rare that they reach such an age.

Way of life

Adaptation to water

Like other seals, the grey seal is well adapted to life in water, and with a 2-4-cm-thick subcutaneous layer, it can easily stay warm in the cold water. They have very long whiskers on each side of the mouth. These are used to detect vibrations in the water, e.g. from prey. This is particularly useful when they hunt at great depths where there is no light. Like other marine mammals, the grey seal is capable of holding its breath much longer than land mammals, among other things because of its ability to store more oxygen in the muscles and the blood. It can also almost completely shut off the blood supply to the organs that can do without oxygen when it dives.

Food

All kinds of fish, squid and crustaceans.

Reproductive biology

The female becomes pregnant with her first pup at the age of 4-5, and thereafter has one pup each year. However, it has been discovered that approximately 60% of the newborn pups die before they turn one. The males do not mate till the age of eight. This late age is due to the fact that young males simply are not large enough to compete with the full-grown males about the females. Before they have turned eight, they are not able to form and defend a harem.

Breeding period

February-March (Baltic Sea), September-October (other areas)

The female grey seal gives birth on land once a year, and during the first 2 1/2 weeks after birth, the pup feeds exclusively on the fatty milk (fat percentage of approx. 60%) and gains almost 3 kg per day. This enables it to become independent at the age of 4-5 weeks, after which a new mating season begins. The strongest males create a harem consisting of typically 5-7 females and defend them ruthlessly. The male mates repeatedly with the females and thereby increases the possibility of getting as many offspring as possible.

Prevalence

In Denmark, the grey seal was quite common until approximately 100 years ago, but intensive hunting killed off the species. Luckily, over the past 15 years the species has re-immigrated, and you can find large groups at Rødsand, Christiansø and Læsø and a few individuals or small groups on the shores near Anholt, Hesselø, Saltholm and in the Wadden Sea. In the Baltic Sea, for a number of years the population has struggled with fishery and environmental toxins, but, despite this, there has been an improvement in recent years, and it is believed that today the population counts around 40-50 thousand individuals.

The grey seal is prevalent in the temperate belt in the North Atlantic, where we know of three main populations, of which the largest is in the sea between the British Isles, Iceland, the Faroe Islands and Norway. In addition, the grey seal is also located along North America's east coast, where it is seen from Labrador in Canada to Boston in the US. In 2014, there were about 100,000 animals in this area. 

The grey seal is happy to go ashore to rest, moult and breed, and the most popular habitats on land are characterized by rocky coasts and sand beaches.