Probably regional dialect. I spent my teenage years "trolling" for just about every part of the female anatomy, but never heard anyone use the word "trawling" in the same way. That said, trolling is a much more popular fishing method where I'm from.

In fishing isn't it a question of volume? IIUC trolling is moving very slowly/wakeless with a small number of lines, and trawling is a big net that catches everything?