This is how the animal kingdom keeps itself in check. It happens on land, at sea, in our backyards and in our ponds and lake. For a quick example we have the Great Blue Heron, Osprey, Racoons, snakes and turtles just to mention a few plus the fish themselves. I know some folks may find it offensive that animals are brutal or savage when they take down their prey, but look at your house cat attacking a mouse or a dog going after a ground hog. I could go on but just wanted to show you this short video of a Jaguar versus a alligator.
We also have a post about a frog and a snake, just a little caution about the pictures.