In 2006, Stellwagen Bank National Marine Sanctuary established the first international sister sanctuary network to protect the same population of a migratory marine mammal, the endangered humpback whale at its northern feeding and nursery grounds and its southern mating and calving grounds in the Dominican Republic. With expansion of the network in 2011 to include the French Antilles and Bermuda, the foundation for the world’s first marine mammal protected areas network was established in the Caribbean. The sanctuary system also has sister sites with the Phoenix Islands Protected Area in Kiribati (with Papahānaumokuākea Marine National Monument) and the Egadi Islands Marine Protected Area in Italy (with Monterey Bay National Marine Sanctuary).

Sister sanctuaries are only one type of international partnerships in which the sanctuary system engages. Agency-to-agency agreements have been signed with France, Italy, and Mexico, which form the foundation for a number of cooperative projects. The sanctuary system also works on specific projects with colleagues from China, Vietnam, South Korea, and Chile. These kinds of relationships are crucial to meeting the mandates of the National Marine Sanctuaries Act by maximizing resource protection; leveraging resources and experiences; and meeting legislative mandates.