Either way black market is illegal so I don't think they will be really bothered with the bag limit.
E.g say a person caught 10 fish in 30min and obviously his going to keep fishing, what if every time that person catches a larger fish he chuck one of his small fish out. When I go LB and check peoples buckets most of the fish are dead already. So they would be chucking out a smaller fish (dead) to replace with a larger one.
What a waste, I can imagine people doing that, it's like you caught just undersize fish and we know that the fish won't survive but still by law we have to chuck it back out.