Wadden Sea is an important habitat for both harbour and grey seals. Harbour porpoises and white-beaked dolphins are the sea's only resident cetaceans. They were once extinct in the southern part of the sea but have also re-colonized that area again. Many other cetaceans only visit seasonally, or occasionally. In early history, North Atlantic right whales and gray whales were present in region, perhaps using the shallow, calm waters for feeding and breeding. It has been theorized that they were hunted to extinction in this region by shore-based whalers in medieval times. They are generally considered long-extinct in the region, but in the Netherlands a possible right whale was observed close to beaches on Texel in the West Frisian Islands and off Steenbanken, Schouwen-Duiveland in July 2005. Recent increases in number of North Atlantic humpback whales and minke whales might have resulted in more visits and possible re-colonization by the species to the areas especially around Marsdiep. Future recovery of once-extinct local bottlenose dolphins is also expected.