Basically, you don't trust the fishermen, and if you don't trust them, monitoring or checking out there should be what it takes.
Now, here's the part where I have a hard time to take that as an answer. I hunt and fish, just recreationally. If I get a deer tag, I'm allowed one deer. If I take more than one deer, I can be checked by an officer or whatever. It's the same thing with fish limits. That's what controls the catch.
Why should it not work the same here?