If the guy just took the fish and left, probably nothing you can really do. You can call DEM, but they’re not going to hunt the guy down. If you notice a pattern (he does it every day, or most days) definitely call DEM.
You could tell the guy that if he put down the sissy stick and fished like a real man, that he might actually catch a legal fish
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Assuming that he wasn’t a big guy and that you though you could take him easily if it escalated, I would have said something to him.