How did he GET there?

We have this shallow indentation in the ground we dug to prevent rain runoff from the neighborhood from flooding the back area. It only holds water during really heavy rains. A week or so ago Beezer found dozens of goldfish floating - dead - in the evaporating shallow water. After a few days, and a lot of sun, he found this little carcass in the mud. I'm wondering how the fish came to be in the "pond" in the first place and what I can do to save any future fishies from drying out.