Ecomare

A seal swims just as readily on its back as on its belly, standing upright or upside down. The front flippers serve as paddles; the body and hind fins provide the propulsion.
Seals can dive down to depths of hundreds of meters. During the first few minutes, they swim actively downwards, after which they go into a kind of gliding flight while they sink even deeper. Their body is totally adapted to long and deep dives. Their blood can absorb much more oxygen than human blood. Furthermore, they can lower their heart rate tremendously during the dive, from 40 to less than 1 beat per minute. This means they don't need to breathe as often. When they ascend from deep depths, they can pump out the inhaled air from the air sacs, preventing deadly nitrogen bubbles from entering the bloodstream. Once above water, the heart beat of a seal increases to 120 beats per minute in order to provide oxygen for the organs.

Seals sleep in the water as well as on land. In the water, they sleep floating in a standing position, like a fishing bobber, or floating horizontally on the surface. Because they are sleeping and not actively swimming, they can stay under water much longer than when hunting for food. There are known incidents where seals stay under water up to half an hour, however on the average, their stay is not longer than fifteen minutes.

The number of seals in the Wadden Sea region is well documented. Scientists from IMARES count the seals in the Dutch Wadden Sea from an airplane by taking photos of them while lying on the sandbanks. One third of the population is then under water, so the scientists add 30% to the total counted in order to reach a final estimation.

Of the seals that inhabit the international wadden region (Netherlands, Germany and Denmark), the large majority are harbour seals, numbering around 35,000 in 2011 (adults and pups). However, harbour seals also inhabit other waters. In the North Sea area, they are found around the Scottish islands, in the Wash, along the English, French and Belgium coasts, in the Danish Skagerrak and along the southern coast of Norway.

Grey seals are found primarily around the Scottish islands, along the British east coast and in Cornwall. In December 1999, there was even a young seal seen in the Thames beyond London. In the Dutch Wadden Sea, a colony of grey seals was established in the 1990s. Since then, it has expanded into the largest colony in the entire Wadden Sea. In the Dutch Wadden Sea alone, a total of 2388 grey seals were counted in 2011. The largest groups are located west of Terschelling, but small groups or individuals are sometimes seen elsewhere. The German islands of Helgoland and Amrum have colonies, numbering more than a hundred animals. Satellite observations of seals with transmitters show that grey seals from the Wadden Sea can swim all the way to Northern Scotland.

Every day, the North Sea receives large loads of disposed wastes originating from agriculture, industry and cities. These materials find their way to the sea via rivers which mouth in the North Sea. Other waste materials come from dumping and shipping accidents at sea, and via the air in the form of acid rain. Plankton absorb the poisons released, which is how they enter the food chain. Eventually, the poisons reach the marine mammals via the benthic animals and the fish. The concentrations of poison are highest in those species of animals living closest to the coastline. This is because the material from the rivers enter the coastal water more or less undiluted. A well-known example of poisoning is the influence of PCBs on the reproduction and immunity systems of the seals in the Wadden Sea.

Another problem for seals in the North Sea and Wadden Sea is the increasing pressure in their haul-out areas, from tourists and military activities. Especially during the summer when the pups are born, this disturbance can form a threat for the seals. The newly born pups cannot be separated from their mothers during the first few weeks. They drink milk from their mother during the hours that the sandbanks are exposed. If mother and child are continually chased into the sea by tourists, then the pup will not get enough to drink, which can have serious consequences. The young animal will grow weak and be more vulnerable to diseases or more readily contaminated with parasites. Sometimes, the mother and pup get separated from each other. During the summer season, the water police must take action against tourists on average four times a day to stop the disturbance of the seals lying on the sandbanks.

Fish that swim into a fyke form an attractive prize for a seal. The seal will swim into the fyke to catch the fish. Once in the fyke, the seal cannot get out and drowns. Since 1994, all fykes in Dutch tidal waters are required to be equipped with a retaining net, specifically to keep out seals. This retaining net is a large-meshed net at the entrance to the fyke, through which eel and other fish can swim, but hold back seals. Nevertheless, seals are still drowning in fykes unequipped with the required retaining nets.