Mystery Preditor Bird Indentified?

Twice this week my wife has come home midday and noticed a very large bird by the pond. Once the bird was sitting on the top of the lamp post over the deep end of the large pond. Today he was sitting on the apple tree over the shallow end of the pond. Luckily the net over the pond appears to have kept him from dining on our goldfish. He flies away very quickly but from the sketchy description, it sound like we have a Hawk with an appetite for goldfish. I hope I can get a good look and maybe a picture of what I'm being challenged by. Once I know, it will be easier to design a defense.