Larger fish including rays, Atlantic salmon and brown trout are still present in several sections of the Wadden Sea, but others like European sea sturgeon only survive in the region through a reintroduction project. The world's only remaining natural population of houting survives in the Danish part of the Wadden Sea and it has been used as a basis for reintroductions further south, but considerable taxonomic confusion remains over its status (whether it is the same as the houting that once lived further south in the Wadden Sea). European oyster once formed large beds in the region and was still present until a few decades ago, when extirpated due to a combination of disease and the continued spread of the invasive Pacific oyster, which now forms large beds in the Wadden Sea. Especially the southwestern part of the Wadden Sea has been greatly reduced. Historically, the Rhine was by far the most important river flowing into this section, but it has been greatly reduced due to dams. As a result, about 90% of all the species which historically inhabited that part of the Wadden Sea are at risk.