You seem to have a serious misconception about either the US Navy or the oceans.
There are actually satellites that do it. Over the course of many days. And it's not easy or quick to act on their feedback. Something like a military or police patrol simply can not happen.
This isn't true at all. I worked for a long time on the first implementation of the Navy's Maritime Domain Awareness System and my wife currently works for the NRO's Enterprise Collection Orchestration system that auto-tasks orbital surveillance platforms in response to automatically detected global events. Many of the capabilities are classified, but I can assure the US Navy both knows about and has some means to respond quickly (not necessarily by dispatching a manned vessel) to anything bigger than a driftwood log that appears on an Earth ocean.
The reason they don't is the Navy is not a law enforcement agency and this is not their mission.
I mean it could, but it would be pretty expensive. We know generally where ships go to fish. You watch those areas and get an idea of the various ships, you stage your ships nearby and wait. If you're really keen on being targeted you could follow ships back to port and over time have assets there wait to ID the individuals on the ship.
The problem is this would cost millions of dollars and take weeks or months unless you got very lucky. The Navy could do it in their sleep but why would they? Nobody who cares enough about this problem has the means to execute it, and nobody with the means cares.
While that may be true in principle, it seems likely to me that price is a very important factor for many or even most people, so much so that they don't look too closely at provenance. Were that not the case then I would expect the market for battery chickens to have dried up by now, for example.
I know nothing about the industry but I did wonder if an innocent excuse is that as a fishing vessel you wouldn't want to tell everyone else where you are fishing?
They do claim that, although they should still share with the appropriate regulatory body where they were fishing. I know at least some of them there are rules about what can be done with fishers' data for exactly that reason.
More likely they don't want to follow quota, rules about what they can or where they can fish.
That often appears to be true in the non-innocent sense that they stop telling everyone else where they're fishing somewhere near the edge of zones they're not allowed to fish in...
Small shops next to large banks almost never get robbed - convenience stores in a neighborhood that cops tend to ignore... that's a different story. Most of the world's oceans are pretty thoroughly patrolled so if the risk is the same you might as well go after the bank.
