I think fish traps is a cert, as all it would take would be careful placement of rocks to trap fish as the tide went out, like an artificial rock pool. Such things are found in a few old-style cultures around the globe, so it's likely to be a very ancient practice.

Oyster beds might be a bit different, I have no idea how shellfish behave. I do know that if I were subsisting on a beach, I'd rather know where I had a reliable supply of shellfish, preferably in a nice sheltered spot. I might even go so far as to try and enhance a natural feature or two to encourage the oysters to move in. It would make it much easier to build a midden if you had a handy oysterfarm nearby.

I guess we'll never know, as the sites would presumably be quite far out to sea by now.