A trawler is for fishing, right? That is not the same as merchant marine, so I would guess civilian. In which case maybe they could be interned by a belligerent, but they are not POW's.
I'll bet one of the regulars here knows how it works.
Quite a few civilian vessels of all kinds were pressed into navy service by all the waring nations. In the Royal Navy, the term “trawler” was kind of a catch-all to describe small vessels that performed routine patrol. Obviously, the U-Boats, Battleships, Aircraft Carriers, etc. got all the newspaper ink.
http://www.harry-tates.org.uk/
The Royal Naval Patrol Service or Harry Tate’s Navy, as it was more popularly known, was a very special service indeed with its own rules and regulations and the unique distinction of having its own exclusive Silver Badge, worn by sea-going officers and ratings alike. Its headquarters were at Sparrow`s Nest, the municipal pleasure ground at Lowestoft, and its fighting fleet consisted of hundreds of requisitioned trawlers, whalers, drifters, paddle steamers, yachts, tugs and the like ‘Minor War Vessels’ as the Admiralty called them.
http://en.wikipedia.org/wiki/Royal_Naval_Patrol_Service
Royal Naval Patrol Service
http://www.rnps.lowestoft.org.uk/more.htm
Royal Naval Patrol Service Association
The discussion about the Trawlers made me think back to some of the WW II “faction” (historical fiction) that I have read over the years.
For all who might like rip-roaring stories of war at sea, this author has no equal. Back forty years ago when I read more for fun, I found his stuff. It is as good as anything C. S. Forrester every wrote.