Birds can drop fish after they've captured them. Not longer than a year now, we were visiting my wife's family, and me and my father in law got to Walmart there was a dead fish in the front, but when we mentioned it to my wife who got there before us (separate cars) she said it was struggling for life when they saw it, I don't think there was a lake nearby enough, but I wouldn't be surprised if birds have accidentally introduced life into new bodies of water.