Hmm, I'm no fishing man, but I think that when you Trawl you are using a big fishing boat that pulls a very long net on the ocean. When you troll, you are rowing a small boat (in a lake perhaps) with a special kind of fishing rod ...
Well, trawling is old spelling of trolling, as best as I can figure. I think they're the same thing. Which is, fishing with a lead-weighted outrigger from a slowly moving boat. This requires heavy poles and balance systems for the extra weight, in some cases (ocean) or can simply be a lure or small sinker on small lakes. The idea is that by dragging your fishing rig (weighted, bait and hooks) through the water, it looks more like a real fish, and a larger fish will swim up and snatch it.
Hence, I also believe that "trolling for newbies" is that someone tosses out a "bait" statement, which then others pounce on.
Then again, "trolls" do live under bridges, and so do some of the residents of message boards, I'm sure. ;-)
