06 April 1937,
The Franco government announced the maritime blockade of Bilbao. Britain and France did not declare it and only accepted control of merchant vessels at 3 miles. The coastal batteries protecting Bilbao dominated this distance and once the 3 mile barrier was overcome, merchant ships knew that they would not be stopped. That same day the first episode took place: The Francoists tried to stop the English ship Thorpehall, but with the help of three English destroyers he managed to reach Bilbao. In the photo, the sailors of the Gipuzkoa bou raising the ikurrina.
Source:
Juan Pardo San Gil, Euzkadi Army. The Navy of Auxiliary War of Euzkadi (1936-39) (Naval Museum, DFG).