In 1945, President Truman issued a proclamation claiming U.S. jurisdiction over U.S. continental shelf resources adjacent to the U.S. coast. This has generally been viewed as the advent of coastal nations extending territorial seas and declaring fishery and economic zones.
Shortly thereafter several Latin American nations proclaimed marine jurisdictions extending 200 miles off their Pacific coasts.
Image Source: http://www.sos.mo.gov/archives/exhibits/TrumanProject/close.asp
Prior to MSA