This means that he was retained (“zurückgeh.” = “zurückgehalten”) by a neutral country until now (“bish.” = “bisher”), as this list of abbreviations for the navy lists suggests. Now he is prisoner of war (“krgef.” = “kriegsgefangen”). “A” could indicate that this information was received from a foreign country (“Ausland“), as “A.N.” is elsewhere used for “Auslands-Nachricht” (message from abroad). It could also refer to American captivity, as “krgef. F.” is used for POW in France or “krgef. E.” for POW in England according to the abbreviation list.
There are also entries like
- bish. v. krgef., v. u. zurückgeh. (until now wounded and POW, [now] wounded and retained)
- bish. krgef., zurückgeh. (until now POW, [now] retained)
There is an earlier (1915) record of someone with the same name (Peter, Robert from Bremerhaven): http://des.genealogy.net/search/show/918673 He is marked as retained there.
I would assume that the ship was in a neutral harbor and the servicemen where later passed over to a enemy country or the former neutral country declared war on Germany.
The Deutsche Dienststelle (WASt) holds 1.6 million records on German navy men (Marine-Archiv), maybe you can get more information from them. See my answer on How to find information on German soldiers from World War I and World War II? for more details.
For other abbreviations in the Verlustlisten related to casualties please see Reference Key for Injuries for WWI German Casualty Lists?