The Caribbean held additional strategic significance to the United States. The United States' Gulf of Mexico coastline, including petroleum facilities and Mississippi River trade, could be defended at two points. The United States was well positioned to defend the Straits of Florida but was less able to prevent access from the Caribbean through the Yucatán Channel. Bauxite was the preferred ore for aluminum, and one of the few strategic raw materials not available within the continental United States. United States military aircraft production depended upon bauxite imported from the Guianas along shipping routes paralleling the Lesser Antilles. The United States defended the Panama Canal with 189 bombers and 202 fighters, and based submarines at Colón, Panama and at Submarine Base, Crown Bay, St. Thomas, U.S. Virgin Islands. United States Navy VP-51 Consolidated PBY Catalinas began neutrality patrols along the Lesser Antillies from San Juan, Puerto Rico on 13 September 1939; and facilities were upgraded at Guantanamo Bay Naval Base and at Naval Air Station Key West.