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isn’t difficult. Getting away from Stavoren is a lot more challenging. Our target was the island of Texel in the Wadden Sea, where Lisa and I were to find Pacific oysters, also known as Japanese cupped oysters. Without a car, we were forced to take what was disparagingly referred to as the “tourist route.” This meant a ferry from Stavoren to Enkhuizen, a train to Hoorn, another train to Heerhugowaard, another train to Den Helder, and then a ferry to Texel.
    The Netherlands is not a really big place. Even so, we got to see an awful lot of water and this was contaminated by an incredible assortment of introduced aquatic species. These include clawed frogs from Africa, water ferns from Brazil, Pacific crabs from Japan, round gobies from Russia, tubenose gobies from Germany, slipper limpets from the U.S., pond turtles from Italy, Wels catfish from Hungary, and soft-shelled clams from Canada.
    The Wadden Sea is 500 kilometres of coastal waters stretching northeast from Den Helder in the Netherlands, past Germany, and on to Denmark. It is a region of low-lying islands, sandbanks, and mud flats, and long recognized as home to incredible biological diversity, making it a sought-after refuge for citizens looking for an unspoiled corner of Europe. Those with a commercial mind think of it as an important nursery for edible North Sea marine life.
    In the 1800s, the Wadden Sea had a thriving oyster fishery based on the native European oyster. But, as so often happens, the harvest was a bit too zealous, and the fishery collapsed. Over the yearsattempts were made to find ecological replacements for the native oyster, including the American oyster and the Portuguese oyster, but these efforts met with failure. Then came the new kid on the block.
    In biology circles, the Pacific oyster has a rather nasty reputation for being where it shouldn’t be, including the Wadden Sea. Around sixty countries have reported it in their coastal waters, and it has become well established in at least twenty-four. In countries including Australia, Canada, Chile, and the Netherlands, the Pacific oyster is considered to be invasive, having a negative impact on the ecosystem and/or the economy.
    The Netherlands has long been a seafaring nation, and Pacific oysters have probably been arriving in the Wadden Sea for centuries, attached to the hulls of ships. For reasons not fully explored, these oysters didn’t establish themselves. Then they were introduced intentionally in the hopes of re-establishing the oyster fisheries where their American and Portuguese counterparts could not. The Pacific oyster followed a pattern typical of the establishment of introduced species; their numbers remained low for a protracted period before rapidly growing in quantity and distribution. Today there are more than 60,000 tonnes of Pacific oysters living on the Wadden Sea’s tidal flats. Thirty years after first being reported as a self-sustaining species, their numbers in the Netherlands show no evidence of levelling off.
    Pacific oysters are filter feeders, extracting plankton and organic material from the water with great efficiency. We are most familiar with them in their adult form, with two hard shells hinged along one side. But like others of their kind, very young Pacific oysters first spend three or four weeks as tiny free-swimming larvae before settling down to mature. When first establishing themselves in an area, Pacific oysters settle on a hard substrate, such as blue mussel shells. Once the oyster population is established, young oysters are perfectly capable of settling on the shells of older oysters. They require about two years to reach a size worth eating. They don’t stop there, living as long as thirty years, during which time theygrow to forty centimetres in length and more than a kilogram in mass. In places, they can grow to a density of between 500 and 1,500 individuals per square metre.
    How bad are Pacific oysters in the Wadden Sea? There may not be