In 1971, the Humboldt Bay National Wildlife Refuge Complex, which includes Castle Rock National Wildlife Refuge, was established to conserve precious habitat for the great diversity of birds, mammals, fish, amphibians, invertebrates, and plants that occur in the Humboldt Bay area. The refuge has several different units totaling almost 4,000 acres. These units consist of a mosaic of mudflats, estuarine eelgrass meadows, saltmarsh, brackish marsh, seasonally flooded freshwater wetlands, riparian wetlands, streams, coastal dunes, and forest. These habitats support over 316 species of birds and 40 species of mammals. The refuge also provides habitat for approximately 100 species of fish and marine invertebrates, many of which contribute to sport and commercial fisheries, including steelhead, coho and chinook salmon, and Dungeness crab.