Piracy on the High Seas always used to be regarded as an act of war.

For some reason the EU in its wisdom decided the pirates were criminals and that piracy is a crime, which means the same procedures of arrest apply to an armed pirate as apply to a shoplifter.

"Q"-Ships and what they catch is purely "luck-of-the-draw" its like playing blind man's bluff. They only can serve as a deterrent provided the pirates are aware that the "Q"-ships are out there, there value is that they cause the pirates to be more circumspect in what they attack.

Attack the anchorages where they are holding the ships. Attack the towns nearby, establish convoy routes, use Maritime Patrol Aircraft and watch, watch, watch. The picture will soon emerge and as soon as you have that picture the anomalies stick out like balls on a billiard table.