Gorad Goch is a fish farm and it works very simply. In front of the buildings there is a pool and when the tide is high the fish stray into it, but then the tide goes out and ... well do I need to tell you the rest? I'm sure the herons will have their share.
Incidentally, I was hoping to show you one of the original sections of the Britannia Bridge tube but it happens to be on the Caernarfonshire side so I shall make a point of finding it next time I am here.