I know the answer to this question, but I want to see how many can answer this. Say you built a man-made pond in your backyard somewhere away from city life and you add nothing to the pond except water thats it. You of course have wildlife visit the pond, to catch a drink or wade in the water, from squirells, to muskrats, ducks, geese, birds of all varieties, racoons, and so forth. The following year you notice you marine life living inside your pond, strangely the majority of the marine life is fish of various species. How did they get there when nothing was added?