The Bahía San Blas Multiple Use Nature Reserve is located on the maritime coast of the Patagones district, in the extreme south of the province of Buenos Aires. San Blas and Anegada Bays are composed of a group of islands, banks connected by an extensive network of shallow water channels, with an extensive intertidal zone. The environment is completed with brackish water marshes, small islands and islets and other estuarine environments, with sandy beaches, other stony beaches and some coastal dunes. The vegetation corresponds to halophilic steppes (adapted to environments with little water availability). There are large crayfish beds in the area, which are feeding grounds for migratory birds. Migratory coastal birds, seabirds, marine mammals, rays, and sharks are found in the reserve. The red shorebird and Nearctic migratory species that visit the area during their migration are threatened. The area is noted for the existence of large colonies of the crabeater gull, an endemic and endangered species of the southwestern Atlantic coast. Together with the Bahía Blanca estuary ecosystem, they concentrate approximately 90% of its breeding population. The reserve’s coasts and waters are used by numerous species of marine mammals, especially the franciscana, an endemic species at great risk of extinction. The area is also home to the most important sea lion colony in the province of Buenos Aires. Its coastal waters are used by a large number of endangered species of rays and sharks.