Yea, I wish there was a way to screenshot people stealing your fish. Zeekio Aco was hanging out at the fishing spot spamming about their being a bot at the fishing spot (which there was not, I was chatting with everyone there), and would run up next to me everytime I had my pet spit out its contents so I could get more bait to keep fishing and he would grab some of my fish. I got him to return them the first time, but the second time he grabbed two piles and then claimed nothing happened - and the other people down there tried to get him to return it too, but I had / have no proof that he took them, and I was out of the other fishers sight range I believe, so...
