While you could use general terms like "fleet" or "group" for a collection of trawlers, there isn't a word that specifically evokes the image of a group of trawlers at sea.
If you're looking for a more evocative term, you could try:
* "A haul of trawlers"
* "A school of trawlers"
* "A fleet of trawlers"
Ultimately, the best term to use will depend on the specific context and the desired effect.