Seals in fresh water

Knowledge repository

As the name suggests, seals live in salt water. Nevertheless, it sometimes happens that we receive a report of a seal in an area with fresh water. Such a report often comes in with the question: "Isn't that a bad thing?"

Luckily, that is not the case. A seal mainly needs fish to survive and they are also in fresh water. There are reports of seals in the Utrecht canal, the Schildmeer and in the major rivers. Seals are smart. They usually find their way back. 

See also

  • Zeehond in zoet water

  • Zeehonden in De Onlanden, Drenthe

Influence of fresh water on health

Seals don't necessarily need to be in salt water to survive. Unlike other marine animals, they are not very sensitive to the salinity of water. It is true that after a while their eyes can get irritated. Apart from that, seals can survive in fresh water for a while. As long as there's plenty of fish to hunt.

Reports of seals in fresh water

The two most recent cases reported to us are seals Hulcky and Thor. We took Hulcky from the Veluwemeer when it turned out that she was suffering from a lungworm infection. She was taken in at the Sealcentre and eventually released back into the sea.

Seal Thor was just seen in the fresh water of De Onlanden after we had caught and released him. Thanks to its tag and its spots on fur, we were able to recognize that it was this specific seal. After a while he found his way back on his own.

Since March 2012 we have had more reports:

  • March 2012: common seal near Goudswaard, South Holland.
  • December 2012: common seal in the Biesbosch.
  • December 2012: common seal in the Schildmeer, Groningen. The animal seemed to swim back and forth between the Schildmeer and Farmsum.
  • January 2013: common seal in the Meuse near Dreumel.
  • April 2013: common seal in river Hunze in Drenthe, moved on by itself.
  • December 2013: common seal in the Schildmeer, Groningen. Unknown if it was the same seal. The animal was seen with fish in its mouth.
  • August 2015: ringed seal in Utrecht canals, collected at seal sanctuary A Seal in Stellendam.
  • October 2015: common seal in Biesbosch.
  • March 2016: common seal in the Maasplassen near the Weerd in Roermond.
  • November 2016: common seal “Hulcky” spotted in Veluwemeer.
  • August 2021: common seal Thor spotted first in Peizerdiep, later in De Onlanden.

Seal species in freshwater

The seal species that occur in the Netherlands all live in salt water. But there are seal species in the world that actually live in fresh water. There are five in total. These populations probably became separated from the sea long ago.

It concerns the following five species:

Lake Iliamna, Alaska Phoca vitulina mellonaea freshwater subspecies of the common seal.
Lake Baikal, Siberiia Pusa sibirica, Baikal seal , a separate species. Also the smallest seal species.
Lake Ladoga, Russia Pusa hispida ladogensis, freshwater subspecies of the ringed seal.
Lake Saimaa, Finland Pusa hispida saimensis, freshwater subspecies of the ringed seal.
Lac des loups marins, Quebec Phoca vitulina mellonae, a freshwater subspecies of the common seal.

Bron: Van Lanen, 2012

Were there more seals in fresh water in the past?

It is believed that more seals visited the Dutch inland waterways in the past, but that has decreased in the last century.

The construction of flood defenses in all major rivers in the Netherlands has probably contributed to this. Because of all the locks, dikes and scuppers, it is quite difficult for a seal to find an easy way to inland. Sometimes seals find a way, but they get lost along the way and can't find their way back. If those animals do not receive enough food, then action must be taken to help them return to the sea. In other cases, seals seem to know very well how to move between sea and lake.

The seal that has already been seen a number of times in the Schildmeer, for example. It was first seen at Farmsum, then at Schildmeer. To get to that lake, the seal first had to swim through a channel to the south, and then swim into a small drainage channel. When the animal was later spotted again at Farmsum, it seemed that the animal knew what it was doing. A year later another seal was seen in the Schildmeer.

It has been said by several people that the seals may be able to learn how locks work.

Water quality deteriorating: less fish means fewer seals?

Due to industry, agriculture and waste water from households, the water quality has deteriorated rapidly in the last century. Due to the deteriorating quality of the freshwater ecosystem, many freshwater bodies became uninhabitable for certain fish species. As a result, diversity in many freshwater bodies also declined sharply. As mentioned before, seals need a lot of fish to survive. If that fish is not there, a seal will not easily choose to hang out in such an area.

Improving water quality + opening rivers for more fish: more seals in the future?

In the past few years, a lot of work has been done to improve the water quality. And that will continue. Various projects are also being developed to increase the species richness in Dutch waters. For example, by leaving the sea locks ajar and creating fish passages in dikes, it is hoped that migrating fish species will return to the Dutch interior. With this return, they hope that the ecosystem will improve.

Perhaps with this improvement we will encounter more seals in Lake Veluwe in a few years? 

On this page

Continue reading

Lees verder

Grijze zeehonden in de duinen

Grey seal

Knowledge repository

Scientific name: Halichoerus grypus
Family: Phocidae
Size:: male: 2.70 meter; female: 2.00 meter
Weight: male: 310 kilos; female: 190 kilos
Habitat:North Atlantic Ocean to the Baltic Sea
Endangered status: not endangered

See also

  • Grijze zeehond moeder en pup

  • Grijze zeehondenpup

  • Grijze zeehonden in duinen

"Because of its striking nose, the grey seal is sometimes also called "sea pig."

External features of the grey seal

The grey seal has an elongated snout. This is flat from the forehead to the tip of the nose. This makes them a bit like a "real" dog. In addition, the adult males have a very bulbous nose, which is especially striking when you look at them from the side.

Did you know...

The literal translation of the grey seal's scientific name means "hook-nosed sea pig?

The nostrils of the grey seal are large, round and set wide apart. They do not run towards each other, but are straight apart. This has no other seal species and it gives grey seals a sort of pig nose. That's where their scientific name of the grey seal, Halichoerus grypus, comes from which means: "hook-nosed sea pig".

It is a fairly large seal species. An adult male living in the seas of Europe grows to about 2.7 meters in length and can weigh up to 310 kilograms. Adult females are much smaller. They grow to about 2 meters in length and weigh up to 190 kilos. Grey seals in the western Atlantic tend to be quite a bit larger. They sometimes grow up to 3 meters in length and can weigh up to 400 kilos.

Did you know...

The largest grey seal ever measured was 3.30 meters long?

Gender differences

Males are much larger than the females. They usually have a much thicker neck than the females (often with scars from fighting with other males). In addition to size, there are major differences in appearance between males and females in grey seals. The term used in science for this is sexual dimorphism.

They also have a different colour. Adult males have dark brown to black fur with some light spots here and there. The females are much lighter brown or blonde in colour, with dark spots. If a seal's fur is completely black, it is called melanism. This is pretty rare.

Distribution and status

Grey seals live in groups on the coast of the Western Atlantic Ocean in the Northern hemisphere through to the Baltic Sea. The seals of the Atlantic Ocean have probably had no contact with the seals that live in and around the Baltic Sea for thousands of years. That is why we also speak of two “subspecies”. Each of these species has its own scientific name.

  • Halichoerus grypus atlantica (in the Atlantic Ocean)
  • Halichoerus grypus grypus (in the Baltic Sea)

They are usually found on rocky shores, beaches, sand banks, islands and even - far enough north - on ice floes. Grey seals share their habitat with many other animal species, for example the common seal (Phoca vitulina).

Grey seals have been an endangered species for many years. Due to centuries of hunting by humans, grey seals had completely disappeared in some areas of the world. In the Netherlands, this species had been gone for hundreds of years before they came back in the eighties. In most of Europe they are still a protected species.

Did you know...

There are two types of seals living in the Netherlands. The grey seal and… Do you know the other one? You can find them here.

Grey seals in the Netherlands

In the Netherlands, grey seals can be seen along the entire coast. Most are in and around the Wadden Sea. A smaller group of seals live in the south of the country, in Zeeland. Here, too, they share these areas with common seals.

The grey seal here is doing better now. In 2021, there were 6,788 animals in the Netherlands. This is almost twenty percent more compared to the previous year. There are more and more grey seals worldwide. According to the 2016 IUCN Red List, there were approximately 316,000 adult grey seals. They therefore have the status "not endangered", this is mainly because they are a protected species in many countries.

Diet and foraging

Grey seals eat anything. Depending on where they live and which prey is most present at that time. They're not that p. They eat Atlantic fish species, squid and crustaceans. A few grey seals have also been seen eating birds or even young seals and porpoises.

When looking for food, they usually dive for 4 to 10 minutes, but sometimes they search for up to 30 minutes. While the average dive goes to a depth of 60 to 100 meters, dives have also been measured from 200 meters deep.

Behavior of grey seals

Like other seal species, grey seals only search for food. Although grey seals often rest in large numbers on the coast in some places, they are not seen as very social animals. They usually try to avoid social contact. Except when females feed their pups or during the mating season.

Here in the Netherlands, there are not really large groups of grey seals. They rest in smaller groups on sandbars or beaches. Every year at the end of spring, larger groups of grey seals come together to molt. They then lose their winter coat. They are then a few weeks more on land. On land, they lose their hair more easily. And on top of that: the sunlight is good for the new coat.

Voortplanting bij grijze zeehonden

Mating behaviour

The female is ready to mate a few days after giving birth. During the mating season, adult grey seals gather on shore or on pack ice to reproduce. The males then compete for females. They do not defend a territory or harem like sea lions. Instead, they fight other males to mate with certain females.

Although their fights seem vicious, grey seals don't fight to the death. Usually, they don't get hurt very badly. Instead, they fight until someone is too tired or realizes they've lost. The strongest males can mate with up to 10 females.

Diapause and pregnancy

The fertilized egg is not moved to the uterus until three to four months after the moment of fertilization. This is called diapause. After that, a grey seal is eight months pregnant. In the time between fertilization and pregnancy, the female must gain a lot of weight. So when her new pup is born, she can take good care of it.

Did you know...

Grey seal pups born with white fur?

Grey seal pups

Grey seal pups are born in winter. Female grey seals give birth to one pup on land or ice. Pups weigh between 11 and 20 kilos at birth and have a white coat in the first few weeks. We call this baby coat “lanugo”. It sometimes takes up to three weeks for the puppy to lose all its white hair, but it usually disappears within a few days.

Birth and nursing period

The nursing period lasts about three weeks. During this time, the pup will gain four times its weight. This is due to the fatty breast milk, which contains approximately 43% fat in grey seals. Because the female has to stay with her pup a lot, she has little time to eat and she loses a lot of weight. Female grey seals sometimes weigh as much as 40% less after this period.

After 3 weeks of nursing, the mother has no breast milk left. She leaves her pup and is not coming back. The first few days the pup will stay on land, because the lanugo coat is not suitable for swimming. The pup will not eat. They then begin to lose the white fur and lose weight until they are finally ready to enter the water.

Then they learn to hunt for themself. Without help from the mother: natural instinct takes over. It is not for nothing that the grey seal is the largest predator in the Netherlands.

On this page

Continue reading

Lees verder


Knowledge repository

Plastic waste does not always stay the same. A plastic bag floating in the sea will break down into smaller pieces. Due to contact with UV light from the sun and the weather, nothing will remain of this bag, but it will disintegrate into an uncountable number of microplastics [1]. We call something macroplastic if you can see it with the naked eye, such as the plastic bag. The plastic bag continues to chip and break down until it is so small that the plastic particles are no longer visible to the naked eye. Plastic that is less than 5 mm in size is called microplastic.

See also

  • Microplastic

  • Ernstige gevolgen van plastic afval

  • Plastic afval

Microplastics in drinking water

It's a big problem if you can't see such incredibly small plastic. People and animals can eat or swallow plastic without realizing it. Unfortunately, there are not yet any water treatment plants that can filter the microplastics out of the water.

Research into drinking water in European cities shows that there is microplastic in our drinking water: 72.2% of the 18 tested drinking water locations contained plastic fibers [2]. That meant that the drinking water consisted of 4.5 fiber per 500 ml of water. The conclusion is that we may be drinking water with plastic particles without realizing it.

Microplastics in household items

A lot of microplastic also ends up in our water through our households. Care products such as shampoo, sunscreen, toothpaste or shower gel may contain microbeads.

Did you know...

You also add microplastics to the water when you wash synthetic clothing? About 63% of our clothing consists of a mix of natural and synthetic fibers [3]. A wash of 5 kilos releases between 600,000 and 17,000,000 of these plastic microfibers [4]. 

Microbeads are small plastic granules that act as a scrub (see Figure 4). When used, these plastic balls wash away into the sewer system via the shower or sink.


Figure4. You see microbeads, the blue plastic balls, that are in a scrub. Source foto: MPCA Photos – microbeads-plastic-particles © ©

From microplastic to nanoplastic

Microplastic can further break down into nanoplastic through weathering. Nanoplastic is smaller than 100 nanometers. It is so small that there is a chance that this plastic will enter your body cells and end up in various organs (5).

Perhaps the very little can be detrimental to your health. Microplastics and nanoplastics can carry chemicals and pathogens. Too little research has been done to know what consequences this may have for our health.


  1. Wageningen World (2016).Ons plastic is zoek. 13.
  2. Synthetic polymer contamination in global drinking water– Kosuth, Wattenberg, Mason, Tyree, Morrison. (2017).
  3. Synthetic fibers – Disasterous for humans and oceansPlastic Soup Foundation
  4. De Falco, F.; Gullo, M.P.; Gentile, G.; Di Pace, E.; Cocca, M.; Gelabert, L.; Brouta-Agnésa, M.; Rovira, A.; Escudero, R.; Villalba, R.; et al. (2018).Evaluation of microplastic release caused by textile washing processes of synthetic fabrics.  Pollut., 236, 916–925.
  5. Nanoplastics gevaarlijkerdan microplastics (2014) Ingrid Zeegers. Magazine over de zee, link >>

On this page

Continue reading

Lees verder

Causes of plastic soup

Knowledge repository

De mens is de veroorzaker van de plastic soep. Wij maken de plastic producten die uiteindelijk in de natuur belanden. Wist je dat 80% van het afval in zee van het land komt? [1]. Via verschillende omwegen zoals de wind, riolen, grachten of rivieren komt het afval uiteindelijk terecht in zee. 

See also

  • Plastic afval

  • Plastic afval

  • Plastic zwerfafval van land naar zee

The image shows that a small part of our waste ends up directly in the sea. Most plastic takes a longer route to end up in the water. So something you throw on the street in the city can still end up in the sea.

Plastic pollution for years

Plastic in the sea is (unfortunately) nothing new. Plastic waste has been ending up in the sea for decades. And this will only get more. Every year, 8 billion (!) kilos of plastic waste is added to the sea [2]. Scientists estimate that there are already 150 billion kilograms of plastic in the oceans [3]. At the same time, the world is producing more and more plastic: in 1964, 15 billion kilos were made and in 2014 this had grown to 311 billion kilos of plastic [4].

More and more

So if nothing changes, there will only be more plastic waste in the sea. The shocking prediction of researchers is that if we do not change anything, the seas will consist of more plastic than fish by weight by 2050 [5].

Disposable plastic

The most common plastic waste in the sea is products that you use every day. A striking feature of all these products is that they are single-use plastic: plastic that is thrown away after one use. Figure 3 shows the top 10 most common marine debris. What objects do you sometimes use?

Top 10 plastic afval

Figure 3: The top 10 most common types of marine litter, by numbers. Bron:

Plastic is part of our society

We live in a world where it is unthinkable to live without plastic. Packaging, clothing, cars, care products, electronics: almost everything is (partly) made of plastic. Now it sounds like if plastic is only bad, but did you know that this material also has a lot of georteen? 

Plastic material:

  • Is cheap,
  • Is versatile,
  • Can be used for many years,
  • Is quite strong [6].

It is precisely these advantages that are also the disadvantages of plastic. Recycling of plastic happens far too little. From 1950 to 2015, only 9% of plastic was given a new life and the rest was landfilled or incinerated to generate energy [7, 8]. In addition, recycling means postponing throwing away a product. This is because used plastic will deteriorate in quality if you want to make it into a complete product again [8].

Plastic can last a lifetime. Plastic that is not recycled can last for hundreds of years and can last even longer if it breaks down into smaller pieces, known as microplastic. Some plastics can even last up to 600 years! [9] If plastic remains floating in the sea for so long, it will continue to pose a threat to marine life now – and in the future.


  1. R. Jambeck et al. Plastic waste inputs from land into the ocean. Science, 13 february 2015.
  2. World Economic Forum, Ellen MacArthur Foundation and McKinsey & Company, The New Plastics Economy– Rethinking the future of plastics (2016), Bladzijde 17.
  3. Ocean Conservancy and McKinsey Center for Business and Environment,Stemming the Tide: Land-based strategies for a plastic-free ocean (2015).
  4. PlasticsEurope, Plastics – the Facts 2013 (2013);PlasticsEurope, Plastics – the Facts 2015 (2015).
  5. Projections for 2015 and 2025 based on Ocean Conservancy, Stemming the Tide (2015)
  6. World Economic Forum, Ellen MacArthur Foundation and McKinsey & Company, The New Plastics Economy – Rethinking the future of plastics (2016),
  7. Production, use, and fateof all plastics ever made – Geyer et al (2017)
  8. Strategy on plastics in the circular economy (2017)European Parlement.
  9. NOAA / Mote Marine Lab. Approximate Time it Takes for Garbage to Decompose in the Environment.(2017). Available at this link >>

On this page

Continue reading

Lees verder

Plastic soup

Knowledge repository

An island has emerged in the Pacific Ocean from about 79,000,000 kilograms of garbage that floats together. It concerns an area that is 3 times the size of France! This plastic collection in the sea was first called the plastic soup. 

See also

  • Plastic in sea

  • Plastic zwerfafval op het strand

  • Verzamelde plastic tijdens beach clean-up

Who discovered the plastic soup?

In 1997, ocean scientist Charles Moore sailed from Hawaii to Southern California, taking a different route than usual. It passed through a "gyre" in the North Pacific Ocean, as you can see in the picture below. A "gyre" is a large circular motion of the sea water caused by the rotation of the earth in combination with certain ocean currents You can compare it with a tornado under water: a gyre has the same rotating movement and the suction force pulls the floating plastic towards itself.  

Charles saw pieces of plastic floating past here every day. He went on an investigation and discovered that a larger amount of plastic floated in this place than in other places in the Pacific Ocean. He also saw that the plastic here not only floated on the water, but also floated under water. Charles called this discovery the "plastic soup", a word that is still known around the world today. 

De North Pacific Gyre

The North Pacific Gyre is where Charles Moore discovered and described the plastic soup. Source: Fangz

On this page

Ontdekking plastic soep

Continue reading

Lees verder


Bearded seal

Knowledge repository

Scientific name: Erignathus barbatus
Family: Phocidae
Size:: 2 - 2,5 meters
Weight: 250 – 300 kilos
Habitat:: Arctic
Endangered status: not endangered

See also


"Bearded seals get their name from their... moustache!"

External characteristics of the bearded seal

Do you know how the bearded seal got its name? Bearded seals have a bunch of thick and long whiskers. When these are dry, the ends of these whiskers curl. It looks like they have a thick moustache.

Bearded seals have a broad head that appears a bit flat when viewed from above. Their eyes are relatively small and close to eachother. They have a large, broad muzzle with the cheeks appearing to droop a little. Their nostrils are wide apart and quite large1,3.

The head is quite small in relation to their body. Bearded seals have a large, long body. Adult animals are between 2 and 2.5 meters long, with an average weight between 250 and 300 kilograms3.

Gender differences

The females sometimes grow a bit larger than the males. But otherwise the females and males are difficult to tell apart. They both have a gray-brown color, sometimes with a rust-colored head and flippers. Because their fingers are all the same length, bearded seals' flippers appear almost square in shape. The nails are wide and short1,2.

Did you know...

Did you know: If there is iron in the seabed, some seal species can develop a rust-colored coat on the head and flippers. This is quite common in bearded seals, but also the common seal (Phoca vitulina) here in the Netherlands sometimes has this!

Distribution and status

Bearded seals are members of the family true seals (Phocidae), and belong to the subfamily of northern true seals (Phocinae). They live in the Arctic. Their remote habitat and way of life make it very difficult to conduct long-term research. As a result, little is known about their behavior.

This seal species has two subspecies. The Eastern bearded seal (Erignathus barbatus barbatus) and the Western bearded seal (Erignathus barbatus nauticus). The first one lives in the Atlantic part of the North Pole. The Western bearded seal also lives in the North Pole, but on the Pacific side1.

It is estimated that there are more than 500,000 bearded seals. They are considered non-threatened, but global warming is causing significant changes in their habitat1. They depend on sea ice during mating season, so its reduction is disadvantageous to the bearded seal population. So they are not in trouble yet. But melting ice caps will mean less and less space for bearded seals to mate and nurse pups2.

Diet and foraging

Bearded seals mainly hunt benthos. Benthos is a collective name for organisms that live in and on the seabed. Their diet is varied. They eat crabs, shrimps, shellfish, squid and fish. They prey on these animals in shallower waters, usually diving around 100 meters deep. Younger, less experienced animals sometimes go a lot deeper (up to 450 meters). Bearded seals can stay underwater for 25 minutes, but most dives don't last more than 10 minutes3.3.

Behaviour of the bearded seal

Bearded seals are not very social. They prefer to spend their time alone. Females also like to be alone when they have and nurse their pups. However, it is not strange to see them lying next to each other on the ice. They prefer not to go into the water during moulting. Then they can often be found in small groups on the ice.

Bearded seals rest close to water. They do this so that they can flee quickly if necessary. So the best places to rest are along the water's edge on the pack ice, on ice floes or around breathing holes on the sea ice3.

Did you know...

Did you know that bearded seals prevent their favorite breathing holes from freezing? They do this by scraping the ice with their short but strong claws. 1

Reproduction in bearded seals

Little is known about this species, both about mating behaviour and about the period of birth. For example, we don't know how long the pregnancy of bearded seals lasts.

Mating behaviour

When they are between 5 and 7 years old, bearded seals become sexually mature. That means that they are adults and can therefore have pups.

Bearded seals mate in the water. Because of this, little is known about their mating behaviour. We do know that males “sing” during the mating season. They do this to attract females and to scare off other males. Their singing usually consists of sharp, whistling sounds. These start very high and then become lower in pitch3.

Birth and nursing period

The pups are born between March and early May on ice floes in shallow water. The pups are around 1.30 meters at birth and then weigh an average of 35 kilos. They have dense, light gray fur and a layer of subcutaneous blubber.

Did you know that most bearded seal pups enter the water with their mother within a few hours of birth? They probably do this to avoid polar bears in the area3.

During the nursing period, the pup spends about half of the time on the ice sleeping. The rest of the time, mother and pup will be in the water together. So bearded seal pups learn to swim and dive from their mother, something most other seal species do not do3.

The milk that the pup gets from the mother is about 50 percent fat. This helps the pup gain weight quickly. The nursing period lasts between 18 and 24 days. At the end of this period, the bearded seal pup can weigh 100 kilos. So pups gain 60-70 kilos in this short time! The mother then leaves her weaner to mate2,3.

Enemies of the bearded seal

Bearded seals are often on the menu of polar bears. Orcas, Greenland sharks and even walruses also sometimes hunt young bearded seals.

These seals are also hunted by humans. Traditional seal hunting is an important source of food, fuel and equipment for native people.

Bearded seals were also hunted commercially in Russia. Between 1950 and 1960, more than 10,000 bearded seals were killed. That has now been greatly reduced. Now only a few hundred animals are allowed to be killed each year. However, the only sources on this are from the Russian government, so it is not clear whether these numbers are correct and whether those rules are also being followed3,4.

1 (Shirihai & Jarrett, 2021 [2006])

2 (NOAA, 2022)

3 (Kovaks, 2018)

4(Cameron et al., 2010)

On this page

  • Classification

  • Characteristics bearded seal

  • Gender differences

  • Distribution and status

  • Diet and foraging

  • Behaviour of the bearded seal

  • Reproduction in bearded seals

  • Enemies of the bearded seal

Continue reading

Lees verder

Zeehond met melanisme


Knowledge repository

Have you ever seen a black seal? That would be very special because black coloured seals are quite rare. These seals have melanismwhich makes them completely black from head to toe. In this article you can read all about what melanism is and learn what a melanistic seal looks like.

See also

  • Melanism

  • Melanistische zeehond

  • Zwarte zeehond

  • Melanisme is het tegenovergestelde van albinisme

What is melanism?

Having melanism means too much of the pigment melanin is produced. This is caused by an abnormality in genes. The pigment melanin is found in humans and animals in the skin, eyes and hair. The more melanin you have, the darker your skin color, eyes and hair are.

The amount of melanin you naturally have can vary per person/animal, but it is quite unique to produce both an excess of melanin (melanism) and no melanin at all (albinism).

Black seals in the Netherlands

Seals that are completely black are called melanistic. These seals have completely black fur and black nails. As far as we know, we have taken care of at least 10 melanistic seals in Pieterburen.

The most recent one came to us in January ,2016. We see that seals with melanism behave the same as all other seals and are no more sensitive or weaker than the rest. We, therefore release seals with melanism back into the sea with confidence.

Watch this video

Curious what a melanistic seal looks like? Then watch the short video below of seal Pepper at our Sealcentre:

Melanism is rare

It is almost never the case that you see a completely black seal. It is difficult to say exactly how little melanism occurs in the seals in the sea. We have only seen melanism in grey seals, not in common seals (yet).

Zwarte zeehond - melanisme

Melanism in other animals

Melanism does not only occur in seals. There are also melanistic foxes, chickens, frogs, guinea pigs and house cats. A very well-known example of a melanistic animal is the black panther.

Did you know...

The black panther is not a separate animal species, but is a collective name for all melanistic felines?

In most cases, the completely black animals are in the minority in felines, but in the feline jaguarundi about 80 percent are melanistic animals [1].

Advantages or disadvantages of melanism

Melanism can be beneficial and/or disadvantageous for animal species and individual. For scientists, melanism is an interesting topic. If animals can differ in appearance, does black fur have certain advantages or disadvantages? It often happens that the colour and/or pattern of the coat matches the colours of the environment.

White pups

Think, for example, of a seal pup with a white coat lying on the ice in the North Pole. This way he is less noticeable to predators who would love to eat the pup. On the other hand, it also works to their advantage for predators. A polar bear is colored white for the same reason, so that seals are less likely to see it coming when it goes to hunt them. If you have the color of your environment, it works like a protective color: you stand out less which increases your chance of survival.

For that reason, melanism should not occur in the North Pole. According to this theory, black animals would stand out in such a snow-white environment. The result: they are eaten earlier and therefore even rarer. The fewer black animals there are, the less likely they are to reproduce and pass on their genes that provide the black coat.

Melanistic advantage in the Netherlands

For a seal in the Netherlands, a black fur has no advantage or disadvantage in any case. This is because the seal is at the very top of the Wadden Sea food chain. That is so because there is no other animal that eats the seal.

Also, there are no other reasons that can cause melanistic seals to increase or decrease. The number of black seals in the seal population depends entirely on how many of these melanistic seals reproduce.

Did you know...

Animals can change colour when their environment changes? A special and well-known example of this is the pepper-and-salt butterfly [2].

Adapt to the environment

The name says it all: the pepper-and-salt butterfly was white with black. But in the 19th century (1800-1899) completely black specimens of this species suddenly appeared in England. These black butterflies are very noticeable when they were on a light tree trunk. Why did more black butterflies appear at that time if that seems to be a disadvantage?

In this century, the Industrial Revolution was in full swing. Due to the large spread of factories and machines, there was a lot of air pollution. The black soot in the air precipitated, turning buildings and trees black. Suddenly you didn't stand out as a black butterfly. In cities, the ratio turned around. Black butterflies of the pepper-and-salt butterfly were mainly found there, while the light variant occurred in nature reserves.

Depending on various factors, such as your environment, melanism can therefore work in your favor! Can you think of in which nature reserves a black seal has an advantage compared to a grey seal?


  2. Majerus, Michael E. N. (2008). “Industrial Melanism in the Peppered Moth, Biston betularia: An Excellent Teaching Example of Darwinian Evolution in Action”.Evolution: Education and Outreach 2 (1): 63–74. doi:10.1007/s12052-008-0107-y

On this page

Continue reading

Lees verder

Seal mother also let pups be

Knowledge repository

Research in the Dollard has been ongoing since 2015. In all these years, several results have been achieved. We have learned more about the behaviour of seal mothers and their pups. We have also been able to use the results to adjust our rehabilitation policy: we now observe seal pups longer and catch fewer pups. Below we share the insights we've gained so far.

See also

  • Pup roept

  • Zeehond - moeder en pup zogen

  • Onderzoek in de Dollard

First insights

We used to think that a seal pup lying alone along the shore had been abandoned by its mother. They were also called 'howlers', because of the sound they made. But were they really abandoned? Our researchers have followed several mothers over the past few years. They looked at how long it took them to return to their pups.

Thus, we found that puppies were regularly left alone for a while (see the picture below). Some mothers even stayed away for eight hours. Then they would reappear and give their pup milk. This means that a pup alone is not necessarily abandoned. Seal mothers also have to look after themselves and hunt for food. It is quite normal that she cannot be with her pup all the time.

Veldonderzoek - observatiemuur in de Dollard

Peculiar behaviour of seal mothers and pups

The researchers also looked at the behaviour of mothers and their pups as well as pups of other mothers. They found out that the pups not only drink from their own mother, but also join in drinking from other mothers. Not all mothers allow this; there are mothers who only give milk to their own pup (see image below). But there are also very social mothers, who let dozens of other pups suckle. One mother gave milk to 33 different pups!

Zeehonden rusten uit op drooggevallen zandbank

Seal pups cannot survive on the milk of other mothers alone. For the first 7-11 days of their life, the pup drinks from its own mother, after which the mother sometimes lets other pups drink from her. If a pup loses their mother within that first 7-11 days time, they can get milk from other mothers. This does not mean that the abandoned pup will survive with only milk from other mothers. In fact, the pup gets most of its milk from its own mother. So other seal mothers cannot replace their own mothers, but collectively they can care for several pups. It is like a daycare centre for seals.

Less rehabilitation, more observation

This research has allowed us to change our seal rehabilitation rules. Thanks to these discoveries, seal guards observe pups that are alone for up to 24 to 48 hours. This is a lot longer than before. This gives pups a greater chance of staying with their mothers. Only pups who have really lost their mothers are now taken in.

On this page

Continue reading

Lees verder

Blind seals

Knowledge repository

Did you ever wonder how a blind seal survives in the wild? Seals are used to hunting with low visibility. Most seal species look for their food dozens, sometimes even hundreds, of metres underwater. Light does not penetrate as deeply into the sea. As a result, seals often hunt in the dark. Although their eyes are large and see more underwater than we do, it is the whiskers that are indispensable when hunting.

See also

Whiskers of a seal

Seals have very sensitive whiskers, which allow them to detect the smallest movements in water. As such, they find their way through the water current and feel the vibrations given off by fish. These are picked up by the hundreds of nerves in each whisker. Minutes after a fish has swum away, a seal can still detect it. This is because a seal's whiskers are not smooth, as a dog's or cat's whisker, but ribbed. Therefore, they glide smoothly through the water.

This allows Blind, the blind seal seen for years by our researchers in the Dollard, to find enough food without seeing anything.

On this page

Whiskers of a seal

Continue reading

Lees verder

Maternity strategy of seals

Knowledge repository

One of the studies that our researchers conduct in the Dollard, is to analyse ten seal mothers. Over several years, they look at whether their mothering strategy changes. For example, they look at whether she feeds more or fewer pups from other mothers and whether she always feeds the same pups or whether this varies. In addition, they study whether the seal mothers give milk each other’s pups.

See also

Zeehond - moeder en pup zogen

Allonursing, what is it?

Allonursing means that a seal gives mothermilk to a pup that is not their own. But why would a seal mother give milk to another pup? The causes and function of allonursing in seals remain mysterious, as giving milk takes an enormous amount of energy for the mother seal. There is also an increased risk of transmission of pathogens between mothers and other pups. It is therefore a crucial research subject.

Why do mother seals conduct allonursing?

There are five hypotheses that could explain why seals nurse their other pups:

  1. Allonursing is the result of misguided parental behaviour.
  2. Seal mothers alternate to nurse each other's pups.
  3. Females nurse foreign pups for additional health benefits.
  4. Females feed foreign pups to discharge milk not drunk by their own.
  5. Inexperienced seals spontaneously give milk without reproducing, or because they have lost their young. In order to improve their mothering.

Dit onderzoek doen we tijdens onze observatie in de Dollard: de periode waarin gewone zeehonden hun pups krijgen. Er worden filmpjes gemaakt van de moeders die melk geven. Met photo identification kunnen we de verschillende pups herkennen.

On this page

Continue reading

Lees verder

Report a seal. Seen a seal in need? Call 144 (24 hours a day available) Read more.