The Afsluitdijk, a 32 kilometre dyke which links the provinces of Noord-Holland and Friesland, is currently undergoing an extensive programme of maintenance to ensure its safety to 2050. Joint effort The migration river is a combined effort by a number of conservation organisations including the Waddenvereniging and It Fryske Gea and the provincial authorities of Noord-Holland and Friesland. Fish numbers had been falling to worrying levels, Waddenvereniging marine biologist Wouter van der Heij told local broadcaster Omrop Fryslân.