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Oysters of the Wadden Sea

The Wadden Sea is brimming with oysters. At low tide, large oyster banks are bared, and you can simply walk out and collect and eat oysters on the spot.

The Pacific oyster was first found in the Danish part of the Wadden Sea in 1996. Since then, the oyster has spread quickly and has formed oyster banks in many parts of the sea. The banks vary in size and accessibility and are a popular tourist attractions today. You can be lucky enough to find these culinary pearls on your own, but you can also hire a guide to take you to oyster banks along the coast of the Wadden Sea. You are allowed to collect Pacific oysters in the sea as long as they are for your own consumption.

Pacific oyster - biology and lifecycle

The Pacific oyster (Crassostrea gigas) is, as indicated by its Latin name, a giant among oysters. The shell can be up to 40 cm long. The oysters can reach up to 30 years of age and are very hardy in that it can tolerate water temperatures ranging from between - 5°C to + 40°C. Due to its tolerance to great temperature fluctuations, the species has spread across the world. The Pacific oyster originates from the Pacific coast of Asia, but it has been successfully introduced in several other continents. The Pacific oyster filters seawater to feed on the phytoplankton in the sea. Studies have shown that the Pacific oyster can filter up to 12 litres of seawater per hour. The females spawn in late summer and release between 20 and 100 million eggs per spawning. However, the Pacific oyster does not spawn every year in the Wadden Sea as it needs water temperatures above 20 °C for a longer continuous period in order to spawn.

From the Pacific Ocean to the Wadden Sea

In 1922 the European flat oyster (Ostrea edulis) fell victim to crayfish plague. The disease led to high mortality rates and in the course of a few years, the European flat oyster had been completely wiped out. The lack of oysters led to the import of Pacific oysters from Japan in the mid 1900s. This oyster had proven to be resistant to the disease that led to the demise of the European flat oyster. The first Pacific oysters were released in France and the Netherlands in the period 1964-1969, and in 1986, Pacific oysters were released in the German Wadden Sea. As a consequence of the release in the waters off the island of Sild, in 1996 Pacific oysters were found the Danish Wadden Sea. Because the Pacific oyster is not an indigenous species in Danish waters and has a negative effect on other indigenous species, it is referred to as an invasive species.  You are therefore helping maintain Denmark’s indigenous nature when you harvest oysters for your own consumption. 

The Pacific oyster affects the eco-system

The many oyster banks in the Wadden Sea act as large reefs that provide numerous hiding places for some of the smaller animal species that live on the seabed. The abundance of hiding places has led to an increase in the number of crabs and starfish, which, in turn, has led to an increased threat for their prey. Thus, the Pacific oyster may indirectly shift the balance of the food chain in the Wadden Sea. Moreover, the Pacific oyster grows in clusters on top of already existing banks of common mussel. This makes the hunt for food more difficult for birds such as the common eider, oystercatcher and herring gull that primarily live off common mussel.

Warning

Shellfish are a delicacy to be enjoyed with some caution. Collecting and eating oysters and mussels from the Wadden Sea may entail a risk.

  • Make sure you check the local conditions before you collect oysters and mussels in the Wadden Sea - and never collect shellfish if the authorities have issued a warning against it. You can check the status for commercial harvest areas. See the links below for more information.
  • Never collect oysters or mussels near the mouth of a stream, near areas where sewage water is discharged or near a harbour entrance.
  • Follow your guide’s instructions when you are on an oyster safari

Algae toxins

Algae toxins can accumulate in oysters and mussels because these creatures feed on micro algae, including poisonous algae, that they filter through their system. If the water contains poisonous algae, the oysters and mussels accumulate the toxins at levels that may make you sick. 

  • Algae toxins are most common during the summer, when the water is calm and temperatures are high, but they may occur at other times of the year.
  • Algae toxins are heat stable, so no amount of cooking will remove them.
  • These toxins may cause diarrhoea, paralysis, numbness (e.g. of fingers), difficulty breathing, coma and, in some severe cases, death.
  • Symptoms of algae poisoning appear just a few hours after exposure and may last up to 2-3 days.     

If you suspect shellfish poisoning, call the emergency services immediately (in Denmark 112).

Pathogenic bacteria and viruses

Oysters and mussels can accumulate pathogenic bacteria and viruses that can cause diarrhoea, vomiting and stomach infection.

  • Pathogenic bacteria and viruses are most often present in waters after heavy rain due to overflow of sewage water.
  • If you heat mussels and oysters to 100o C for a minute, you will kill any viruses and pathogenic bacteria.

If you experience a severe stomach upset, call the doctor.

Walking and driving on the tidal flats

Walking and driving on the tidal flats is not without risk. The tidal waters and sudden changes in the weather can be dangerous. Therefore, you should always pay heed to any warnings about walking and driving in the Wadden Sea. It’s a good idea to go with a guide if you don’t know the area.

Read more

Read more (in Danish) about collecting oysters and mussels and warnings issued by the authorities on the website of the Danish Veterinary and Food Administration:

Fødevarestyrelsens retningslinjer

Fødevarestyrelsen varslinger


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VisitIf you want to learn more, visit one of the National Park’s exciting exhibition venues.

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