Female harbour seals are sexually mature at age 4; males are only mature at age 6. They mate from early August to mid September. Although the pregnancy of the harbour seal lasts for 11 months, the implantation is delayed during the first months. The fertilized egg enters the womb after three months and then begins to grow. The actual pregnancy is only around 7 to 8 months.

The pups are born during low tide on exposed sandbanks. They need to be able to swim almost immediately, as the tide rises. The pups drink milk from the mother for three weeks and in this period, the pups grow from 10 to 24 kilograms. They grow so quickly because the mother's milk is very nutricious and has a fatty content of 45%. When nursing, the mother and pup lie on a sandbank during low tide. The nursing usually takes place at the same time that people are the most active on the mud-flats with boats and mud-flat hikes. That is why special haul-out areas have been established, where people are forbidden to enter during the birthing and nursing period. This way, the young can nurse undisturbed with its mother and gain lots of weight. After the nursing period, the mother leaves her pup who must now fend for itself. It must find its own food and learn to catch fish. That extra weight is necessary because most young seals need time to become handy at catching a meal.

Seals eat mostly what's available at the time. However, each seal seems to have its own preference for specific species of fish. Therefore, the variation is large and their diet changes with every season. They prefer to eat fish that live close to the bottom, such as flatfish, cod species, gobies and lesser sandeel. Adult seals that live off of flatfish, need to eat up to 8 kilograms per day. Animals in captivity are fed fattier fish, such as mackerel or herring, and therefore eat only an average of 3 to 4 kilograms per day.

Every year, the number of seals in the entire wadden region are counted. In 2012, there was a total of 26.220 harbour seals counted in the Netherlands, Germany and Denmark, of which 7267 were pups. The seals are counted during low tide, when most of the animals lie on the sandbanks. One third of the seals are likely swimming during the count, so that this amount must be accounted for in the total. That is why the total amount in the entire Wadden Sea is estimated at 38,500 animals. In the graph, you can see how many seals have been counted since 1975. You can clearly recognize the two virus epidemics, one in 1988 and the other in 2002.

There used to be a large population of seals in the Dutch delta region. Based upon the number of seals shot at the beginning of the 20th century, the seal population here was between 6000 and 12,000 animals. In the preceding centuries, hunters in the delta caught hundreds of seals yearly, but the population was apparently large and strong enough to make up for the loss. Around 1935, the seals were counted for the first time, around 1300 animals. The population had declined due to hunting with rifles.

Just like the Wadden Sea, the delta region was also an important area for the harbour seal in the Netherlands. Unfortunately, life for the seals in the delta region declined. In 1990, there were only 14 seals. This was probably related to the heavy pressure from hunting up till 1961, followed by very poor water quality in the area. In addition, recreation, tourism, fisheries and shipping continued to increase, which continually disturbed the seals. They also lost a large part of their habitat when the Delta Works were built. Since 1994, life for the seals in the delta slowly started to improve. More than 150 seals were counted in 2005 and 530 seals in 2011. Food is not a limiting factor for the animals. In the adjacent Voordelta and North Sea coastal zone, there is the same amount or even more fish than in the Wadden Sea.

People at sea often see seals lying in groups along the shore or on tidal flats. That's why for a long time, we used to believe that the seals were strongly bonded to these banks and shores. But when a number of seals were tagged with a transmitter, it became clear that seals swim over a large area, and they don't only stay in the vicinity of their haul-out areas.

IMARES is working in the Netherlands to study seals with transmitters. The first group of nine seals were provided with transmitters placed on their heads in 1997. The seals were born in captivity and released in the Briels Channel, in the South-Holland delta region. From these transmitters, the scientists received information concerning seal behavior: where do they go to hunt, where are their preferred haul-outs, do they wander far away and if so, where to?

SInce then, only seals from the wild have been given transmitters. The scientists want to learn more about the areas that are important for seals and whether they are affected by human activities, such as target practice or ramming. The transmitters are attached to the seal's fur. When the seal molts, the transmitter falls off. Newer technique means smaller transmitters so they are now placed in the neck instead of on the head.

In 1988 and 2002, the harbour seals in the North Sea and wadden region were infected with a virus. Both times, more than half of the harbour seal population in the Dutch Wadden Sea died. Although the seal virus is not dangerous to people, it is always a good idea to stay away from a sick or dead seal. Dead animals can carry other germs that are harmful for our health. In addition, some seal diseases can also be contagious for dogs.