Dear Mr. Redden,
Thank you for posting your request on History Hub!
We were unable to locate any immediately accessible information on whether any US merchant ships were seized in Japanese ports when the war started between Japan and the United States, which (because of the international date line) was December 8, 1941 in Japan.
If such an event occured, records concerning it might be in one of multiple record series.
We searched the World War II Prisoners of War Data File, 12/7/1941 - 11/19/1946 available through Access to Archival Database (AAD) and located 515 records for merchant mariners held by Japan. However, this database does not indicate the date that they were captured.
We searched the National Archives Catalog and located Records Relating to Civilian Internee Files, 1942 - 1945 and Subject Files, 1939 - 1955 in the General Records of the Department of State (Record Group 59); as well as World War II Individual Civilian Internment Cards, 1941 - 1946 in the Records of the National Archives and Records Administration (Record Group 64). These records pertain to civilians detained by Axis powers, to include Japan. We also located Central Correspondence Files, 1936 - 1950 in the Records of the U.S. Maritime Commission (Record Group 178) that may have information on this topic. Next, there are multiple other records series in several other record groups relating to prisoners of war and Axis mistreatment of prisoners, including Record of the Provost Marshal General (Record Group 389), the Records of the Judge Advocate General (Record Group 153) and the National Archives Collection of World War II War Crimes Records (Record Group 238). We do not know if the US government would have classified merchant mariners seized the day the war started as civilian internees or prisoners of war, given that they would have been civilians at the time the were captured, but that the Merchant Marine was militarized during the war and was (for some purposes) treated as a branch of the military. For more information about these records, please contact the National Archives at College Park - Textual Reference (RDT2) at the Archives2reference@nara.gov.
Additionally, we located Reports Concerning the Sinking of Merchant Vessels During World War II, 1938 - 2002, Records Relating to Merchant Vessel Casualties During World War II, 1941 - 1950, and Messages and Memorandums Regarding Merchant Ship Losses, ca. 1940 - 1991 in the Records of the U.S. Coast Guard (Record Group 26). For more information about these records, please email the National Archives at Washington, DC - Textual Reference (RDT1) at Archives1reference@nara.gov.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RDT2 and RDT1. We apologize for this inconvenience and appreciate your understanding and patience.
We hope this information is helpful. Best of luck with your research!